fix(receiver): non-blocking receiver opens, inplace type gate, dry-run/B4/B5/B6

Address confirmed receiver security findings B1-B6:

B1 (HIGH): add O_NONBLOCK to the three receiver read-opens that opened an
existing destination/basis entry before the S_ISREG gate
(incremental_check_open_destination, basis_open_regular, hardlink_read_source)
so a client-planted FIFO can no longer block the receive thread forever while
the post-open type gate still rejects it.

B2 (HIGH/MED): --inplace now fstatat(AT_SYMLINK_NOFOLLOW)-probes the target and
refuses any existing non-regular entry, opens with O_NONBLOCK, and re-checks
S_ISREG on the opened fd.  This stops a FIFO from hanging the open and stops a
char/block device from being written directly (bypassing --write-devices).

B3 (MED): under --dry-run the incremental quick-skip no longer reads/hashes the
destination file for --checksum/--delta; it decides from metadata only and
reports would-transfer when the comparison is inconclusive, closing the
read-only-module content-hash oracle.

B4 (LOW): xattr_name_appliable() now gates the two system.posix_acl_* names on
preserve_acls (--acls), not the derived use_xattrs (--xattrs OR --acls).  The
receiver drops (never applies) ACL entries when -A was not negotiated while
keeping user.* working for -X.

B5 (INFO): receive_manifest_section() charges a per-entry overhead against
MAX_MANIFEST_BYTES and the aggregate entry count across all three sections is
capped at MAX_MANIFEST_ENTRIES.

B6 (MED): data_charge_session() reserves decompressed/chunk-copy bytes against
the owning ProtocolSession (MAX_CONNECTION_MEMORY) and records them on the Data
so data_destroy() releases them via the Data.owner path.  Applied to the
whole-file/append/delta decompression sites and chunk_deserialize() per-file
copies; a missing session owner degrades to the previous uncharged behavior.

Tests: FIFO destination/basis non-hang (with alarm), --inplace FIFO/device
refusal, dry-run no-read oracle test plus updated metadata-only dry-run tests,
ACL-without--acls drop, manifest total-entry cap, and chunk session charging.

src/shared/chunk.c

@@ -1,6 +1,7 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <limits.h>
+#include <stdatomic.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -20,6 +21,33 @@
 #define MAX_FILE_DATA_SIZE (64ULL * 1024 * 1024)
 #define MAX_FILES_PER_CHUNK 65536U
 
+/* Reserve `charge` against `session`'s connection budget.  This mirrors the
+   static protocol_reserve_memory() in protocol.c: the receive-side call sites
+   only have the Data.owner pointer (a ProtocolSession*), and protocol.c is out
+   of scope for this fix, so the same atomic CAS accounting is reproduced here.
+   The matching release always goes through data_destroy()'s Data.owner path. */
+static bool chunk_session_reserve(ProtocolSession* session, size_t charge) {
+  unsigned long long allocated = atomic_load(&session->total_allocated_bytes);
+  while (true) {
+    if (allocated > MAX_CONNECTION_MEMORY ||
+        (unsigned long long)charge > MAX_CONNECTION_MEMORY - allocated)
+      return false;
+    if (atomic_compare_exchange_weak(&session->total_allocated_bytes, &allocated,
+                                     allocated + (unsigned long long)charge))
+      return true;
+  }
+}
+
+bool data_charge_session(Data* data, ProtocolSession* session, size_t charge) {
+  if (!data || charge == 0 || session == NULL)
+    return true;
+  if (!chunk_session_reserve(session, charge))
+    return false;
+  data->owner = session;
+  data->protocol_charge = charge;
+  return true;
+}
+
 Chunk* chunk_create(File** items, int element_count) {
   if (element_count < 0 || (element_count > 0 && items == NULL))
     return NULL;
@@ -370,6 +398,15 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
     Data* replacement = data_create(file_data, file_data_size);
     if (replacement == NULL)
       goto error;
+    /* Charge the retained per-file copy to the connection budget (when the
+       inbound chunk carries an owning session) so the queued copies are not
+       held outside MAX_CONNECTION_MEMORY (B6).  A NULL owner (e.g. a local
+       batch apply) leaves the copy uncharged. */
+    if (!data_charge_session(replacement, data->owner, allocation_size)) {
+      log_message(LOG_LEVEL_ERROR, "Per-connection memory limit exceeded for chunk file data");
+      data_destroy(replacement);
+      goto error;
+    }
     data_destroy(file->data);
     file->data = replacement;
     data_pointer += file_data_size;
@@ -466,12 +503,21 @@ Chunk* receive_chunk_data(int fd, const Config* config) {
   }
   Data* data_to_process = chunk_data;
   if (config->use_compression) {
+    /* Preserve the inbound session across decompression so the (larger)
+       decompressed chunk is charged to the same connection budget; the
+       compressed buffer's own charge is released by data_destroy below. */
+    ProtocolSession* owner = chunk_data->owner;
     data_to_process = data_decompress_limited(chunk_data, MAX_CHUNK_SIZE);
     data_destroy(chunk_data);
     if (data_to_process == NULL) {
       log_message(LOG_LEVEL_ERROR, "Failed to decompress chunk");
       return NULL;
     }
+    if (!data_charge_session(data_to_process, owner, data_to_process->size)) {
+      log_message(LOG_LEVEL_ERROR, "Per-connection memory limit exceeded for decompressed chunk");
+      data_destroy(data_to_process);
+      return NULL;
+    }
   }
 
   // Reject chunks larger than the maximum allowed size to prevent OOM.

src/shared/chunk.h

@@ -23,4 +23,15 @@ Data* chunk_compress_with_threads(Chunk* chunk, int compression_level, bool use_
                                   int compression_threads);
 Chunk* receive_chunk_data(int fd, const Config* config);
 
+/* Charge `charge` retained bytes of `data` against `session`'s per-connection
+ * budget (MAX_CONNECTION_MEMORY), mirroring the protocol layer's accounting, and
+ * record them on `data` so data_destroy() returns the charge through the
+ * Data.owner path.  Returns false (leaving `data` uncharged) when the ceiling
+ * would be exceeded.  A NULL/zero-size charge or a NULL session is a no-op
+ * success.  The receive-side decompression and chunk-copy paths know the owning
+ * session only through the Data.owner of the buffer they are processing, so
+ * this is the entry point that lets them participate in the connection budget
+ * without a session handle (B6). */
+bool data_charge_session(Data* data, ProtocolSession* session, size_t charge);
+
 #endif

src/shared/file.c

@@ -890,13 +890,35 @@ static bool file_to_disk_secure_impl(const char* path, const void* data,
   if (inplace) {
     /* --inplace writes directly into the destination; a scratch --temp-dir
        does not apply and must never redirect these writes. */
-    fd = openat(dirfd, leaf, O_WRONLY | O_CREAT | O_CLOEXEC | O_NOFOLLOW, 0644);
+    /* Type gate BEFORE opening: an existing destination entry that is not a
+       regular file (FIFO, socket, char/block device, directory) must never be
+       opened for writing.  Opening a FIFO would block the receive thread
+       forever and writing into a device would bypass the --write-devices /
+       super-mode gate (a client-controlled device write).  fstatat with
+       AT_SYMLINK_NOFOLLOW does not follow a symlink and does not block. */
+    struct stat pre_stat;
+    if (fstatat(dirfd, leaf, &pre_stat, AT_SYMLINK_NOFOLLOW) == 0 && !S_ISREG(pre_stat.st_mode)) {
+      close(dirfd);
+      free(leaf);
+      return false;
+    }
+    /* O_NONBLOCK: a no-op for a regular file, but a raced-in FIFO cannot block
+       the open before the post-open S_ISREG re-check rejects it. */
+    fd = openat(dirfd, leaf, O_WRONLY | O_CREAT | O_CLOEXEC | O_NOFOLLOW | O_NONBLOCK, 0644);
     if (fd >= 0) {
       struct stat destination_stat;
+      /* Re-check the opened descriptor: a concurrent replacement between the
+         fstatat probe and the open (or a device/FIFO raced in) must never be
+         written through. */
+      if (fstat(fd, &destination_stat) != 0 || !S_ISREG(destination_stat.st_mode)) {
+        close(fd);
+        close(dirfd);
+        free(leaf);
+        return false;
+      }
       bool newer = false;
-      if (update && metadata && fstat(fd, &destination_stat) == 0 &&
-          S_ISREG(destination_stat.st_mode)) {
-        newer = stat_is_newer(&destination_stat, metadata);
+      if (update && metadata && stat_is_newer(&destination_stat, metadata)) {
+        newer = true;
       }
       if (newer) {
         ok = true;

src/shared/file_receive.c

@@ -12,6 +12,7 @@
 #include "array_list.h"
 #include "charset.h"
 #include "chmod.h"
+#include "chunk.h"
 #include "compression.h"
 #include "config.h"
 #include "data.h"
@@ -27,6 +28,11 @@
 
 #define MAX_SERVER_DELETE_COUNT 100000U
 #define MAX_FILE_DATA_SIZE MAX_RECEIVE_WHOLE_FILE_SIZE
+/* Retained cost of one delete-manifest entry beyond its path bytes: the
+   ArrayList pointer slot plus an approximate malloc header/rounding for the
+   heap copy.  Charged against MAX_MANIFEST_BYTES so a frame full of tiny paths
+   cannot retain far more than the byte budget (B5). */
+#define MANIFEST_ENTRY_OVERHEAD (sizeof(char*) + 16)
 
 bool file_save_to_disk(const char* root_directory, const File* file, const Config* config) {
   return file_save_to_disk_full(root_directory, file, config) != FILE_SAVE_ERROR;
@@ -127,7 +133,10 @@ static bool hardlink_read_source(const char* path, void** out_buf, unsigned long
     *source_absent = errno == ENOENT || errno == ENOTDIR;
     return false;
   }
-  int fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
+  /* O_NONBLOCK is a no-op for a regular file but makes openat() fail/succeed
+     immediately for a client-planted FIFO instead of blocking the receive
+     thread forever; the post-open S_ISREG gate below is the actual type check. */
+  int fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW | O_NONBLOCK);
   int saved_errno = errno;
   free(leaf);
   close(parent_fd);
@@ -944,7 +953,7 @@ static bool receive_file_xattrs(File* file, int fd, const Config* config) {
   if (!config->use_xattrs)
     return true;
   int xok = 0;
-  FileXattrList* list = xattr_receive(fd, &xok);
+  FileXattrList* list = xattr_receive(fd, &xok, config->preserve_acls);
   if (!xok) {
     xattr_list_free(list);
     return false;
@@ -1009,6 +1018,7 @@ static File* receive_delta_file(int fd, const Config* config, const char* check_
         !compression_should_skip_with_suffixes(
             check_path, config->skip_compress_suffixes,
             config->skip_compress_set ? config->skip_compress_count : -1)) {
+      ProtocolSession* owner = delta_data->owner;
       raw_delta = data_decompress_limited(delta_data, MAX_RECEIVE_WHOLE_FILE_SIZE);
       data_destroy(delta_data);
       if (!raw_delta) {
@@ -1017,6 +1027,15 @@ static File* receive_delta_file(int fd, const Config* config, const char* check_
         *failed = true;
         return NULL;
       }
+      /* Charge the decompressed delta to the connection budget (the paired
+         wire buffer's charge was just released). */
+      if (!data_charge_session(raw_delta, owner, raw_delta->size)) {
+        data_destroy(raw_delta);
+        free(old_data);
+        delta_signature_destroy(sig);
+        *failed = true;
+        return NULL;
+      }
     }
 
     Delta* delta = delta_deserialize(raw_delta);
@@ -1131,12 +1150,20 @@ static File* receive_delta_file(int fd, const Config* config, const char* check_
             file->path, config->skip_compress_suffixes,
             config->skip_compress_set ? config->skip_compress_count : -1)) {
       Data* uncompressed = data_decompress_limited(file_data, MAX_RECEIVE_WHOLE_FILE_SIZE);
+      ProtocolSession* owner = file_data->owner;
       data_destroy(file_data);
       if (uncompressed == NULL) {
         file_destroy(file);
         *failed = true;
         return NULL;
       }
+      if (!data_charge_session(uncompressed, owner, uncompressed->size)) {
+        data_destroy(uncompressed);
+        file_destroy(file);
+        send_status(fd, STATUS_ERROR);
+        *failed = true;
+        return NULL;
+      }
       if (uncompressed->size > MAX_FILE_DATA_SIZE) {
         data_destroy(uncompressed);
         file_destroy(file);
@@ -1193,7 +1220,9 @@ static bool basis_open_regular(const char* path, unsigned long long expected_siz
   int parent_fd = file_open_secure_parent(path, &leaf, false);
   if (parent_fd < 0)
     return false;
-  int fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
+  /* O_NONBLOCK: a client-planted FIFO must not block the receiver's openat()
+     forever; the fstat()/S_ISREG gate below rejects it immediately. */
+  int fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW | O_NONBLOCK);
   free(leaf);
   close(parent_fd);
   if (fd < 0)
@@ -1637,11 +1666,17 @@ static File* receive_full_file(int fd, const Config* config, const char* path) {
                                              config->skip_compress_set ? config->skip_compress_count
                                                                        : -1)) {
     Data* uncompressed = data_decompress_limited(file_data, MAX_RECEIVE_WHOLE_FILE_SIZE);
+    ProtocolSession* owner = file_data->owner;
     data_destroy(file_data);
     if (uncompressed == NULL) {
       file_destroy(file);
       return NULL;
     }
+    if (!data_charge_session(uncompressed, owner, uncompressed->size)) {
+      data_destroy(uncompressed);
+      file_destroy(file);
+      return NULL;
+    }
     if (uncompressed->size > MAX_FILE_DATA_SIZE) {
       data_destroy(uncompressed);
       file_destroy(file);
@@ -1776,7 +1811,9 @@ static IncrementalCheckOutcome incremental_check_open_destination(IncrementalChe
   char* leaf = NULL;
   int parent_fd = file_open_secure_parent(full_path, &leaf, false);
   if (parent_fd >= 0) {
-    state->old_fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
+    /* O_NONBLOCK: an existing FIFO at the destination must not block this
+       openat(); the S_ISREG gate below rejects the non-regular entry. */
+    state->old_fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW | O_NONBLOCK);
     free(leaf);
     close(parent_fd);
     state->has_old_file = state->old_fd >= 0 && fstat(state->old_fd, &state->old_st) == 0 &&
@@ -1815,7 +1852,15 @@ static IncrementalCheckOutcome incremental_check_quick_skip(IncrementalCheckStat
   bool try_delta = config->use_delta && !config->whole_file && has_old_file &&
                    delta_should_attempt(old_size, state->check_size, config->delta_max_file_size);
   bool checksum_needs_read = size_equal && !config->ignore_times && config->checksum;
-  bool need_old_data = checksum_needs_read || try_delta;
+  /* --dry-run must never read the destination file's CONTENTS: a client could
+     otherwise use `--dry-run --checksum` against a read-only module as a
+     1-bit content oracle (hash match / mismatch) and force arbitrary reads.
+     Decide from metadata alone; when metadata is inconclusive (checksum or
+     delta would have required the body) report would-transfer.  The real
+     (non-dry-run) behavior below is unchanged. */
+  bool need_old_data = !config->dry_run && (checksum_needs_read || try_delta);
+  if (config->dry_run)
+    try_delta = false;
   *out_try_delta = try_delta;
 
   if (need_old_data && has_old_file && old_size > 0 && old_size <= MAX_RECEIVE_WHOLE_FILE_SIZE &&
@@ -1836,7 +1881,12 @@ static IncrementalCheckOutcome incremental_check_quick_skip(IncrementalCheckStat
   }
 
   bool match = false;
-  if (checksum_needs_read) {
+  if (config->dry_run) {
+    /* Metadata-only decision: a size match plus a matching mtime is treated as
+       up to date; --checksum/--delta cannot be verified without reading, so an
+       otherwise inconclusive comparison is a would-transfer. */
+    match = size_equal && !config->ignore_times && (config->size_only || match_by_metadata);
+  } else if (checksum_needs_read) {
     uint8_t old_digest[CHECKSUM_MAX_DIGEST_LEN];
     size_t old_len = 0;
     bool hashed = checksum_digest((ChecksumAlgo)config->checksum_algo, config->checksum_seed,
@@ -2050,7 +2100,7 @@ static IncrementalCheckOutcome incremental_check_try_append_resume(IncrementalCh
   }
   if (config->use_xattrs) {
     int xok = 0;
-    append_xattrs = xattr_receive(fd, &xok);
+    append_xattrs = xattr_receive(fd, &xok, config->preserve_acls);
     if (!xok) {
       xattr_list_free(append_xattrs);
       return INCREMENTAL_ERROR;
@@ -2066,11 +2116,17 @@ static IncrementalCheckOutcome incremental_check_try_append_resume(IncrementalCh
                                              config->skip_compress_set ? config->skip_compress_count
                                                                        : -1)) {
     Data* uncompressed = data_decompress_limited(tail, MAX_RECEIVE_WHOLE_FILE_SIZE);
+    ProtocolSession* owner = tail->owner;
     data_destroy(tail);
     if (uncompressed == NULL) {
       xattr_list_free(append_xattrs);
       return INCREMENTAL_ERROR;
     }
+    if (!data_charge_session(uncompressed, owner, uncompressed->size)) {
+      data_destroy(uncompressed);
+      xattr_list_free(append_xattrs);
+      return INCREMENTAL_ERROR;
+    }
     if (uncompressed->size > MAX_FILE_DATA_SIZE) {
       data_destroy(uncompressed);
       xattr_list_free(append_xattrs);
@@ -2301,11 +2357,17 @@ File* file_receive(const Config* config, int file_descriptor) {
                                              config->skip_compress_set ? config->skip_compress_count
                                                                        : -1)) {
     Data* file_data_uncompressed = data_decompress_limited(file_data, MAX_RECEIVE_WHOLE_FILE_SIZE);
+    ProtocolSession* owner = file_data->owner;
     data_destroy(file_data);
     if (file_data_uncompressed == NULL) {
       file_destroy(file);
       return NULL;
     }
+    if (!data_charge_session(file_data_uncompressed, owner, file_data_uncompressed->size)) {
+      data_destroy(file_data_uncompressed);
+      file_destroy(file);
+      return NULL;
+    }
     if (file_data_uncompressed->size > MAX_FILE_DATA_SIZE) {
       data_destroy(file_data_uncompressed);
       file_destroy(file);
@@ -2694,19 +2756,21 @@ File* file_receive_special(int file_descriptor) {
    manifest).  Returns an owned DeleteManifest, or NULL after sending STATUS_ERROR
    when the frame is malformed (bad count, empty/absolute path, path traversal,
    or an aggregate size beyond MAX_MANIFEST_BYTES). */
-static bool receive_manifest_section(int fd, ArrayList* list, size_t* manifest_bytes) {
+static bool receive_manifest_section(int fd, ArrayList* list, size_t* manifest_bytes,
+                                     size_t* manifest_entries) {
   int count;
   if (!receive_int(fd, &count)) {
     send_status(fd, STATUS_ERROR);
     return false;
   }
-  if (count < 0 || count > MAX_MANIFEST_ENTRIES) {
+  if (count < 0 || count > MAX_MANIFEST_ENTRIES ||
+      (size_t)count > MAX_MANIFEST_ENTRIES - *manifest_entries) {
     send_status(fd, STATUS_ERROR);
     return false;
   }
   for (int i = 0; i < count; i++) {
     char* s = receive_wire_str(fd);
-    size_t entry_size = s ? strlen(s) : 0;
+    size_t entry_size = s ? strlen(s) + MANIFEST_ENTRY_OVERHEAD : 0;
     if (!s || s[0] == '\0' || s[0] == '/' || has_path_traversal(s) ||
         entry_size > MAX_MANIFEST_BYTES - *manifest_bytes ||
         (*manifest_bytes += entry_size) > MAX_MANIFEST_BYTES || !array_list_add(list, s)) {
@@ -2715,6 +2779,7 @@ static bool receive_manifest_section(int fd, ArrayList* list, size_t* manifest_b
       return false;
     }
   }
+  *manifest_entries += (size_t)count;
   return true;
 }
 
@@ -2733,9 +2798,10 @@ DeleteManifest* receive_manifest_entries(int fd) {
     return NULL;
   }
   size_t manifest_bytes = 0;
-  if (!receive_manifest_section(fd, manifest->keeps, &manifest_bytes) ||
-      !receive_manifest_section(fd, manifest->protected, &manifest_bytes) ||
-      !receive_manifest_section(fd, manifest->missing, &manifest_bytes)) {
+  size_t manifest_entries = 0;
+  if (!receive_manifest_section(fd, manifest->keeps, &manifest_bytes, &manifest_entries) ||
+      !receive_manifest_section(fd, manifest->protected, &manifest_bytes, &manifest_entries) ||
+      !receive_manifest_section(fd, manifest->missing, &manifest_bytes, &manifest_entries)) {
     delete_manifest_free(manifest);
     return NULL;
   }

src/shared/xattr.c

@@ -73,13 +73,17 @@ bool xattr_list_append(FileXattrList* list, const char* name, const void* value,
 
 /* A Linux xattr name is "namespace.name" with an optional leading "trusted.",
  * "system.", "security.", "user.", or "trusted." prefix.  We only ever touch
- * the unprivileged "user.*" namespace and the two POSIX ACL xattrs carried in
- * the "system." namespace.  Everything else -- especially "security.*" (ACLs,
- * capabilities, SELinux labels) and "trusted.*" -- is refused so a client can
- * never compel the receiver to apply a privileged attribute it would not
- * otherwise be able to set (and which would be a local privilege escalation if
- * it could). */
-bool xattr_name_appliable(const char* name) {
+ * the unprivileged "user.*" namespace and, only when --acls/-A was negotiated,
+ * the two POSIX ACL xattrs carried in the "system." namespace.  Everything else
+ * -- especially "security.*" (ACLs, capabilities, SELinux labels) and
+ * "trusted.*" -- is refused so a client can never compel the receiver to apply a
+ * privileged attribute it would not otherwise be able to set (and which would be
+ * a local privilege escalation if it could).
+ *
+ * The ACL gate is deliberate: --xattrs/-X alone derives use_xattrs but must NOT
+ * authorize the ACL names, otherwise a -X client could plant an ACL the
+ * receiver never opted into (B4). */
+bool xattr_name_appliable(const char* name, bool preserve_acls) {
   if (!name || name[0] == '\0')
     return false;
   size_t len = strlen(name);
@@ -95,12 +99,21 @@ bool xattr_name_appliable(const char* name) {
   if (strncmp(name, "user.", 5) == 0)
     return name[5] != '\0';
   if (strcmp(name, "system.posix_acl_access") == 0)
-    return true;
+    return preserve_acls;
   if (strcmp(name, "system.posix_acl_default") == 0)
-    return true;
+    return preserve_acls;
   return false;
 }
 
+/* The two POSIX ACL xattr names: the only names whose applicablity is
+ * conditional (they require --acls).  Used by the receiver to distinguish "not
+ * negotiated" (drop the entry, keep user.* working for -X) from a genuinely
+ * disallowed namespace (hard reject). */
+static bool xattr_name_is_posix_acl(const char* name) {
+  return name != NULL && (strcmp(name, "system.posix_acl_access") == 0 ||
+                          strcmp(name, "system.posix_acl_default") == 0);
+}
+
 /* ---- SENDER: capture ---- */
 
 FileXattrList* xattr_capture_path(const char* path) {
@@ -130,7 +143,9 @@ FileXattrList* xattr_capture_path(const char* path) {
     if (name_len == 0)
       break; /* trailing double NUL not expected; stop */
     offset += (ssize_t)name_len + 1;
-    if (!xattr_name_appliable(name))
+    /* Capture is sender-side: the scanner has already gated on -X/-A, so the
+       per-name whitelist here allows the ACL names (true). */
+    if (!xattr_name_appliable(name, true))
       continue;
     ssize_t value_size = getxattr(path, name, NULL, 0);
     if (value_size < 0)
@@ -190,7 +205,7 @@ bool xattr_send(int fd, const FileXattrList* list) {
   return true;
 }
 
-FileXattrList* xattr_receive(int fd, int* ok) {
+FileXattrList* xattr_receive(int fd, int* ok, bool preserve_acls) {
   if (ok)
     *ok = 0;
   int count;
@@ -232,11 +247,19 @@ FileXattrList* xattr_receive(int fd, int* ok) {
       xattr_list_free(list);
       return NULL;
     }
-    if (!xattr_name_appliable(name)) {
-      log_message(LOG_LEVEL_ERROR, "rejected xattr block: disallowed namespace for '%s'", name);
-      free(name);
-      xattr_list_free(list);
-      return NULL;
+    bool skip = false;
+    if (!xattr_name_appliable(name, preserve_acls)) {
+      if (!preserve_acls && xattr_name_is_posix_acl(name)) {
+        /* -X without -A: the sender may still carry ACLs, but the receiver must
+           never apply an ACL it was not asked to preserve.  Consume and drop the
+           entry (keeping -X compatibility) rather than failing the transfer. */
+        skip = true;
+      } else {
+        log_message(LOG_LEVEL_ERROR, "rejected xattr block: disallowed namespace for '%s'", name);
+        free(name);
+        xattr_list_free(list);
+        return NULL;
+      }
     }
     int32_t value_len32;
     if (!receive_n_data(fd, &value_len32, sizeof(value_len32))) {
@@ -273,6 +296,12 @@ FileXattrList* xattr_receive(int fd, int* ok) {
         return NULL;
       }
     }
+    if (skip) {
+      free(value);
+      free(name);
+      budget += (size_t)name_len32 + (size_t)value_len32;
+      continue;
+    }
     if (!xattr_list_append(list, name, value, (size_t)value_len32)) {
       free(value);
       free(name);

src/shared/xattr.h

@@ -59,9 +59,11 @@ void xattr_list_free(FileXattrList* list);
 bool xattr_list_append(FileXattrList* list, const char* name, const void* value, size_t value_len);
 
 /* True when `name` is a well-formed xattr name AND belongs to a namespace this
- * build is authorized to apply (user.* or the two POSIX ACL xattrs).  Used for
- * both capture and receiver-side validation. */
-bool xattr_name_appliable(const char* name);
+ * build is authorized to apply.  `user.*` is always accepted for -X; the two
+ * POSIX ACL xattrs are accepted only when `preserve_acls` (--acls/-A) is set, so
+ * a plain -X run can never carry or apply an ACL the receiver did not ask for.
+ * Used for both capture and receiver-side validation. */
+bool xattr_name_appliable(const char* name, bool preserve_acls);
 
 /* Sender: read the whitelisted xattrs of `path` into a new list.  Returns NULL
  * when the path has no appliable xattrs (or the filesystem has no xattr
@@ -70,9 +72,12 @@ FileXattrList* xattr_capture_path(const char* path);
 
 /* Wire: bounded serialization.  xattr_send returns false on write failure; an
  * empty/NULL list transmits a zero-count block.  xattr_receive returns NULL and
- * sets *ok = 0 on any malformed / oversized / non-whitelisted entry. */
+ * sets *ok = 0 on any malformed / oversized / non-whitelisted entry.  When
+ * `preserve_acls` is false, any POSIX ACL entries are consumed and DROPPED (so
+ * a -X transfer still succeeds and never applies an ACL it did not negotiate);
+ * a genuinely disallowed namespace is still rejected. */
 bool xattr_send(int fd, const FileXattrList* list);
-FileXattrList* xattr_receive(int fd, int* ok);
+FileXattrList* xattr_receive(int fd, int* ok, bool preserve_acls);
 
 /* Receiver: apply every entry fd-relative (fsetxattr) to the just-written file
  * descriptor.  A per-attribute failure (e.g. ACL set refused for non-root on a

tests/fuzz/fuzz_xattr_block.c

@@ -75,7 +75,7 @@ static void write_best_effort(int fd, const void* data, size_t size) {
 }
 
 static void receive_stream(const unsigned char* prefix, size_t prefix_len, const uint8_t* data,
-                           size_t size) {
+                           size_t size, bool preserve_acls) {
   int sv[2];
   if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) != 0)
     return;
@@ -91,7 +91,7 @@ static void receive_stream(const unsigned char* prefix, size_t prefix_len, const
   shutdown(sv[0], SHUT_WR);
 
   int ok = 0;
-  FileXattrList* list = xattr_receive(sv[1], &ok);
+  FileXattrList* list = xattr_receive(sv[1], &ok, preserve_acls);
   xattr_list_free(list);
 
   close(sv[0]);
@@ -102,14 +102,18 @@ int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   if (!g_block_ready)
     build_canonical_block();
 
-  /* Raw bytes as the whole block. */
-  receive_stream(NULL, 0, data, size);
+  /* Raw bytes as the whole block.  Exercise both the -X-only (no ACLs) and the
+   * -A (ACL names accepted) receiver gates. */
+  for (int acls = 0; acls < 2; acls++) {
+    bool preserve_acls = acls != 0;
+    receive_stream(NULL, 0, data, size, preserve_acls);
 
-  /* Valid framing so the fuzzer mutates the entry list, the first value and
-   * the second entry respectively instead of stopping at the count. */
-  receive_stream(g_block, g_off_after_entry0, data, size);
-  receive_stream(g_block, g_off_value0, data, size);
-  receive_stream(g_block, g_off_after_count, data, size);
+    /* Valid framing so the fuzzer mutates the entry list, the first value and
+     * the second entry respectively instead of stopping at the count. */
+    receive_stream(g_block, g_off_after_entry0, data, size, preserve_acls);
+    receive_stream(g_block, g_off_value0, data, size, preserve_acls);
+    receive_stream(g_block, g_off_after_count, data, size, preserve_acls);
+  }
 
   return 0;
 }

tests/integration/test_features.py

@@ -444,9 +444,10 @@ class TestRemoteDryRun:
         self._seed(source)
         clean_dir(dest)
 
-        # Populate the destination with a real transfer, then make exactly one
-        # file differ (content+size) and add a brand-new file.
-        result, _ = run_client(source, dest, port=shared_server.port)
+        # Populate the destination with a real transfer that preserves mtimes
+        # (--preserve), then make exactly one file differ (content+size) and add
+        # a brand-new file.
+        result, _ = run_client(source, dest, flags=["--preserve"], port=shared_server.port)
         assert result.returncode == 0, f"seed transfer failed: {result.stderr[:200]}"
         received = get_dest_received_dir(dest, source)
 
@@ -456,9 +457,11 @@ class TestRemoteDryRun:
             f.write(b"newly added\n")
 
         before = _snapshot_tree(received)
-        # --checksum makes the up-to-date decision content-based (the seed
-        # transfer did not preserve mtimes), so keep.txt/deep.txt report skip.
-        result, _ = run_client(source, dest, flags=["--dry-run", "--checksum"],
+        # --checksum must NOT read destination contents in a dry-run (B3), so
+        # the up-to-date decision is metadata-only.  The --preserve seed made
+        # keep.txt and deep.txt size+mtime-identical; the dry-run must also
+        # transmit metadata (--preserve) for that metadata to be comparable.
+        result, _ = run_client(source, dest, flags=["--dry-run", "--checksum", "--preserve"],
                                port=shared_server.port)
         assert result.returncode == 0, f"remote dry-run failed: {result.stderr[:300]}"
         assert "Dry run:" in result.stdout, result.stdout[:200]
@@ -470,6 +473,34 @@ class TestRemoteDryRun:
         assert "deep.txt" not in result.stdout, result.stdout
         assert _snapshot_tree(received) == before, "remote dry-run mutated the destination"
 
+    @pytest.mark.ci
+    def test_remote_dry_run_checksum_does_not_read_destination(self, shared_server):
+        """B3: --dry-run --checksum against a read-only module must not read the
+        destination file's content (a 1-bit hash oracle).  A same-size/same-content
+        file whose mtime differs is therefore reported as would-transfer because
+        the metadata-only decision is inconclusive, instead of being hashed and
+        silently skipped."""
+        source = os.path.join(TEST_DATA_DIR, "remote_dry_oracle_src")
+        dest = os.path.join(TEST_DATA_DIR, "remote_dry_oracle_dst")
+        self._seed(source)
+        clean_dir(dest)
+        result, _ = run_client(source, dest, flags=["--preserve"], port=shared_server.port)
+        assert result.returncode == 0, result.stderr[:200]
+        received = get_dest_received_dir(dest, source)
+
+        target = os.path.join(received, "keep.txt")
+        # Identical size and content, but a deliberately different mtime.
+        os.utime(target, (1000000000, 1000000000))
+        before = _snapshot_tree(received)
+
+        result, _ = run_client(source, dest, flags=["--dry-run", "--checksum", "--preserve"],
+                               port=shared_server.port)
+        assert result.returncode == 0, result.stderr[:300]
+        assert "keep.txt" in result.stdout, (
+            f"dry-run --checksum must not read the destination to prove equality: {result.stdout}"
+        )
+        assert _snapshot_tree(received) == before, "dry-run mutated the destination"
+
     @pytest.mark.ci
     def test_remote_dry_run_into_empty_dest_creates_nothing(self, shared_server):
         source = os.path.join(TEST_DATA_DIR, "remote_dry_empty_src")

tests/test_chunk.c

@@ -1,8 +1,10 @@
 
 #include "chunk.h"
+#include "protocol.h"
 #include "test_utils.h"
 #include "utils.h"
 #include <string.h>
+#include <sys/socket.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
@@ -279,6 +281,51 @@ static void test_chunk_special_rdev_out_of_range_rejected() {
   chunk_destroy(chunk);
 }
 
+/* B6: chunk_deserialize() charges each retained per-file copy to the owning
+ * session's connection budget (MAX_CONNECTION_MEMORY) so queued chunk payloads
+ * are not held outside the per-connection ceiling; destroying the chunk returns
+ * the charge through the Data.owner path. */
+static void test_chunk_deserialize_charges_session_budget() {
+  const char* path = "temp_chunk_charge.txt";
+  const char* content = "charge me to the connection budget";
+  unlink(path);
+  file_write_to_disk(path, content, strlen(content), false, false);
+  struct stat st;
+  EXPECT_EQ_INT(stat(path, &st), 0);
+
+  int p[2];
+  EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);
+  ProtocolSession session;
+  protocol_session_init(&session, p[0], p[1]);
+  protocol_session_set_max_alloc(&session, 4ULL * 1024 * 1024);
+
+  File* f = file_create(path);
+  EXPECT_NOT_NULL(f);
+  f->data->size = (unsigned long long)st.st_size;
+  EXPECT_TRUE(file_load_data(f));
+  File* files[1] = {f};
+  Chunk* chunk = chunk_create(files, 1);
+  EXPECT_NOT_NULL(chunk);
+  Data* serialized = chunk_serialize(chunk, false);
+  EXPECT_NOT_NULL(serialized);
+  /* Simulate a received buffer carrying its owning session. */
+  serialized->owner = &session;
+
+  Chunk* deserialized = chunk_deserialize(serialized, false);
+  EXPECT_NOT_NULL(deserialized);
+  unsigned long long charged = atomic_load(&session.total_allocated_bytes);
+  EXPECT_EQ_INT((int)charged, (int)strlen(content));
+  chunk_destroy(deserialized);
+  /* The copy's charge is released with the File/Data on destroy. */
+  EXPECT_EQ_INT((int)atomic_load(&session.total_allocated_bytes), 0);
+
+  data_destroy(serialized);
+  chunk_destroy(chunk);
+  close(p[0]);
+  close(p[1]);
+  unlink(path);
+}
+
 void test_chunk() {
   test_file_operations();
   test_chunk_operations();
@@ -286,4 +333,5 @@ void test_chunk() {
   test_chunk_symlink_roundtrip();
   test_chunk_special_rdev_roundtrip();
   test_chunk_special_rdev_out_of_range_rejected();
+  test_chunk_deserialize_charges_session_budget();
 }

tests/test_file.c

@@ -14,6 +14,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/sysmacros.h>
 #include <sys/wait.h>
 #include <time.h>
 #include <unistd.h>
@@ -994,6 +995,94 @@ static void test_inplace_overwrite_truncates_shorter_payload() {
   rmdir(root);
 }
 
+/* B2: --inplace must refuse an existing non-regular destination entry.  A FIFO
+   would block open(O_WRONLY) forever and a device node would be written
+   directly, bypassing the --write-devices/super gate.  Forked with an alarm so
+   a regression is a prompt failure instead of a hung suite. */
+static void test_inplace_refuses_fifo_destination() {
+  const char* root = "test_inplace_fifo_tmp";
+  const char* path = "test_inplace_fifo_tmp/fifo";
+  unlink(path);
+  rmdir(root);
+  EXPECT_EQ_INT(mkdir(root, 0700), 0);
+  EXPECT_EQ_INT(mkfifo(path, 0600), 0);
+
+  pid_t pid = fork();
+  if (pid == 0) {
+    alarm(10);
+    File* f = file_create("fifo");
+    if (!f)
+      _exit(1);
+    const char* content = "payload";
+    f->data->data = malloc(strlen(content));
+    if (!f->data->data)
+      _exit(1);
+    memcpy(f->data->data, content, strlen(content));
+    f->data->size = strlen(content);
+    Config* cfg = config_create();
+    if (!cfg)
+      _exit(1);
+    cfg->inplace = true;
+    bool written = file_save_to_disk(root, f, cfg);
+    file_destroy(f);
+    config_delete(cfg);
+    _exit(written ? 1 : 0); /* must be refused */
+  }
+  int status;
+  waitpid(pid, &status, 0);
+  EXPECT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0);
+  struct stat st;
+  EXPECT_EQ_INT(lstat(path, &st), 0);
+  EXPECT_TRUE(S_ISFIFO(st.st_mode)); /* left untouched */
+  unlink(path);
+  rmdir(root);
+}
+
+/* B2: an existing char device must not be written by --inplace.  mknod needs
+   privilege, so a non-root run skips gracefully.  /dev/null's (1:3) rdev makes
+   the negative case harmless if it ever regresses. */
+static void test_inplace_refuses_device_destination() {
+  const char* root = "test_inplace_dev_tmp";
+  const char* path = "test_inplace_dev_tmp/dev";
+  unlink(path);
+  rmdir(root);
+  EXPECT_EQ_INT(mkdir(root, 0700), 0);
+  if (mknod(path, S_IFCHR | 0600, makedev(1, 3)) != 0) {
+    rmdir(root);
+    return; /* no privilege to create a device node: skip */
+  }
+
+  pid_t pid = fork();
+  if (pid == 0) {
+    alarm(10);
+    File* f = file_create("dev");
+    if (!f)
+      _exit(1);
+    const char* content = "payload";
+    f->data->data = malloc(strlen(content));
+    if (!f->data->data)
+      _exit(1);
+    memcpy(f->data->data, content, strlen(content));
+    f->data->size = strlen(content);
+    Config* cfg = config_create();
+    if (!cfg)
+      _exit(1);
+    cfg->inplace = true;
+    bool written = file_save_to_disk(root, f, cfg);
+    file_destroy(f);
+    config_delete(cfg);
+    _exit(written ? 1 : 0); /* must be refused */
+  }
+  int status;
+  waitpid(pid, &status, 0);
+  EXPECT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0);
+  struct stat st;
+  EXPECT_EQ_INT(lstat(path, &st), 0);
+  EXPECT_TRUE(S_ISCHR(st.st_mode)); /* still a device, not replaced */
+  unlink(path);
+  rmdir(root);
+}
+
 /* Explicit directory entries (--dirs) create the directory under the receive
    root through the same save funnel, creating parents as needed, and reject
    traversal the same way a file path does. */
@@ -1606,4 +1695,6 @@ void test_file() {
   test_inplace_overwrite_clears_special_mode_bits();
   test_inplace_overwrite_metadata_strips_special_bits();
   test_inplace_overwrite_truncates_shorter_payload();
+  test_inplace_refuses_fifo_destination();
+  test_inplace_refuses_device_destination();
 }

tests/test_server.c

@@ -844,6 +844,172 @@ static void test_special_socket_path_log_escaped() {
   EXPECT_NOT_NULL(strstr(output, "socket not recreated: evil\\#012path"));
 }
 
+/* B1: a client-planted FIFO at the destination must not block the receiver's
+ * incremental-check open.  With the O_NONBLOCK open plus the post-open S_ISREG
+ * gate the FIFO is simply "no existing regular file", so the receiver proceeds
+ * to a full transfer; without O_NONBLOCK the child blocks in openat() and the
+ * alarm(30) kills it. */
+static void test_incremental_check_fifo_destination_does_not_hang() {
+  Config* cfg = config_create();
+  EXPECT_NOT_NULL(cfg);
+  char* root = make_check_root("qffo");
+  EXPECT_NOT_NULL(root);
+  cfg->receive_root_directory = str_dup(root);
+  char path[1024];
+  snprintf(path, sizeof(path), "%s/file.txt", root);
+  EXPECT_EQ_INT(mkfifo(path, 0600), 0);
+
+  int p[2];
+  EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);
+  io_set_fds(p[0], p[1]);
+  io_set_bwlimit(0);
+
+  pid_t pid = fork();
+  if (pid == 0) {
+    alarm(30);
+    close(p[1]);
+    io_set_fds(p[0], p[0]);
+    bool skipped = false;
+    File* file = receive_incremental_check(p[0], cfg, &skipped);
+    bool ok = file != NULL && !skipped;
+    file_destroy(file);
+    config_delete(cfg);
+    close(p[0]);
+    _exit(ok ? 0 : 1);
+  } else {
+    close(p[0]);
+    io_set_fds(p[1], p[1]);
+    EXPECT_TRUE(send_str(p[1], "file.txt"));
+    unsigned long long size = 4;
+    long long mtime = 42;
+    long long mtime_nsec = 0;
+    EXPECT_TRUE(send_n_data(p[1], &size, sizeof(size)));
+    EXPECT_TRUE(send_n_data(p[1], &mtime, sizeof(mtime)));
+    EXPECT_TRUE(send_n_data(p[1], &mtime_nsec, sizeof(mtime_nsec)));
+    Status s;
+    EXPECT_TRUE(receive_status(p[1], &s));
+    EXPECT_EQ_INT(s, STATUS_NEXT);
+
+    Data* body = data_create_reserve(4);
+    EXPECT_NOT_NULL(body);
+    body->data = malloc(4);
+    EXPECT_NOT_NULL(body->data);
+    memcpy(body->data, "data", 4);
+    body->size = 4;
+    EXPECT_TRUE(send_data(p[1], body));
+    data_destroy(body);
+
+    int status;
+    waitpid(pid, &status, 0);
+    close(p[1]);
+    config_delete(cfg);
+    unlink(path);
+    rmdir(root);
+    free(root);
+    EXPECT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0);
+  }
+}
+
+/* B1: a FIFO planted in a --link-dest basis directory must not block
+ * basis_open_regular() either; the basis match is simply declined. */
+static void test_incremental_check_basis_fifo_does_not_hang() {
+  Config* cfg = config_create();
+  EXPECT_NOT_NULL(cfg);
+  char* root = make_check_root("qbfi");
+  EXPECT_NOT_NULL(root);
+  cfg->receive_root_directory = str_dup(root);
+  char basis_dir[1024];
+  char basis_path[2048];
+  snprintf(basis_dir, sizeof(basis_dir), "%s/basis", root);
+  EXPECT_EQ_INT(mkdir(basis_dir, 0700), 0);
+  snprintf(basis_path, sizeof(basis_path), "%s/file.txt", basis_dir);
+  EXPECT_EQ_INT(mkfifo(basis_path, 0600), 0);
+  EXPECT_EQ_INT(config_basis_append(cfg, BASIS_DEST_LINK, "basis"), 0);
+
+  int p[2];
+  EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);
+  io_set_fds(p[0], p[1]);
+  io_set_bwlimit(0);
+
+  pid_t pid = fork();
+  if (pid == 0) {
+    alarm(30);
+    close(p[1]);
+    io_set_fds(p[0], p[0]);
+    bool skipped = false;
+    File* file = receive_incremental_check(p[0], cfg, &skipped);
+    bool ok = file != NULL && !skipped;
+    file_destroy(file);
+    config_delete(cfg);
+    close(p[0]);
+    _exit(ok ? 0 : 1);
+  } else {
+    close(p[0]);
+    io_set_fds(p[1], p[1]);
+    EXPECT_TRUE(send_str(p[1], "file.txt"));
+    unsigned long long size = 4;
+    long long mtime = 42;
+    long long mtime_nsec = 0;
+    EXPECT_TRUE(send_n_data(p[1], &size, sizeof(size)));
+    EXPECT_TRUE(send_n_data(p[1], &mtime, sizeof(mtime)));
+    EXPECT_TRUE(send_n_data(p[1], &mtime_nsec, sizeof(mtime_nsec)));
+    /* config_has_basis() makes the request carry the source digest. */
+    uint8_t wire_len = 8;
+    uint8_t digest[8] = {0};
+    EXPECT_TRUE(send_n_data(p[1], &wire_len, sizeof(wire_len)));
+    EXPECT_TRUE(send_n_data(p[1], digest, sizeof(digest)));
+    Status s;
+    EXPECT_TRUE(receive_status(p[1], &s));
+    EXPECT_EQ_INT(s, STATUS_NEXT);
+
+    Data* body = data_create_reserve(4);
+    EXPECT_NOT_NULL(body);
+    body->data = malloc(4);
+    EXPECT_NOT_NULL(body->data);
+    memcpy(body->data, "data", 4);
+    body->size = 4;
+    EXPECT_TRUE(send_data(p[1], body));
+    data_destroy(body);
+
+    int status;
+    waitpid(pid, &status, 0);
+    close(p[1]);
+    config_delete(cfg);
+    unlink(basis_path);
+    rmdir(basis_dir);
+    rmdir(root);
+    free(root);
+    EXPECT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0);
+  }
+}
+
+/* B5: the aggregate entry count across the three manifest sections is capped at
+ * MAX_MANIFEST_ENTRIES, and a section that would push the total over the cap is
+ * rejected before its entries are read (so a tiny first section followed by a
+ * huge claimed second section fails fast). */
+static void test_receive_manifest_total_entry_cap() {
+  Config* cfg = config_create();
+  EXPECT_NOT_NULL(cfg);
+  cfg->receive_root_directory = str_dup("/tmp/dst");
+  int p[2];
+  EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);
+  io_set_fds(p[0], p[1]);
+
+  EXPECT_TRUE(send_int(p[1], 1));
+  EXPECT_TRUE(send_str(p[1], "keep.txt"));
+  /* The second section alone is within its per-section cap, but 1 + it exceeds
+     the cross-section cap; the receiver must reject at the count. */
+  EXPECT_TRUE(send_int(p[1], MAX_MANIFEST_ENTRIES));
+  EXPECT_NULL(receive_manifest_entries(p[0]));
+  Status status;
+  EXPECT_TRUE(receive_status(p[1], &status));
+  EXPECT_EQ_INT(status, STATUS_ERROR);
+
+  close(p[0]);
+  close(p[1]);
+  config_delete(cfg);
+}
+
 void test_server() {
   test_special_socket_path_log_escaped();
   if (!is_running_under_valgrind()) {
@@ -856,6 +1022,9 @@ void test_server() {
     test_incremental_check_size_mismatch_full_transfer();
     test_incremental_check_dry_run_reports_transfer_without_writing();
     test_incremental_check_delta_oversize_reports_failure();
+    test_incremental_check_fifo_destination_does_not_hang();
+    test_incremental_check_basis_fifo_does_not_hang();
+    test_receive_manifest_total_entry_cap();
     test_late_manifest_abort_frees_keepset();
     test_late_manifest_eof_frees_keepset();
     test_late_second_manifest_frees_both();

tests/test_xattr.c

@@ -17,7 +17,7 @@
 
 static void run_recv_helper(int fd) {
   int ok = 0;
-  FileXattrList* list = xattr_receive(fd, &ok);
+  FileXattrList* list = xattr_receive(fd, &ok, false);
   if (!ok)
     _exit(1);
   if (!list) {
@@ -64,7 +64,7 @@ static void test_xattr_wire_roundtrip() {
 
 static void run_recv_must_fail(int fd) {
   int ok = 0;
-  FileXattrList* list = xattr_receive(fd, &ok);
+  FileXattrList* list = xattr_receive(fd, &ok, false);
   /* A NULL list with ok==0 is the expected rejection. */
   if (ok == 0 && list == NULL)
     _exit(0);
@@ -148,15 +148,64 @@ static void test_xattr_count_bound() {
 /* The captured list on a plain file reflects only whitelisted namespaces
  * (Linux only; skipped when the filesystem has no xattr support). */
 static void test_xattr_capture_and_appliable() {
-  EXPECT_FALSE(xattr_name_appliable(NULL));
-  EXPECT_FALSE(xattr_name_appliable(""));
-  EXPECT_FALSE(xattr_name_appliable("security.selinux"));
-  EXPECT_FALSE(xattr_name_appliable("trusted.blob"));
-  EXPECT_TRUE(xattr_name_appliable("user.foo"));
+  EXPECT_FALSE(xattr_name_appliable(NULL, false));
+  EXPECT_FALSE(xattr_name_appliable("", false));
+  EXPECT_FALSE(xattr_name_appliable("security.selinux", false));
+  EXPECT_FALSE(xattr_name_appliable("trusted.blob", false));
+  EXPECT_TRUE(xattr_name_appliable("user.foo", false));
+  EXPECT_TRUE(xattr_name_appliable("user.foo", true));
   /* The reserved fake-super key is receiver-only and never forwarded/applied. */
-  EXPECT_FALSE(xattr_name_appliable("user.fastsync.stat"));
-  EXPECT_TRUE(xattr_name_appliable("system.posix_acl_access"));
-  EXPECT_TRUE(xattr_name_appliable("system.posix_acl_default"));
+  EXPECT_FALSE(xattr_name_appliable("user.fastsync.stat", false));
+  EXPECT_FALSE(xattr_name_appliable("user.fastsync.stat", true));
+  /* B4: the ACL names require --acls; -X alone must not authorize them. */
+  EXPECT_FALSE(xattr_name_appliable("system.posix_acl_access", false));
+  EXPECT_FALSE(xattr_name_appliable("system.posix_acl_default", false));
+  EXPECT_TRUE(xattr_name_appliable("system.posix_acl_access", true));
+  EXPECT_TRUE(xattr_name_appliable("system.posix_acl_default", true));
+}
+
+/* B4: a `-X`-only receiver (preserve_acls false) must NOT apply an incoming
+ * ACL xattr, while a user.* attribute in the same block still survives.  The
+ * ACL entry is dropped, not applied (and the -X transfer is not failed). */
+static void run_recv_drops_acl_keeps_user(int fd) {
+  int ok = 0;
+  FileXattrList* list = xattr_receive(fd, &ok, false);
+  if (!ok || list == NULL)
+    _exit(1);
+  bool saw_user = false;
+  for (int i = 0; i < list->count; i++) {
+    if (strcmp(list->items[i].name, "system.posix_acl_access") == 0)
+      _exit(1); /* ACL must have been dropped */
+    if (strcmp(list->items[i].name, "user.keep") == 0)
+      saw_user = true;
+  }
+  xattr_list_free(list);
+  _exit(saw_user ? 0 : 1);
+}
+
+static void test_xattr_receive_drops_acl_without_preserve_acls() {
+  int p[2];
+  EXPECT_EQ_INT(pipe(p), 0);
+  io_set_fds(p[0], p[1]);
+  io_set_bwlimit(0);
+  pid_t pid = fork();
+  if (pid == 0) {
+    close(p[1]);
+    io_set_fds(p[0], p[0]);
+    run_recv_drops_acl_keeps_user(p[0]);
+  }
+  close(p[0]);
+  io_set_fds(p[1], p[1]);
+  FileXattrList* list = xattr_list_new();
+  EXPECT_NOT_NULL(list);
+  EXPECT_TRUE(xattr_list_append(list, "system.posix_acl_access", "\x02\x00\x00\x00", 4));
+  EXPECT_TRUE(xattr_list_append(list, "user.keep", "yes", 3));
+  xattr_send(p[1], list);
+  xattr_list_free(list);
+  int status;
+  waitpid(pid, &status, 0);
+  close(p[1]);
+  EXPECT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0);
 }
 
 /* MINOR-2: a --link-dest / -H copy fallback (linkat refused) must still apply
@@ -360,6 +409,7 @@ void test_xattr() {
   test_xattr_reject_oversized_value();
   test_xattr_count_bound();
   test_xattr_capture_and_appliable();
+  test_xattr_receive_drops_acl_without_preserve_acls();
   test_link_copy_fallback_preserves_xattrs();
   test_fake_super_restore();
   test_fake_super_owner_gate();