fix(p5-remote-option): wire server --trust-sender, align save-layer gates, add hostile-sender test

src/server/server.c

@@ -22,6 +22,7 @@
 static char* authorized_root;
 static int authorized_root_fd = -1;
 static bool allow_delete;
+static bool trust_sender;
 static bool allow_unauthenticated;
 static const char* required_client_cn;
 
@@ -221,11 +222,14 @@ void handler(int file_descriptor) {
      across the per-connection forked processes). */
   file_set_keep_dirlinks(config->keep_dirlinks);
   /* --trust-sender is a LOCAL receiver policy: it never crosses the wire (so a
-     wire peer can never enable it), the receiving process applies it here from
-     its own config.  Set before any multithreaded receiver/writer threads are
-     spawned so the fd-walk reads a stable value during the whole transfer, and
-     never bleeds across the per-connection forked processes.  Off by default. */
-  file_set_trust_sender(config->trust_sender);
+     wire peer can never enable it).  The standalone server only honours it when
+     its own CLI was started with --trust-sender (the client forwards that switch
+     into the remote argv via --remote-option=--trust-sender; the server then
+     parses it here and applies the policy below).  Set before any multithreaded
+     receiver/writer threads are spawned so the fd-walk reads a stable value
+     during the whole transfer, and never bleeds across the per-connection
+     forked processes.  Off by default. */
+  file_set_trust_sender(trust_sender);
   if (config->use_multithreading) {
     Queue* q = queue_create(100, file_destroy);
     if (q == NULL) {
@@ -348,6 +352,7 @@ static void print_server_usage(void) {
   printf("  --client-cn <name>  Required TLS client certificate CN\n");
   printf("  --destination-root <path>  Authorized destination root (default: .)\n");
   printf("  --allow-delete      Permit manifest deletion\n");
+  printf("  --trust-sender      Trust the remote sender's file list\n");
   printf("  --allow-unauthenticated  Allow plaintext/anonymous network clients\n");
   printf("  -v, --verbose       Enable debug logging\n");
   printf("  --help              Show this help\n");
@@ -384,6 +389,8 @@ int main(int argc, char* argv[]) {
       destination_root = argv[++i];
     } else if (strcmp(argv[i], "--allow-delete") == 0) {
       allow_delete = true;
+    } else if (strcmp(argv[i], "--trust-sender") == 0) {
+      trust_sender = true;
     } else if (strcmp(argv[i], "--allow-unauthenticated") == 0) {
       allow_unauthenticated = true;
     } else if (strcmp(argv[i], "-p") == 0 && i + 1 < argc) {

src/shared/file_receive.c

@@ -329,8 +329,12 @@ bool file_special_rdev_valid(int32_t major, int32_t minor, mode_t mode) {
  */
 static FileSaveResult file_save_special_to_disk(const char* root_directory, const File* file,
                                                 const Config* config) {
+  /* The empty-path and structural checks stay unconditional; the redundant
+     ".." list-path re-check is skipped under --trust-sender exactly like the
+     receive layer (confinement is deferred to the secure parent walk below,
+     which is never disabled). */
   if (!root_directory || !file || !file->path || file->path[0] == '\0' ||
-      has_path_traversal(file->path) || !file->metadata)
+      (!file_get_trust_sender() && has_path_traversal(file->path)) || !file->metadata)
     return FILE_SAVE_ERROR;
 
   mode_t mode = file->metadata->mode;
@@ -461,7 +465,7 @@ static FileSaveResult file_save_special_to_disk(const char* root_directory, cons
  * entry is skipped), never aborts. */
 static FileSaveResult file_save_write_device(const char* root_directory, const File* file) {
   if (!root_directory || !file || !file->path || file->path[0] == '\0' ||
-      has_path_traversal(file->path))
+      (!file_get_trust_sender() && has_path_traversal(file->path)))
     return FILE_SAVE_ERROR;
   if (!file->data)
     return FILE_SAVE_ERROR;
@@ -538,7 +542,7 @@ FileSaveResult file_save_to_disk_full(const char* root_directory, const File* fi
   char *backup_path = NULL, *parent_copy = NULL;
 
   if (!file || !file->path || !file->data || (file->data->size != 0 && !file->data->data) ||
-      has_path_traversal(file->path) ||
+      (!file_get_trust_sender() && has_path_traversal(file->path)) ||
       (backup_enabled &&
        (!backup_suffix || backup_suffix[0] == '\0' || strchr(backup_suffix, '/') != NULL ||
         strcmp(backup_suffix, ".") == 0 || strcmp(backup_suffix, "..") == 0))) {
@@ -560,7 +564,7 @@ FileSaveResult file_save_to_disk_full(const char* root_directory, const File* fi
      immediately (they are never staged by --delay-updates, matching rsync,
      where directory creation is not delayed). */
   if (file->is_dir) {
-    if (file->path[0] == '\0' || has_path_traversal(file->path)) {
+    if (file->path[0] == '\0' || (!file_get_trust_sender() && has_path_traversal(file->path))) {
       log_message(LOG_LEVEL_ERROR, "Invalid directory path received");
       return FILE_SAVE_ERROR;
     }
@@ -577,7 +581,8 @@ FileSaveResult file_save_to_disk_full(const char* root_directory, const File* fi
      threads start, so it is stable throughout this walk.) */
 
   if (file->is_symlink) {
-    if (!file->symlink_target || file->path[0] == '\0' || has_path_traversal(file->path)) {
+    if (!file->symlink_target || file->path[0] == '\0' ||
+        (!file_get_trust_sender() && has_path_traversal(file->path))) {
       log_message(LOG_LEVEL_ERROR, "Invalid symlink entry received");
       return FILE_SAVE_ERROR;
     }

tests/runner.c

@@ -55,6 +55,7 @@ int main() {
   RUN_TEST(test_metadata);
   RUN_TEST(test_glob);
   RUN_TEST(test_file);
+  RUN_TEST(test_trust_sender);
   RUN_TEST(test_delay_updates);
   RUN_TEST(test_file_sendfile);
   RUN_TEST(test_multiprocessing);

tests/test_file.c

@@ -6,6 +6,7 @@
 #include "protocol.h"
 #include "test_utils.h"
 #include <fcntl.h>
+#include <limits.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
@@ -1022,6 +1023,199 @@ static void test_dir_entry_save_to_disk() {
   rmdir(root);
 }
 
+/* ---- Phase 5 (--trust-sender) safety-floor tests ----
+ *
+ * --trust-sender is a receiver-local policy that never crosses the wire: a real
+ * receiver enables it from its own process (the standalone server's --trust-
+ * sender CLI switch, which a client forwards as --remote-option=--trust-sender),
+ * so these tests force file_set_trust_sender(true) directly.  Trust must RELAX
+ * only the redundant list-level re-validation (an escaping symlink TARGET is
+ * copied verbatim, rsync -l parity) and must NEVER disable the low-level
+ * fd-relative confinement floor: file_open_secure_parent's ".." rejection, the
+ * O_NOFOLLOW parent walk, leaf/destination confinement, and the ungated
+ * has_path_traversal on the link's own placement path in file_symlink_at_secure
+ * stay hard.  A hostile sender therefore still cannot place a file, directory
+ * or symlink outside the receive root even with trust on. */
+
+static void test_trust_sender_relaxes_symlink_target() {
+  const char* root = "test_trust_sender_root";
+  const char* link = "test_trust_sender_root/escape_link";
+  unlink(link);
+  rmdir(root);
+  EXPECT_EQ_INT(mkdir(root, 0755), 0);
+
+  /* Control: without trust an absolute (escaping) target is refused and the
+     link is never placed. */
+  file_set_trust_sender(false);
+  EXPECT_FALSE(file_symlink_at_secure(link, "/etc/passwd"));
+  struct stat st;
+  EXPECT_EQ_INT(lstat(link, &st), -1);
+
+  /* Trust ON: the escaping target is copied verbatim (rsync -l parity) ... */
+  file_set_trust_sender(true);
+  EXPECT_TRUE(file_symlink_at_secure(link, "/etc/passwd"));
+  EXPECT_EQ_INT(lstat(link, &st), 0);
+  EXPECT_TRUE(S_ISLNK(st.st_mode));
+  /* ...but the link itself still lands beneath the receive root. */
+  char target[128];
+  ssize_t target_len = readlink(link, target, sizeof(target) - 1);
+  EXPECT_TRUE(target_len > 0);
+  // cppcheck-suppress knownConditionTrueFalse
+  if (target_len > 0) {
+    target[target_len] = '\0';
+    EXPECT_EQ_STR(target, "/etc/passwd");
+  }
+  unlink(link);
+
+  /* Same relaxation through the real save funnel (file_save_to_disk_full). */
+  Config* config = config_create();
+  EXPECT_NOT_NULL(config);
+  const char* save_link = "test_trust_sender_root/save_link";
+  unlink(save_link);
+
+  File* sym = file_create("save_link");
+  EXPECT_NOT_NULL(sym);
+  sym->is_symlink = true;
+  sym->symlink_target = str_dup("/etc/passwd");
+  EXPECT_NOT_NULL(sym->symlink_target);
+
+  file_set_trust_sender(false);
+  EXPECT_EQ_INT(file_save_to_disk_full(root, sym, config), FILE_SAVE_SKIPPED);
+  EXPECT_EQ_INT(lstat(save_link, &st), -1);
+
+  file_set_trust_sender(true);
+  EXPECT_EQ_INT(file_save_to_disk_full(root, sym, config), FILE_SAVE_WRITTEN);
+  EXPECT_EQ_INT(lstat(save_link, &st), 0);
+  EXPECT_TRUE(S_ISLNK(st.st_mode));
+
+  file_destroy(sym);
+  config_delete(config);
+  unlink(save_link);
+  rmdir(root);
+}
+
+static void test_trust_sender_confines_hostile_paths() {
+  const char* root = "test_trust_sender_root";
+  const char* escaped_file = "../test_trust_sender_escaped_file.txt";
+  const char* escaped_dir = "../test_trust_sender_escaped_dir";
+  const char* escaped_link = "../test_trust_sender_escaped_link";
+  unlink(escaped_file);
+  rmdir(escaped_dir);
+  unlink(escaped_link);
+  unlink(root);
+  rmdir(root);
+  EXPECT_EQ_INT(mkdir(root, 0755), 0);
+
+  Config* config = config_create();
+  EXPECT_NOT_NULL(config);
+  file_set_trust_sender(true);
+  struct stat st;
+
+  /* A hostile regular-file path that would escape the root is contained: the
+     save-layer ".." re-check is relaxed under trust, so the attempt reaches the
+     secure floor, which refuses the walk -- nothing appears outside. */
+  File* file = file_create(escaped_file);
+  EXPECT_NOT_NULL(file);
+  file->data->data = malloc(5);
+  EXPECT_NOT_NULL(file->data->data);
+  memcpy(file->data->data, "evil", 4);
+  file->data->size = 4;
+  EXPECT_EQ_INT(file_save_to_disk_full(root, file, config), FILE_SAVE_ERROR);
+  file_destroy(file);
+  EXPECT_EQ_INT(lstat(escaped_file, &st), -1);
+
+  /* A hostile directory entry is contained the same way. */
+  File* dir = file_create(escaped_dir);
+  EXPECT_NOT_NULL(dir);
+  dir->is_dir = true;
+  EXPECT_EQ_INT(file_save_to_disk_full(root, dir, config), FILE_SAVE_ERROR);
+  file_destroy(dir);
+  EXPECT_EQ_INT(lstat(escaped_dir, &st), -1);
+
+  /* A hostile symlink whose OWN placement path escapes the root is refused even
+     under trust: the ungated has_path_traversal in file_symlink_at_secure never
+     turns off. */
+  EXPECT_FALSE(file_symlink_at_secure("test_trust_sender_root/../escaped_link", "/etc/passwd"));
+  EXPECT_EQ_INT(lstat(escaped_link, &st), -1);
+
+  /* file_open_secure_parent still refuses a ".." component outright. */
+  char* leaf = NULL;
+  EXPECT_EQ_INT(file_open_secure_parent("test_trust_sender_root/../../etc/passwd", &leaf, true),
+                -1);
+  free(leaf);
+
+  config_delete(config);
+  rmdir(root);
+}
+
+/* The same guarantees under a configured authorized root: a within-root link
+   with an escaping target is created (relaxed), while a placement path that is
+   a clean absolute path OUTSIDE the authorized root (no ".." anywhere) is
+   refused by the leaf/destination confinement. */
+static void test_trust_sender_authorized_root_confinement() {
+  const char* root = "test_trust_sender_root";
+  const char* sibling = "test_trust_sender_sibling";
+  unlink(root);
+  rmdir(root);
+  rmdir(sibling);
+  EXPECT_EQ_INT(mkdir(root, 0755), 0);
+  EXPECT_EQ_INT(mkdir(sibling, 0755), 0);
+
+  char root_abs[PATH_MAX];
+  char sibling_abs[PATH_MAX];
+  EXPECT_NOT_NULL(realpath(root, root_abs));
+  EXPECT_NOT_NULL(realpath(sibling, sibling_abs));
+  int root_fd = open(root_abs, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+  EXPECT_TRUE(root_fd >= 0);
+  // cppcheck-suppress knownConditionTrueFalse
+  if (root_fd < 0) {
+    rmdir(root);
+    rmdir(sibling);
+    return;
+  }
+  EXPECT_TRUE(file_set_authorized_root(root_fd, root_abs));
+
+  file_set_trust_sender(true);
+  struct stat st;
+
+  /* Within the authorized root, an escaping symlink TARGET is copied verbatim. */
+  char* inside_link = path_cat(root_abs, "authorized_escape_link");
+  EXPECT_NOT_NULL(inside_link);
+  unlink(inside_link);
+  EXPECT_TRUE(file_symlink_at_secure(inside_link, "/etc/passwd"));
+  EXPECT_EQ_INT(lstat(inside_link, &st), 0);
+  EXPECT_TRUE(S_ISLNK(st.st_mode));
+  unlink(inside_link);
+
+  /* A clean absolute path in a sibling directory (outside the authorized root)
+     is still refused even under trust. */
+  char* outside_link = path_cat(sibling_abs, "test_trust_sender_outside_link");
+  EXPECT_NOT_NULL(outside_link);
+  unlink(outside_link);
+  EXPECT_FALSE(file_symlink_at_secure(outside_link, "/etc/passwd"));
+  EXPECT_EQ_INT(lstat(outside_link, &st), -1);
+
+  free(outside_link);
+  free(inside_link);
+  file_set_authorized_root(-1, NULL);
+  close(root_fd);
+  unlink("test_trust_sender_outside_link");
+  rmdir(sibling);
+  rmdir(root);
+}
+
+void test_trust_sender() {
+  /* The final reset lines always run (a failing EXPECT only returns from the
+     helper), so a later group never inherits a stray trust/authorized-root
+     policy. */
+  file_set_trust_sender(false);
+  test_trust_sender_relaxes_symlink_target();
+  test_trust_sender_confines_hostile_paths();
+  test_trust_sender_authorized_root_confinement();
+  file_set_trust_sender(false);
+  file_set_authorized_root(-1, NULL);
+}
+
 void test_file() {
   test_file_create();
   test_file_special_rdev_valid();

tests/test_file.h

@@ -2,5 +2,6 @@
 #define TEST_FILE_H
 
 void test_file();
+void test_trust_sender();
 
 #endif