[vfs] Make sticky bit check consistent with Linux.

Our implementation of vfs.CheckDeleteSticky was not consistent with Linux,
specifically not consistent with fs/linux.h:check_sticky().

One of the biggest differences was that the vfs implementation did not
allow the owner of the sticky directory to delete files inside it that belonged
to other users.

This change makes our implementation consistent with Linux.

Also adds an integration test to check for this. This bug is also present in
VFS1.

Updates #3027

PiperOrigin-RevId: 355557425

1 files changed, 96 insertions, 0 deletions

diff --git a/images/basic/integrationtest/test_sticky.c b/images/basic/integrationtest/test_sticky.c
new file mode 100644
index 000000000..58dcf91d3
--- /dev/null
+++ b/images/basic/integrationtest/test_sticky.c
@@ -0,0 +1,96 @@
+#include <err.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+void createFile(const char* path) {
+  int fd = open(path, O_WRONLY | O_CREAT, 0777);
+  if (fd < 0) {
+    err(1, "open(%s)", path);
+    exit(1);
+  } else {
+    close(fd);
+  }
+}
+
+void waitAndCheckStatus(pid_t child) {
+  int status;
+  if (waitpid(child, &status, 0) == -1) {
+    err(1, "waitpid() failed");
+    exit(1);
+  }
+
+  if (WIFEXITED(status)) {
+    int es = WEXITSTATUS(status);
+    if (es) {
+      err(1, "child exit status %d", es);
+      exit(1);
+    }
+  } else {
+    err(1, "child did not exit normally");
+    exit(1);
+  }
+}
+
+void deleteFile(uid_t user, const char* path) {
+  pid_t child = fork();
+  if (child == 0) {
+    if (setuid(user)) {
+      err(1, "setuid(%d)", user);
+      exit(1);
+    }
+
+    if (unlink(path)) {
+      err(1, "unlink(%s)", path);
+      exit(1);
+    }
+    exit(0);
+  }
+  waitAndCheckStatus(child);
+}
+
+int main(int argc, char** argv) {
+  const char kUser1Dir[] = "/user1dir";
+  const char kUser2File[] = "/user1dir/user2file";
+  const char kUser2File2[] = "/user1dir/user2file2";
+
+  const uid_t user1 = 6666;
+  const uid_t user2 = 6667;
+
+  if (mkdir(kUser1Dir, 0755) != 0) {
+    err(1, "mkdir(%s)", kUser1Dir);
+    exit(1);
+  }
+  // Enable sticky bit for user1dir.
+  if (chmod(kUser1Dir, 01777) != 0) {
+    err(1, "chmod(%s)", kUser1Dir);
+    exit(1);
+  }
+  createFile(kUser2File);
+  createFile(kUser2File2);
+
+  if (chown(kUser1Dir, user1, getegid())) {
+    err(1, "chown(%s)", kUser1Dir);
+    exit(1);
+  }
+  if (chown(kUser2File, user2, getegid())) {
+    err(1, "chown(%s)", kUser2File);
+    exit(1);
+  }
+  if (chown(kUser2File2, user2, getegid())) {
+    err(1, "chown(%s)", kUser2File2);
+    exit(1);
+  }
+
+  // User1 should be able to delete any file inside user1dir, even files of
+  // other users due to the sticky bit.
+  deleteFile(user1, kUser2File);
+
+  // User2 should naturally be able to delete its own file even if the file is
+  // inside a sticky dir owned by someone else.
+  deleteFile(user2, kUser2File2);
+}