linux: fix copy_from_fd ownership

[eriksjolund]

Fix some issues regarding file descriptor ownership.

Initial investigation in #2002
(I found these when reading the source code)

[gemini-code-assist]

Summary of Changes

Hello @eriksjolund, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a file descriptor ownership problem within the crun project, specifically related to temporary copy-up operations. It refactors the handling of file descriptors passed between process_single_mount, handle_tmpcopyup, and copy_recursive_fd_to_fd to prevent potential double-closing or leaks. The changes clarify which function is responsible for closing specific file descriptors, improving the robustness and correctness of file handling.

Highlights

• File Descriptor Ownership: The handle_tmpcopyup function now correctly manages the copy_from_fd by initializing tmpfd with cleanup_close, ensuring the file descriptor is automatically closed when tmpfd goes out of scope.
• Redundant Close Calls: Explicit close(destfd) and close(tmpfd) calls were removed from handle_tmpcopyup, as the copy_recursive_fd_to_fd function is now responsible for closing these file descriptors.
• Caller Responsibility: The process_single_mount function now explicitly sets copy_from_fd to -1 after calling handle_tmpcopyup, indicating that handle_tmpcopyup (and subsequently copy_recursive_fd_to_fd) has taken ownership and closed the descriptor.

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Changelog

• src/libcrun/linux.c
  • The handle_tmpcopyup function was modified to use cleanup_close for tmpfd and remove manual close() calls, delegating closing responsibility to copy_recursive_fd_to_fd.
  • In process_single_mount, copy_from_fd is now explicitly set to -1 after being passed to handle_tmpcopyup to reflect its closure.
• src/libcrun/utils.c
  • A comment was added to copy_recursive_fd_to_fd to explicitly state that it closes the srcfd and destfd file descriptors it operates on.

Activity

• No human activity (comments, reviews, etc.) has been recorded on this pull request yet.

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[eriksjolund]

(I haven't tested that it actually works).

[gemini-code-assist]

Code Review

This pull request correctly fixes a file descriptor ownership bug, preventing a double-close on copy_from_fd. The changes in handle_tmpcopyup and process_single_mount properly manage the file descriptor's lifecycle, ensuring it's closed exactly once. The use of cleanup_close combined with setting the descriptor to -1 after its ownership is transferred is a robust pattern that correctly handles error paths. The added comments clarifying which function is responsible for closing the file descriptors are a welcome addition for code clarity and maintainability. The fix appears solid and well-implemented.

[packit-as-a-service]

Ephemeral COPR build failed. @containers/packit-build please check.

[packit-as-a-service]

Ephemeral COPR build failed. @containers/packit-build please check.

[packit-as-a-service]

TMT tests failed. @containers/packit-build please check.

[eriksjolund]

For improved readability, I could also

diff --git a/src/libcrun/linux.c b/src/libcrun/linux.c
index 7438b8c8..46b5c901 100644
--- a/src/libcrun/linux.c
+++ b/src/libcrun/linux.c
@@ -2124,16 +2124,16 @@ static int
 handle_tmpcopyup (libcrun_container_t *container, const char *rootfs, const char *target,
                   int copy_from_fd, libcrun_error_t *err)
 {
-  int destfd, ret;
+  int  ret;
   cleanup_close int tmpfd = copy_from_fd;
-  destfd = safe_openat (get_private_data (container)->rootfsfd, rootfs, target,
+  cleanup_close int destfd = safe_openat (get_private_data (container)->rootfsfd, rootfs, target,
                         O_CLOEXEC | O_DIRECTORY, 0, err);
   if (UNLIKELY (destfd < 0))
     return crun_error_wrap (err, "open `%s` to write for tmpcopyup", target);
 
   // copy_recursive_fd_to_fd closes tmpfd and destfd
   ret = copy_recursive_fd_to_fd (tmpfd, destfd, target, target, err);
-  tmpfd = -1;
+  tmpfd = destfd = -1;
 
   return ret;
 }

but this makes the code less efficient.
For me either way is fine.

[giuseppe]

LGTM