File permissions are not persisted after creating hard link if system crashes

File permissions are not persisted after creating hard link if system crashes

[Vyacheslav Kovalevsky]

Detailed description
====================

Hello, there seems to be an issue with btrfs crash behavior:

1. Create and sync a new file.
2. Open the file and change permissions.
3. Sync the file.
4. Create new hard link to file.
5. Sync the root directory.

After crash the file will have old (original) permissions, though the 
changes were synced.


System info
===========

Linux version 7.0-rc2, also tested on 6.19.2


How to reproduce
================

```
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main() {
   int status;
   int file_fd0;
   int file_fd1;
   int root_fd;

   status = creat("file1", S_IRWXU | S_IRGRP | S_IXGRP | S_IXOTH);
   printf("CREAT: %d\n", status);
   file_fd0 = status;

   status = close(file_fd0);
   printf("CLOSE: %d\n", status);

   sync();

   status = open("file1", O_RDONLY);
   printf("OPEN: %d\n", status);
   file_fd1 = status;

   status = fchmod(file_fd1, S_IRWXU | S_IRWXG | S_IRWXO);
   printf("FCHMOD: %d\n", status);

   status = fsync(file_fd1);
   printf("FSYNC: %d\n", status);

   status = link("file1", "file2");
   printf("LINK: %d\n", status);

   status = open(".", O_RDONLY);
   printf("OPEN: %d\n", status);
   root_fd = status;

   status = fsync(root_fd);
   printf("FSYNC: %d\n", status);
}
// after crash file `file1` / `file2` has old permissions (0751 instead 
of 0777)
```

Steps:

1. Create and mount new btrfs file system in default configuration.
2. Change directory to root of the file system and run the compiled test.
3. Cause hard system crash (e.g. QEMU `system_reset` command).
4. Remount file system after crash.
5. Observe that file permissions were not persisted.

Re: File permissions are not persisted after creating hard link if system crashes

[Filipe Manana]

On Tue, Mar 3, 2026 at 4:47 PM Vyacheslav Kovalevsky
<slava.kovalevskiy.2014@gmail.com> wrote:
>
> Detailed description
> ====================
>
> Hello, there seems to be an issue with btrfs crash behavior:
>
> 1. Create and sync a new file.
> 2. Open the file and change permissions.
> 3. Sync the file.
> 4. Create new hard link to file.
> 5. Sync the root directory.
>
> After crash the file will have old (original) permissions, though the
> changes were synced.

For this test to pass you'll need the patch I sent for another report
you made before:

https://lore.kernel.org/linux-btrfs/af8c15fa-4e41-4bb2-885c-0bc4e97532a6@gmail.com/

The patch is: https://lore.kernel.org/linux-btrfs/1c66bd7efe749cd3b421fc3dff5ef646dda8c7aa.1771350720.git.fdmanana@suse.com/

It's not in any released kernel yet (not even in Linus' master
branch), only in the for-next github branch:
https://github.com/btrfs/linux/commits/for-next/

Whenever you have new things to report, try to see if every patch I
pointed you to in previous reports is applied in the kernel you are
testing.

Thanks.

>
>
> System info
> ===========
>
> Linux version 7.0-rc2, also tested on 6.19.2
>
>
> How to reproduce
> ================
>
> ```
> #include <errno.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <string.h>
> #include <sys/stat.h>
> #include <sys/types.h>
> #include <unistd.h>
>
> int main() {
>    int status;
>    int file_fd0;
>    int file_fd1;
>    int root_fd;
>
>    status = creat("file1", S_IRWXU | S_IRGRP | S_IXGRP | S_IXOTH);
>    printf("CREAT: %d\n", status);
>    file_fd0 = status;
>
>    status = close(file_fd0);
>    printf("CLOSE: %d\n", status);
>
>    sync();
>
>    status = open("file1", O_RDONLY);
>    printf("OPEN: %d\n", status);
>    file_fd1 = status;
>
>    status = fchmod(file_fd1, S_IRWXU | S_IRWXG | S_IRWXO);
>    printf("FCHMOD: %d\n", status);
>
>    status = fsync(file_fd1);
>    printf("FSYNC: %d\n", status);
>
>    status = link("file1", "file2");
>    printf("LINK: %d\n", status);
>
>    status = open(".", O_RDONLY);
>    printf("OPEN: %d\n", status);
>    root_fd = status;
>
>    status = fsync(root_fd);
>    printf("FSYNC: %d\n", status);
> }
> // after crash file `file1` / `file2` has old permissions (0751 instead
> of 0777)
> ```
>
> Steps:
>
> 1. Create and mount new btrfs file system in default configuration.
> 2. Change directory to root of the file system and run the compiled test.
> 3. Cause hard system crash (e.g. QEMU `system_reset` command).
> 4. Remount file system after crash.
> 5. Observe that file permissions were not persisted.
>
>