Re: XFS reports lchmod failure, but changes file system contents

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Feb 12, 2020 at 12:48:49PM +0100, Florian Weimer wrote:
> In principle, Linux supports lchmod via O_PATH descriptors and chmod
> on /proc/self/fd.  (lchmod is the non-symbolic-link-following variant
> of chmod.)
> 
> This helper program can be used to do this:
> 
> #define _GNU_SOURCE
> #include <err.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/stat.h>
> #include <unistd.h>
> 
> int
> main (int argc, char **argv)
> {
>   if (argc != 3)
>     {
>       fprintf (stderr, "usage: %s MODE FILE\n", argv[0]);
>       return 2;
>     }
> 
>   unsigned int mode;
>   if (sscanf (argv[1], "%o", &mode) != 1
>       || mode != (mode_t) mode)
>     errx (1, "invalid mode: %s", argv[1]);
> 
>   int fd = open (argv[2], O_PATH | O_NOFOLLOW);
>   if (fd < 0)
>     err (1, "open");
> 
>   char *fd_path;
>   if (asprintf (&fd_path, "/proc/self/fd/%d", fd) < 0)
>     err (1, "asprintf");
> 
>   if (chmod (fd_path, mode) != 0)
>     err (1, "chmod");
> 
>   free (fd_path);
>   if (close (fd) != 0)
>     err (1, "close");
> 
>   return 0;
> }
> 
> When changing the permissions of on XFS in this way, the chmod
> operation fails:
> 
> $ ln -s does-not-exist /var/tmp/symlink
> $ ls -l /var/tmp/symlink 
> lrwxrwxrwx. 1 fweimer fweimer 14 Feb 12 12:41 /var/tmp/symlink -> does-not-exist
> $ strace ./lchmod 0 /var/tmp/symlink
> […]
> openat(AT_FDCWD, "/var/tmp/symlink", O_RDONLY|O_NOFOLLOW|O_PATH) = 3
> […]
> chmod("/proc/self/fd/3", 000)           = -1 EOPNOTSUPP (Operation not supported)
> write(2, "lchmod: ", 8lchmod: )                 = 8
> write(2, "chmod", 5chmod)                    = 5
> write(2, ": Operation not supported\n", 26: Operation not supported
> ) = 26
> exit_group(1)                           = ?
> 
> But the file system contents has changed nevertheless:
> 
> $ ls -l /var/tmp/symlink 
> l---------. 1 fweimer fweimer 14 Feb 12 12:41 /var/tmp/symlink -> does-not-exist
> $ echo 3 | sudo tee /proc/sys/vm/drop_caches 
> $ ls -l /var/tmp/symlink 
> l---------. 1 fweimer fweimer 14 Feb 12 12:41 /var/tmp/symlink -> does-not-exist
> 
> This looks like an XFS bug to me.  With tmpfs, the chmod succeeds and
> is reflected in the file system.
> 
> This bug also affects regular files, not just symbolic links.
> 
> It causes the io/tst-lchmod glibc test to fail (after it has been
> fixed, the in-tree version has another bug).

xfs_setattr_nonsize calls posix_acl_chmod which returns EOPNOTSUPP
because the xfs symlink inode_operations do not include a ->set_acl
pointer.

I /think/ that posix_acl_chmod code exists to enforce that the file mode
reflects any acl that might be set on the inode, but in this case the
inode is a symbolic link.

I don't remember off the top of my head if ACLs are supposed to apply to
symlinks, but what do you think about adding get_acl/set_acl pointers to
xfs_symlink_inode_operations and xfs_inline_symlink_inode_operations ?

--D