* Florian Weimer:
> 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).
Sorry, I forgot to mention that I see this with Debian's
4.19.67-2+deb10u2 kernel and Fedora's 5.4.17-200.fc31.x86_64 kernel.
