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Linux: Emulate fchmodat with AT_SYMLINK_NOFOLLOW using O_PATH [BZ #14578]

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/proc/self/fd files are special and chmod on O_PATH descriptors
in that directory operates on the symbolic link itself (like lchmod).
This commit is contained in:
Florian Weimer 2020-01-22 19:01:20 +01:00
parent 6b89c385d8
commit 752dd17443
1 changed files with 47 additions and 10 deletions
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57
sysdeps/unix/sysv/linux/fchmodat.c
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@ -18,24 +18,61 @@
#include <errno.h>
#include <fcntl.h>
#include <stddef.h>
#include <not-cancel.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <alloca.h>
#include <sysdep.h>
#include <unistd.h>
int
fchmodat (int fd, const char *file, mode_t mode, int flag)
{
if (flag & ~AT_SYMLINK_NOFOLLOW)
if (flag == 0)
return INLINE_SYSCALL (fchmodat, 3, fd, file, mode);
else if (flag != AT_SYMLINK_NOFOLLOW)
return INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL);
#ifndef __NR_lchmod /* Linux so far has no lchmod syscall. */
if (flag & AT_SYMLINK_NOFOLLOW)
return INLINE_SYSCALL_ERROR_RETURN_VALUE (ENOTSUP);
#endif
else
{
/* The kernel system call does not have a mode argument.
However, we can create an O_PATH descriptor and change that
via /proc (which does not resolve symbolic links). */
return INLINE_SYSCALL (fchmodat, 3, fd, file, mode);
int pathfd = __openat_nocancel (fd, file,
O_PATH | O_NOFOLLOW | O_CLOEXEC);
if (pathfd < 0)
/* This may report errors such as ENFILE and EMFILE. The
caller can treat them as temporary if necessary. */
return pathfd;
char buf[32];
if (__snprintf (buf, sizeof (buf), "/proc/self/fd/%d", pathfd) < 0)
{
/* This also may report strange error codes to the caller
(although snprintf really should not fail). */
__close_nocancel (pathfd);
return -1;
}
/* This operates directly on the symbolic link if it is one.
/proc/self/fd files look like symbolic links, but they are
not. (fchmod and fchmodat do not work on O_PATH descriptors,
similar to fstat before Linux 3.6.) */
int ret = __chmod (buf, mode);
if (ret != 0)
{
if (errno == ENOENT)
/* /proc has not been mounted. Without /proc, there is no
way to upgrade the O_PATH descriptor to a full
descriptor. It is also not possible to re-open the
file without O_PATH because the file name may refer to
another file, and opening that without O_PATH may have
side effects (such as blocking, device rewinding, or
releasing POSIX locks). */
__set_errno (EOPNOTSUPP);
}
__close_nocancel (pathfd);
return ret;
}
}
libc_hidden_def (fchmodat)