On 09/21/2016 01:50 PM, Florian Weimer wrote:
AT_EMPTY_PATH is supposed to be able to give names to files created with
O_TMPFILE unless O_EXCL was specified at creation time.
However, since this commit
commit 11a7b371b64ef39fc5fb1b6f2218eef7c4d035e3
Author: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Date: Sat Jan 29 18:43:42 2011 +0530
fs: allow AT_EMPTY_PATH in linkat(), limit that to CAP_DAC_READ_SEARCH
linkat bails out early with AT_EMPTY_PATH and !CAP_DAC_READ_SEARCH,
never looking at O_EXCL.
The /proc/self/fd kludge works for unprivileged users, but only if
*both* paths use AT_FDCWD. It fails if the first path uses a real
descriptor for /proc/self/fd, or if the second path uses a real
descriptor for the current directory, or both. For privileged users,
only AT_EMPTY_PATH case with an AT_FDCWD target works.
The attached test program prints under a non-privileged user:
error: linkat (fd, "", AT_FDCWD, out_name, AT_EMPTY_PATH):
No such file or directory
error: linkat (fd, "", current_fd, out_name, AT_EMPTY_PATH):
No such file or directory
success: linkat (AT_FDCWD, proc_name, AT_FDCWD, out_name,
AT_SYMLINK_FOLLOW)
error: linkat (AT_FDCWD, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW):
No such file or directory
error: linkat (proc_fd, proc_name, AT_FDCWD, out_name, AT_SYMLINK_FOLLOW):
No such file or directory
error: linkat (proc_fd, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW):
No such file or directory
successes: 1, failures: 5
And under a privileged user:
success: linkat (fd, "", AT_FDCWD, out_name, AT_EMPTY_PATH)
error: linkat (fd, "", current_fd, out_name, AT_EMPTY_PATH):
No such file or directory
error: linkat (AT_FDCWD, proc_name, AT_FDCWD, out_name, AT_SYMLINK_FOLLOW):
No such file or directory
error: linkat (AT_FDCWD, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW):
No such file or directory
error: linkat (proc_fd, proc_name, AT_FDCWD, out_name, AT_SYMLINK_FOLLOW):
No such file or directory
error: linkat (proc_fd, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW):
No such file or directory
successes: 1, failures: 5
(Seen on tmpfs and XFS, 4.7.x kernels.)
I double-checked with strace, and the test case does not appear to be
broken. But the exhibited behavior is truly bizarre, and it means that
it is very difficult to give a name to an O_TMPFILE file.
The test case is broken because it does not account for the fact that an
O_TMPFILE file can only be linked once in this way. This is still a bit
counter-intuitive, but it means that O_TMPFILE works.
Florian
#include <unistd.h>
#include <fcntl.h>
#include <err.h>
#include <stdio.h>
static int failures;
static int successes;
#define CHECK(ret) \
do \
if ((ret) < 0) \
{ \
printf ("error: %s:\n %m\n", #ret); \
++failures; \
} \
else \
{ \
printf ("success: %s\n", #ret); \
++successes; \
} \
while (0)
static const char *const out_name = "linkat.out";
void
setup_fd (int *pfd)
{
if (*pfd >= 0)
close (*pfd);
*pfd = open (".", O_RDWR | O_TMPFILE, 0);
if (*pfd < 0)
err (1, "open");
}
int
main (void)
{
unlink (out_name);
int current_fd = open (".", O_RDONLY | O_DIRECTORY);
if (current_fd < 0)
err (1, "open (O_DIRECTORY)");
int proc_fd = open ("/proc/self/fd", O_RDONLY | O_DIRECTORY);
if (proc_fd < 0)
err (1, "open (O_DIRECTORY)");
int fd = -1;
setup_fd (&fd);
CHECK (linkat (fd, "", AT_FDCWD, out_name, AT_EMPTY_PATH));
unlink (out_name);
setup_fd (&fd);
CHECK (linkat (fd, "", current_fd, out_name, AT_EMPTY_PATH));
unlink (out_name);
char proc_name[100];
snprintf (proc_name, sizeof (proc_name), "/proc/self/fd/%d", fd);
setup_fd (&fd);
CHECK (linkat (AT_FDCWD, proc_name, AT_FDCWD, out_name,
AT_SYMLINK_FOLLOW));
unlink (out_name);
setup_fd (&fd);
CHECK (linkat (AT_FDCWD, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW));
unlink (out_name);
snprintf (proc_name, sizeof (proc_name), "%d", fd);
setup_fd (&fd);
CHECK (linkat (proc_fd, proc_name, AT_FDCWD, out_name,
AT_SYMLINK_FOLLOW));
unlink (out_name);
setup_fd (&fd);
CHECK (linkat (proc_fd, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW));
unlink (out_name);
printf ("successes: %d, failures: %d\n", successes, failures);
return 0;
}
