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 [off-list ref]
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.
Is this really the intended behavior? Has it always been this way?
Thanks,
Florian
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 [off-list ref]
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
On 09/23/2016 10:41 AM, Florian Weimer wrote:
On 09/21/2016 01:50 PM, Florian Weimer wrote:
quoted
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 [off-list ref]
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, could you elaborate on the "can only be linked once in
this way"? In my experiments, it's possible to link multiple times
to the O_TMPFILE file.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/