* Michael Kerrisk: > On 09/23/2016 10:41 AM, Florian Weimer wrote: >>> 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. It's in the quoted bits above. After the first success, subsequent attempts at linking the same file descriptor (with AT_EMPTY_PATH or the /proc/self/fd kludge) fail. This is what confused me so much, and makes the behavior seem extremely erratic. -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
