On 09/23/2016 10:41 AM, Florian Weimer wrote: > 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, 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/ -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
