On 12/30/2019 3:55 PM, Brian Inglis wrote: > On 2019-12-30 12:53, Ken Brown wrote: >> On 12/30/2019 2:18 PM, Brian Inglis wrote: >>> On 2019-12-29 10:56, Ken Brown wrote: >>>> Currently, opening a symlink with O_NOFOLLOW fails with ELOOP. >>>> Following Linux, the first patch in this series allows the call to >>>> succeed if O_PATH is also specified. >>>> >>>> According to the Linux man page for 'open', the file descriptor >>>> returned by the call should be usable as the dirfd argument in calls >>>> to fstatat and readlinkat with an empty pathname, to have >>>> the calls operate on the symbolic link. The second and third patches >>>> achieve this. For fstatat, we do this by adding support >>>> for the AT_EMPTY_PATH flag. >>>> >>>> Note: The man page mentions fchownat and linkat also. linkat already >>>> supports the AT_EMPTY_PATH flag, so nothing needs to be done. But I >>>> don't understand how this could work for fchownat, because fchown >>>> fails with EBADF if its fd argument was opened with O_PATH. So I >>>> haven't touched fchownat. >>>> >>>> Am I missing something? >>> >>> WSL $ man 2 chown >>> ... >>> "AT_EMPTY_PATH (since Linux 2.6.39) >>> If pathname is an empty string, operate on the file referred to >>> by dirfd (which may have been obtained using the open(2) O_PATH >>> flag). In this case, dirfd can refer to any type of file, not >>> just a directory. If dirfd is AT_FDCWD, the call operates on >>> the current working directory. This flag is Linux-specific; de‐ >>> fine _GNU_SOURCE to obtain its definition." >>> >>> says chown the dirfd, regardless of what it is, >>> except if AT_FDCWD, chown the CWD. >>> >>> WSL $ man 2 open >>> "O_PATH (since Linux 2.6.39) >>> Obtain a file descriptor that can be used for two purposes: to >>> indicate a location in the filesystem tree and to perform >>> operations that act purely at the file descriptor level. The >>> file itself is not opened, and other file operations (e.g., >>> read(2), write(2), fchmod(2), fchown(2), fgetxattr(2), >>> ioctl(2), mmap(2)) fail with the error EBADF." >>> >>> O_PATH does not open the file, so fchown returns EBADF, >>> as it requires an fd of an open file. >> >> I think you've just confirmed what I already said: If fchownat is called with >> AT_EMPTY_PATH, with an empty pathname, and with dirfd referring to a file >> that >> was opened with O_PATH, then fchownat will fail with EBADF. >> >> So for the purposes of this patch series, I don't see the point of adding >> support for AT_EMPTY_PATH in fchownat. >> >> Am I missing something? > > That is the user's problem: it is their responsibility to pass an fd open for > reading or searching, not one opened with O_PATH (on Linux or Cygwin), or > AT_FDCWD; it is Cygwin's responsibility to ensure that valid args succeed and > invalid args return the expected errno.
Yes, but Cygwin doesn't claim to support the AT_EMPTY_PATH flag except in linkat. So there is no expected errno. The only way there would be an expected errno is if we decide to add support for AT_EMPTY_PATH to fchownat. I'm saying that I don't see the point in doing that, and I'm asking whether I'm missing something. If you think I should add that support, please explain why. Ken