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Re: [PATCH v2 0/3] Support opening a symlink with O_PATH | O_NOFOLLOW

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.


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