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

[Brian Inglis <Brian dot Inglis at SystematicSw dot ab dot ca>]

On 2019-12-30 14:47, Ken Brown wrote:
> 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.

To allow perms changed on the cwd, directories or files with an open fd, to
avoid race conditions, like the other ...at functions.
I don't get why you don't see those as useful cases.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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