Re: Behaviour of /proc/self/fd/NN

[Nikolaus Rath <Nikolaus@rath.org>]

[Trimming Cc since this gets fuse specific]

On Nov 14 2018, Miklos Szeredi <miklos@szeredi.hu> wrote:
> On Wed, Nov 14, 2018 at 1:10 PM, Nikolaus Rath <Nikolaus@rath.org> wrote:
>> On Nov 14 2018, Miklos Szeredi <miklos@szeredi.hu> wrote:
>>> On Wed, Nov 14, 2018 at 11:45 AM, Nikolaus Rath <Nikolaus@rath.org> wrote:
>>>> If I understand Documentation/filesystems/path-lookup.md correctly, then
>>>> the "symlinks" in /proc/self/fd/NN are not true symlinks but just
>>>> presented as such to userspace. Does this mean that I can access (e.g.,
>>>> open(), setxattr(), or chmod()) these files without the possibility of
>>>> race conditions? I.e., there is no way for someone to rename the target
>>>> after the kernel has "looked up" the target but before the operation is
>>>> applied?
>>>
>>> Exactly.   The only limitation is that if the target is a symlink (fd
>>> opened with O_PATH) then the neither the symlink following, nor the
>>> non-following variants will do the right thing (see *xattr ops in
>>> passthrough_ll for example).
>>
>> That said, I also noticed the following code to handle utimens for symlinks:
>>
>> res = utimensat(inode->fd, "", tv,
>>                 AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW);
>> if (res == -1 && errno == EINVAL) {
>>         /* Sorry, no race free way to set times on symlink. */
>>         errno = EPERM;
>> }
>>
>> So it looks as it at least utimensat() does the right thing under at
>> least some conditions...?
>
> Not yet upstream.  Here's the patch:
>
>   https://marc.info/?l=linux-kernel&m=154158217810354&w=2

Thanks!

Is there any reason that we don't try to open *files* as O_RDWR or such
and only fall back to O_PATH if that doesn't work? That should enable us
to use fsetxattr et all in many cases, right? The kernel already does
permission checking (so that's not a concern), and we should never need
a dirfd for a file...


Best,
-Nikolaus

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