Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N

[Andy Lutomirski]

On Mon, Apr 21, 2014 at 10:00 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Mon, Apr 21, 2014 at 9:30 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>> On Mon, Apr 21, 2014 at 09:22:48AM -0700, Andy Lutomirski wrote:
>>
>>> +static int proc_may_follow(struct nameidata *nd, struct file *f)
>>> +{
>>> +     if (!nd)
>>> +             return 0;  /* This is readlink, */
>>> +
>>> +     if ((nd->flags & LOOKUP_WRITE) && !(f->f_mode & FMODE_WRITE))
>>> +             return -EACCES;
>>> +
>>> +     return 0;
>>> +}
>>
>> And this is just plain wrong.  WTF are you making the traversal of symlink
>> in the middle of pathname dependent on the open flags?
>
> Can you give me a hint?  There are three cases that I need to
> distinguish, I think:
>
> 1. readlink.  Currently handled by nd == NULL.  It's ugly, and I'll clean it up.
>
> 2. Traversal in the middle of a path.  This can be either literally in
> the middle (e.g. "/proc/self/fd/3/something_else") or in a symlink
> that's the last component of the literal path (e.g. "fd3null" where
> "fd3null" is a symlink to "/proc/self/fd/3/null" and "null" is either
> a file or a symlink to /dev/null).  I have the latter type wrong in
> this patch.
>
> 3. Actually opening /proc/self/fd/N.  This can be direct or by opening
> a symlink to /proc/self/fd/N.  I think I have this case correct.
>
> What's the best way to fix this?  Should I be checking nd->depth?

No, I think that's wrong, too.  I think that will cause me to screw up
symlinks to /proc/self/fd/3.  What's the right way to tell that
follow_link is happening on the very last pathname component?

Hmm.  I wonder what happens, or even what should happen, if the file
descriptor is a symlink opened with O_PATH | O_NOFOLLOW.

--Andy
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