Re: [PATCH v3 3/3] namei: aggressively check for nd->root escape on ".." resolution

[Al Viro]

On Sat, Oct 13, 2018 at 07:53:26PM +1100, Aleksa Sarai wrote:

> I didn't know about path_is_under() -- I just checked and it appears to
> not take &rename_lock? From my understanding, in order to protect
> against the rename attack you need to take &rename_lock (or check
> against &rename_lock at least and retry if it changed).
> 
> I could definitely use path_is_under() if you prefer, though I think
> that in this case we'd need to take &rename_lock (right?). Also is there
> a speed issue with taking the write-side of a seqlock when we are just
> reading -- is this more efficient than doing a retry like in __d_path?

???

1) it uses is_subdir(), which does deal with rename_lock
2) what it does is taking mount_lock.lock.  I.e. the same
thing as the second retry in __d_path().  _If_ it shows
up in profiles, we can switch it to read_seqbegin_or_lock(),
but I'd like to see the profiling data first.