[PATCH v15 6/9] namei: LOOKUP_{IN_ROOT,BENEATH}: permit limited ".." resolution

[Aleksa Sarai]

On 2019-11-13, Al Viro <viro at zeniv.linux.org.uk> wrote:
> On Tue, Nov 05, 2019 at 08:05:50PM +1100, Aleksa Sarai wrote:
> 
> > One other possible alternative (which previous versions of this patch
> > used) would be to check with path_is_under() if there was a racing
> > rename or mount (after re-taking the relevant seqlocks). While this does
> > work, it results in possible O(n*m) behaviour if there are many renames
> > or mounts occuring *anywhere on the system*.
> 
> BTW, do you realize that open-by-fhandle (or working nfsd, for that matter)
> will trigger arseloads of write_seqlock(&rename_lock) simply on d_splice_alias()
> bringing disconnected subtrees in contact with parent?

I wasn't aware of that -- that makes path_is_under() even less viable.
I'll reword it to be clearer that path_is_under() isn't a good idea and
why we went with -EAGAIN over an in-kernel retry.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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