Re: [PATCH 0/2] uprobes,mm: speculative lockless VMA-to-uprobe lookup

[Andrii Nakryiko]

On Tue, Sep 10, 2024 at 9:06 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Fri, Sep 6, 2024 at 7:12 AM Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
> > Implement speculative (lockless) resolution of VMA to inode to uprobe,
> > bypassing the need to take mmap_lock for reads, if possible. Patch #1 by Suren
> > adds mm_struct helpers that help detect whether mm_struct were changed, which
> > is used by uprobe logic to validate that speculative results can be trusted
> > after all the lookup logic results in a valid uprobe instance.
>
> Random thought: It would be nice if you could skip the MM stuff
> entirely and instead go through the GUP-fast path, but I guess going
> from a uprobe-created anon page to the corresponding uprobe is hard...
> but maybe if you used the anon_vma pointer as a lookup key to find the
> uprobe, it could work? Though then you'd need hooks in the anon_vma
> code... maybe not such a great idea.

So I'm not crystal clear on all the details here, so maybe you can
elaborate a bit. But keep in mind that a) there could be multiple
uprobes within a single user page, so lookup has to take at least
offset within the page into account somehow. But also b) single uprobe
can be installed in many independent anon VMAs across many processes.
So anon vma itself can't be part of the key.

Though maybe we could have left some sort of "cookie" stashed
somewhere to help with lookup. But then again, multiple uprobes per
page.

It does feel like lockless VMA to inode resolution would be a cleaner
solution, let's see if we can get there somehow.