Re: [PATCH v4 1/4] x86/ibt: factor out cfi and fineibt offset

[Menglong Dong <menglong8.dong@gmail.com>]

On Thu, Mar 6, 2025 at 11:39 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Wed, Mar 5, 2025 at 6:59 PM Menglong Dong <menglong8.dong@gmail.com> wrote:
> >
> > I'm not sure if it works. However, indirect call is also used
> > in function graph, so we still have better performance. Isn't it?
> >
> > Let me have a look at the code of the function graph first :/
>
> Menglong,
>
> Function graph infra isn't going to help.
> "call foo" isn't a problem either.
>
> But we have to step back.
> per-function metadata is an optimization and feels like
> we're doing a premature optimization here without collecting
> performance numbers first.
>
> Let's implement multi-fentry with generic get_metadata_by_ip() first.
> get_metadata_by_ip() will be a hashtable in such a case and
> then we can compare its performance when it's implemented as
> a direct lookup from ip-4 (this patch) vs hash table
> (that does 'ip' to 'metadata' lookup).

Hi, Alexei

You are right, I should do such a performance comparison.

>
> If/when we decide to do this per-function metadata we can also
> punt to generic hashtable for cfi, IBT, FineIBT, etc configs.
> When mitigations are enabled the performance suffers anyway,
> so hashtable lookup vs direct ip-4 lookup won't make much difference.
> So we can enable per-function metadata only on non-mitigation configs
> when FUNCTION_ALIGNMENT=16.
> There will be some number of bytes available before every function
> and if we can tell gcc/llvm to leave at least 5 bytes there
> the growth of vmlinux .text will be within a noise.

Sounds great! It's so different to make the per-function metadata
work in all the cases. Especially, we can't implement it in arm64
if CFI_CLANG is enabled. And the fallbacking to the hash table makes
it much easier in these cases.

>
> So let's figure out the design of multi-fenty first with a hashtable
> for metadata and decide next steps afterwards.

Ok, I'll develop a version for fentry multi-link with both hashtable
and function metadata, and do some performance testing. Thank
you for your advice :/

Thanks!
Menglong Dong