Re: [PATCH v1 6/9] uretprobes: Return probe exit, invoke handlers

[Oleg Nesterov <oleg@redhat.com>]

On 04/13, Srikar Dronamraju wrote:
>
> > > Oh yes, this should be documented more explicitly in the changelog of
> > > this patch or 7/9 (which tries to document the limitations but should
> > > be more clear).
> > >
> > > Currently we do not support longjmp() and we assume that the probed
> > > function should do the regular return. We should certainly try to improve
> > > this, but I really think that this should go into the next series.
> > >
> > > Because this is nontrivial, needs more discussion, and I'm afraid should
> > > be per-arch. Even on x86 (which can check the stack) this is not simple,
> > > in general we can't know how to check that (to simplify) the first frame
> > > is already invalid. Just for example, we could check regs->sp and detect
> > > that longjmp() was called but sigaltstack() can easily fool this logic.
> > >
>
> Yes, its perfectly fine to keep this logic for the next patchset.

OK, great.

> Can you tell me why sigaltstack() can fool us if we rely on regs->sp.

Because we can't simply compare resg->sp and ret_instance->sp and decide
if we should ignore this ri or not, the task can hit retprobe, then take
a signal, switch to altstack and hit another rp. I'll write another email
(hopefully patches) later.

> Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

Thanks Srikar.

OK. Everything is acked, I'll send git-pull-request.

Oleg.