Re: [PATCH bpf 1/2] uprobes/x86: Fix red zone clobbering in nop5 optimization

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, May 12, 2026 at 12:38:34PM -0700, Andrii Nakryiko wrote:
> On Tue, May 12, 2026 at 12:27 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Tue, May 12, 2026 at 10:07 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> > >
> > > +       /*
> > > +        * We have nop10 (with first byte overwritten to int3),
> > > +        * change it to:
> > > +        *   lea 0x80(%rsp), %rsp
> > > +        *   call tramp
> > > +        *
> > > +        * The first lea instruction skips the stack redzone so the call
> > > +        * instruction can safely push return address on stack.
> > > +        */
> >
> > typo: lea -128(%rsp), %rsp

ugh, thanks

> >
> > you can also do:
> >
> > add $-128, %rsp + call tramp = 4 + 5 = 9 bytes instead of 10.
> 
> When I asked AI about this it explained that add instruction modifies
> flags, so it's not a good fit here. lea doesn't touch flags.
> 
> >
> > Initially I didn't like this approach, since we just introduced
> > usdt nop5 and now need to recompile everything again,
> > but looking at the fix it's definitely simpler than alternatives
> > and doesn't have annoying limitations.
> 
> 
> yeah, limitations are annoying, especially with those global "DO NOT
> OPTIMIZE" flags... Jiri, let's polish your version and land it?

ok, will send it out

jirka