linux-kernel - Re: [PATCH v4] perf,x86: avoid missing caller address in stack traces captured in uprobe

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Message-ID: <CAEf4BzZyskNTxq3jUgJ=TzCvZuCVTv5qbUwZrCtF5_+EGo7-mg@mail.gmail.com>
Date: Wed, 10 Jul 2024 12:24:53 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Andrii Nakryiko <andrii@...nel.org>, 
	linux-trace-kernel@...r.kernel.org, rostedt@...dmis.org, mhiramat@...nel.org, 
	x86@...nel.org, mingo@...hat.com, tglx@...utronix.de, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, rihams@...com, 
	linux-perf-users@...r.kernel.org, rick.p.edgecombe@...el.com
Subject: Re: [PATCH v4] perf,x86: avoid missing caller address in stack traces
 captured in uprobe

On Wed, Jul 10, 2024 at 9:24 AM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Wed, Jul 10, 2024 at 08:11:57AM -0700, Andrii Nakryiko wrote:
> > On Wed, Jul 10, 2024 at 4:39 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Tue, Jul 09, 2024 at 10:50:00AM -0700, Andrii Nakryiko wrote:
> > > > You can see it replaced the first byte, the following 3 bytes are
> > > > remnants of endb64 (gdb says it's a nop? :)), and then we proceeded,
> > > > you can see I stepped through a few more instructions.
> > > >
> > > > Works by accident?
> > >
> > > Yeah, we don't actually have Userspace IBT enabled yet, even on hardware
> > > that supports it.
> >
> > OK, I don't know what the implications are, but it's a good accident :)
> >
> > Anyways, what should I do for v4? Drop is_endbr6() check or keep it?
>
> Given the current behavior of uprobe overwriting ENDBR64 with INT3, the
> is_endbr6() check still makes sense, otherwise is_uprobe_at_func_entry()
> would never return true on OSes which have the ENDBR64 compiled in.
>
> However, once userspace IBT actually gets enabled, uprobe should skip
> the ENDBR64 and patch the subsequent instruction.  Then the is_endbr6()
> check would no longer be needed.
>

Ok, I'll keep it then, thanks.

> --
> Josh