Re: [PATCH v2] perf,x86: avoid missing caller address in stack traces captured in uprobe

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Tue, Jul 02, 2024 at 05:06:14PM -0700, Andrii Nakryiko wrote:
> > > Should it also check for ENDBR64?
> > >
> 
> Sure, I can add a check for endbr64 as well. endbr64 probably can be
> used not just at function entry, is that right? So it might be another
> case of false positive (which I think is ok, see below).

Yeah, at least theoretically they could happen in the middle of a
function for implementing C switch jump tables.

> > > When compiled with -fcf-protection=branch, the first instruction of the
> > > function will almost always be ENDBR64.  I'm not sure about other
> > > distros, but at least Fedora compiles its binaries like that.
> >
> > BTW, there are some cases (including leaf functions and some stack
> > alignment sequences) where a "push %rbp" can happen inside a function.
> > Then it would presumably add a bogus trace entry.  Are such false
> > positives ok?
> 
> I think such cases should be rare. People mostly seem to trace user
> function entry/exit, rarely if ever they trace something within the
> function, except for USDT cases, where it will be a nop instruction
> that they trace.
> 
> In general, even with false positives, I think it's overwhelmingly
> better to get correct entry stack trace 99.9% of the time, and in the
> rest 0.01% cases it's fine having one extra bogus entry (but the rest
> should still be correct), which should be easy for humans to recognize
> and filter out, if necessary.

Agreed, this is a definite improvement overall.

BTW, soon there will be support for sframes instead of frame pointers,
at which point these checks should only be done for the frame pointer
case.

-- 
Josh