linux-kernel - Re: [PATCH v16 0/4] perf: Support the deferred unwinding infrastructure

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Message-ID: <20251105130833.GN3245006@noisy.programming.kicks-ass.net>
Date: Wed, 5 Nov 2025 14:08:33 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Florian Weimer <fweimer@...hat.com>
Cc: Jens Remus <jremus@...ux.ibm.com>, Steven Rostedt <rostedt@...nel.org>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	bpf@...r.kernel.org, x86@...nel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrii Nakryiko <andrii@...nel.org>,
	Indu Bhagat <indu.bhagat@...cle.com>,
	"Jose E. Marchesi" <jemarch@....org>,
	Beau Belgrave <beaub@...ux.microsoft.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sam James <sam@...too.org>, Kees Cook <kees@...nel.org>,
	Carlos O'Donell <codonell@...hat.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v16 0/4] perf: Support the deferred unwinding
 infrastructure

On Tue, Nov 04, 2025 at 12:22:01PM +0100, Florian Weimer wrote:
> * Peter Zijlstra:
> 
> > +/*
> > + * Heuristic-based check if uprobe is installed at the function entry.
> > + *
> > + * Under assumption of user code being compiled with frame pointers,
> > + * `push %rbp/%ebp` is a good indicator that we indeed are.
> > + *
> > + * Similarly, `endbr64` (assuming 64-bit mode) is also a common pattern.
> > + * If we get this wrong, captured stack trace might have one extra bogus
> > + * entry, but the rest of stack trace will still be meaningful.
> > + */
> > +bool is_uprobe_at_func_entry(struct pt_regs *regs)
> 
> Is this specifically for uprobes?  Wouldn't it make sense to tell the
> kernel when the uprobe is installed whether the frame pointer has been
> set up at this point?  Userspace can typically figure this out easily
> enough (it's not much more difficult to find the address of the
> function).

Yeah, I suppose so. Not sure the actual user interface for this allows
for that. Someone would have to dig into that a bit.