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

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Message-ID: <20240709152448.GQ27299@noisy.programming.kicks-ass.net>
Date: Tue, 9 Jul 2024 17:24:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-trace-kernel@...r.kernel.org,
	rostedt@...dmis.org, x86@...nel.org, mingo@...hat.com,
	tglx@...utronix.de, jpoimboe@...hat.com,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, rihams@...com,
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v4] perf,x86: avoid missing caller address in stack
 traces captured in uprobe

On Tue, Jul 09, 2024 at 11:10:17PM +0900, Masami Hiramatsu wrote:
> On Tue, 9 Jul 2024 12:11:33 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Mon, Jul 08, 2024 at 04:11:27PM -0700, Andrii Nakryiko wrote:
> > > +#ifdef CONFIG_UPROBES
> > > +/*
> > > + * 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.
> > > + */
> > > +static bool is_uprobe_at_func_entry(struct pt_regs *regs)
> > > +{
> > > +	struct arch_uprobe *auprobe;
> > > +
> > > +	if (!current->utask)
> > > +		return false;
> > > +
> > > +	auprobe = current->utask->auprobe;
> > > +	if (!auprobe)
> > > +		return false;
> > > +
> > > +	/* push %rbp/%ebp */
> > > +	if (auprobe->insn[0] == 0x55)
> > > +		return true;
> > > +
> > > +	/* endbr64 (64-bit only) */
> > > +	if (user_64bit_mode(regs) && *(u32 *)auprobe->insn == 0xfa1e0ff3)
> > > +		return true;
> > 
> > I meant to reply to Josh suggesting this, but... how can this be? If you
> > scribble the ENDBR with an INT3 things will #CP and we'll never get to
> > the #BP.
> 
> Hmm, kprobes checks the instruction and reject if it is ENDBR.
> Shouldn't uprobe also skip the ENDBR too?

Should, yes, but I can't find in a hurry if we actually do.