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

[Peter Zijlstra <peterz@infradead.org>]

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.

Also, we tried very hard to not have a literal encode ENDBR (I really
should teach objtool about this one :/). If it somehow makes sense to
keep this clause, please use: gen_endbr()