linux-kernel - Re: [PATCH v5] perf,x86: avoid missing caller address in stack traces captured in uprobe

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Message-ID: <CAEf4BzaTEUkRU37fsuGy+-otWk9K0-c9=hs0APRz7pJy7rq-5w@mail.gmail.com>
Date: Thu, 18 Jul 2024 08:29:23 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: linux-trace-kernel@...r.kernel.org, peterz@...radead.org, 
	rostedt@...dmis.org, mhiramat@...nel.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 v5] perf,x86: avoid missing caller address in stack traces
 captured in uprobe

On Wed, Jul 10, 2024 at 12:36 PM Andrii Nakryiko <andrii@...nel.org> wrote:
>
> When tracing user functions with uprobe functionality, it's common to
> install the probe (e.g., a BPF program) at the first instruction of the
> function. This is often going to be `push %rbp` instruction in function
> preamble, which means that within that function frame pointer hasn't
> been established yet. This leads to consistently missing an actual
> caller of the traced function, because perf_callchain_user() only
> records current IP (capturing traced function) and then following frame
> pointer chain (which would be caller's frame, containing the address of
> caller's caller).
>
> So when we have target_1 -> target_2 -> target_3 call chain and we are
> tracing an entry to target_3, captured stack trace will report
> target_1 -> target_3 call chain, which is wrong and confusing.
>
> This patch proposes a x86-64-specific heuristic to detect `push %rbp`
> (`push %ebp` on 32-bit architecture) instruction being traced. Given
> entire kernel implementation of user space stack trace capturing works
> under assumption that user space code was compiled with frame pointer
> register (%rbp/%ebp) preservation, it seems pretty reasonable to use
> this instruction as a strong indicator that this is the entry to the
> function. In that case, return address is still pointed to by %rsp/%esp,
> so we fetch it and add to stack trace before proceeding to unwind the
> rest using frame pointer-based logic.
>
> We also check for `endbr64` (for 64-bit modes) as another common pattern
> for function entry, as suggested by Josh Poimboeuf. Even if we get this
> wrong sometimes for uprobes attached not at the function entry, it's OK
> because stack trace will still be overall meaningful, just with one
> extra bogus entry. If we don't detect this, we end up with guaranteed to
> be missing caller function entry in the stack trace, which is worse
> overall.
>
> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> ---
>  arch/x86/events/core.c  | 63 +++++++++++++++++++++++++++++++++++++++++
>  include/linux/uprobes.h |  2 ++
>  kernel/events/uprobes.c |  2 ++
>  3 files changed, 67 insertions(+)
>

Ping. What's the status of this patch? Is it just waiting until after
the merge window, or it got lost?

> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 5b0dd07b1ef1..780b8dc36f05 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -41,6 +41,8 @@
>  #include <asm/desc.h>
>  #include <asm/ldt.h>
>  #include <asm/unwind.h>
> +#include <asm/uprobes.h>
> +#include <asm/ibt.h>
>
>  #include "perf_event.h"
>
> @@ -2813,6 +2815,46 @@ static unsigned long get_segment_base(unsigned int segment)

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