linux-kernel - Re: [PATCHv2 1/3] uprobe: Add uretprobe syscall to speed up return probe

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Date: Sat, 6 Apr 2024 19:55:59 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Jiri Olsa <olsajiri@...il.com>,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
	Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Borislav Petkov (AMD)" <bp@...en8.de>, x86@...nel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCHv2 1/3] uprobe: Add uretprobe syscall to speed up return
 probe

On 04/06, Masami Hiramatsu wrote:
>
> On Fri, 5 Apr 2024 13:02:30 +0200
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > With or without this patch userpace can also do
> >
> > 	foo() { <-- retprobe1
> > 		bar() {
> > 			jump to xol_area
> > 		}
> > 	}
> >
> > handle_trampoline() will handle retprobe1.
>
> This is OK because the execution path has been changed to trampoline,

Agreed, in this case the misuse is more clear. But please see below.

> but the above will continue running bar() after sys_uretprobe().

.. and most probably crash

> > sigreturn() can be "improved" too. Say, it could validate sigcontext->ip
> > and return -EINVAL if this addr is not valid. But why?
>
> Because sigreturn() never returns, but sys_uretprobe() will return.

You mean, sys_uretprobe() returns to the next insn after syscall.

Almost certainly yes, but this is not necessarily true. If one of consumers
changes regs->sp sys_uretprobe() "returns" to another location, just like
sys_rt_sigreturn().

That said.

Masami, it is not that I am trying to prove that you are "wrong" ;) No.

I see your points even if I am biased, I understand that my objections are
not 100% "fair".

I am just trying to explain why, rightly or not, I care much less about the
abuse of sys_uretprobe().

Thanks!

Oleg.