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

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Date: Fri, 5 Apr 2024 13:02:30 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
	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 04/05, Jiri Olsa wrote:
>
> On Fri, Apr 05, 2024 at 10:22:03AM +0900, Masami Hiramatsu wrote:
> >
> > I think this expects setjmp/longjmp as below
> >
> > foo() { <- retprobe1
> > 	setjmp()
> > 	bar() { <- retprobe2
> > 		longjmp()
> > 	}
> > } <- return to trampoline
> >
> > In this case, we need to skip retprobe2's instance.

Yes,

> > My concern is, if we can not find appropriate return instance, what happen?
> > e.g.
> >
> > foo() { <-- retprobe1
> >    bar() { # sp is decremented
> >        sys_uretprobe() <-- ??
> >     }
> > }
> >
> > It seems sys_uretprobe() will handle retprobe1 at that point instead of
> > SIGILL.
>
> yes, and I think it's fine, you get the consumer called in wrong place,
> but it's your fault and kernel won't crash

Agreed.

With or without this patch userpace can also do

	foo() { <-- retprobe1
		bar() {
			jump to xol_area
		}
	}

handle_trampoline() will handle retprobe1.

> this can be fixed by checking the syscall is called from the trampoline
> and prevent handle_trampoline call if it's not

Yes, but I still do not think this makes a lot of sense. But I won't argue.

And what should sys_uretprobe() do if it is not called from the trampoline?
I'd prefer force_sig(SIGILL) to punish the abuser ;) OK, OK, EINVAL.

I agree very much with Andrii,

       sigreturn()  exists only to allow the implementation of signal handlers.  It should never be
       called directly.  Details of the arguments (if any) passed to sigreturn() vary depending  on
       the architecture.

this is how sys_uretprobe() should be treated/documented.

sigreturn() can be "improved" too. Say, it could validate sigcontext->ip
and return -EINVAL if this addr is not valid. But why?

Oleg.