linux-kernel - Re: [PATCH v8 10/12] unwind_user/sframe: Enable debugging in uaccess regions

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Message-ID: <3mf3ytjmoanrfjcyz7qjljwrbxolqrkocqy4asmunqeiqxpqbj@mqkeuyan67q5>
Date: Tue, 8 Jul 2025 11:57:04 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, 
	Steven Rostedt <rostedt@...nel.org>, linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, x86@...nel.org, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>, 
	Indu Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>, 
	Beau Belgrave <beaub@...ux.microsoft.com>, Jens Remus <jremus@...ux.ibm.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>, 
	Florian Weimer <fweimer@...hat.com>, Sam James <sam@...too.org>
Subject: Re: [PATCH v8 10/12] unwind_user/sframe: Enable debugging in uaccess
 regions

On Tue, Jul 08, 2025 at 12:31:20PM -0400, Steven Rostedt wrote:
> On Tue, 8 Jul 2025 08:53:56 -0700
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > On Tue, 8 Jul 2025 at 07:41, Steven Rostedt <rostedt@...dmis.org> wrote:
> > >
> > > Would something like this work? If someone enables the config to enable the
> > > validation, I don't think we need dynamic printk to do it (as that requires
> > > passing in the format directly and not via a pointer).  
> > 
> > I really think you should just not use 'user_access_begin()" AT ALL if
> > you need to play these kinds of games.
> > 
> 
> Looking at the code a bit deeper, I don't think we need to play these games
> and still keep the user_read_access_begin().
> 
> The places that are more performance critical (where it reads the sframe
> during normal stack walking during profiling) has no debug output, and
> there's nothing there that needs to take it out of the user_read_access
> area.
> 
> It's the validator that adds these hacks. I don't think it needs to. It can
> just wrap the calls to the code that requires user_read_access and then
> check the return value. The validator is just a debugging feature and
> performance should not be an issue.
> 
> But I do think performance is something to care about during normal
> operations where the one big user_read_access_begin() can help.
> 
> What about something like this? It adds "safe" versions of the user space
> access functions and uses them only in the slow (we don't care about
> performance) validator:

Looks good to me, thanks!

-- 
Josh