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

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Message-ID: <CAHk-=wjOnFbRJ9V81VEeSK+=HuD8ODSq+S3STTh1JwYQLWXXXg@mail.gmail.com>
Date: Tue, 8 Jul 2025 08:52:17 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.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, 8 Jul 2025 at 07:34, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> I had found those debug printks really useful for debugging
> corrupt/missing .sframe data, but yeah, this patch is ridiculously bad.
> Sorry for putting that out into the world ;-)

I suspect that code that is still needs that level of debugging should
just not use the 'unsafe' user access helpers.

They really are meant for "this sequence turns into three CPU
instructions" kind of uses, and the "unsafe" part of the naming was
very much intended to be a "please don't use this unless you are being
very careful and limited" marker.

Now, I do think that the "goto label for exceptions" part of the
unsafe accessors can be very convenient, so maybe we should make
_that_ part of the interface more widely available. IOW, without the
whole user_read_access_begin/user_read_access_end dance?

That model is already used by "__{get,put}_kernel_nofault()", but I
think it's limited to just some unusual code in mm/maccess.c.

             Linus