linux-kernel - Re: [PATCH v6 5/6] tracing: Show inode and device major:minor in deferred user space stacktrace

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Message-ID: <CAHk-=wi3muqW7XAEawu+xvvqADMmoqyvmDPYUC_64aCnqz-WLg@mail.gmail.com>
Date: Fri, 29 Aug 2025 10:13:33 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Steven Rostedt <rostedt@...nel.org>, Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>, 
	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>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>, 
	Arnaldo Carvalho de Melo <acme@...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>, Florian Weimer <fweimer@...hat.com>, 
	Sam James <sam@...too.org>, Kees Cook <kees@...nel.org>, "Carlos O'Donell" <codonell@...hat.com>
Subject: Re: [PATCH v6 5/6] tracing: Show inode and device major:minor in
 deferred user space stacktrace

On Fri, 29 Aug 2025 at 10:02, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> Note, the ring buffer can be mapped to user space. So anything written into
> the buffer is already exposed.

Oh, good point. Yeah, that means that you have to do the hashing
immediately. Too bad. Because while 'vma->vm_file' is basically free
(since you have to access the vma for other reasons anyway), a good
hash isn't.

siphash is good and fast for being what it is, but it's not completely
free. It's something like 50 shift/xor pairs, and it obviously needs
to also access that secret hash value that is likely behind a cache
miss..

Still, I suspect it's the best we've got.

(If hashing is noticeable, it *might* be worth it to use
'siphash_1u32()' and only hash 32 bits of the pointers. That makes the
hashing slightly cheaper, and since the low bits of the pointer will
be zero anyway due to alignment, and the high bits don't have a lot of
information in them either, it doesn't actually remove much
information. You might get collissions if the two pointers are exactly
32 GB apart or whatever, but that sounds really really unlucky)

                Linus