linux-hardening - Re: [RFC v4 6/7] sprintf: Add [V]SPRINTF

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Message-ID: <CAHk-=wiYistgF+BBeHY_Q58-7-MZLHsvtKybrwtiF97w+aU-UQ@mail.gmail.com>
Date: Thu, 10 Jul 2025 08:52:13 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Alejandro Colomar <alx@...nel.org>
Cc: linux-mm@...ck.org, linux-hardening@...r.kernel.org, 
	Kees Cook <kees@...nel.org>, Christopher Bazley <chris.bazley.wg14@...il.com>, 
	shadow <~hallyn/shadow@...ts.sr.ht>, linux-kernel@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, kasan-dev@...glegroups.com, 
	Dmitry Vyukov <dvyukov@...gle.com>, Alexander Potapenko <glider@...gle.com>, Marco Elver <elver@...gle.com>, 
	Christoph Lameter <cl@...ux.com>, David Rientjes <rientjes@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>, 
	Andrew Clayton <andrew@...ital-domain.net>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Michal Hocko <mhocko@...e.com>, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC v4 6/7] sprintf: Add [V]SPRINTF_END()

On Wed, 9 Jul 2025 at 19:49, Alejandro Colomar <alx@...nel.org> wrote:
>
> +#define SPRINTF_END(a, fmt, ...)  sprintf_end(a, ENDOF(a), fmt, ##__VA_ARGS__)
> +#define VSPRINTF_END(a, fmt, ap)  vsprintf_end(a, ENDOF(a), fmt, ap)

So I like vsprintf_end() more as a name ("like more" not being "I love
it", but at least it makes me think it's a bit more self-explanatory).

But I don't love screaming macros. They historically scream because
they are unsafe, but they shouldn't be unsafe in the first place.

And I don't think those [V]SPRINTF_END() and ENDOF() macros are unsafe
- they use our ARRAY_SIZE() macro which does not evaluate the
argument, only the type, and is safe to use.

So honestly, this interface looks easy to use, but the screaming must stop.

And none of this has *anything* to do with "end" in this form anyway.

IOW, why isn't this just

  #define sprintf_array(a,...) snprintf(a, ARRAY_SIZE(a), __VA_ARGS__)

which is simpler and more direct, doesn't use the "end" version that
is pointless (it's _literally_ about the size of the array, so
'snprintf' is the right thing to use), doesn't scream, and has a
rather self-explanatory name.

Naming matters.

                Linus