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

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Message-ID: <yxa4mb4tq4uamjc5atvhfefvxyu6fl6e6peuozd5j5cemaqd2t@pfwybj4oyscs>
Date: Thu, 10 Jul 2025 23:21:28 +0200
From: Alejandro Colomar <alx@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.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()

Hi Linus,

On Thu, Jul 10, 2025 at 08:30:59PM +0200, Alejandro Colomar wrote:
> > IOW, why isn't this just
> > 
> >   #define sprintf_array(a,...) snprintf(a, ARRAY_SIZE(a), __VA_ARGS__)
> 
> Agree.  This is a better name for the kernel.

Oops, I misread.  I thought you were implementing it as

	#define sprintf_array(a, ...)  sprintf_end(a, ENDOF(a), __VA_ARGS__)

So, I prefer my implementation because it returns NULL on truncation.
Compare usage:

	if (linus_sprintf_array(a, "foo") >= ARRAY_SIZE(a))
		goto fail;

	if (alex_sprintf_array(a, "foo") == NULL)
		goto fail;

Another approach would be to have

	if (third_sprintf_array(a, "foo") < 0)  // -E2BIG
		goto fail;

Which was my first approach, but since we have sprintf_end(), let's just
reuse it.


Cheers,
Alex

-- 
<https://www.alejandro-colomar.es/>

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