linux-kernel - Re: [RFC v1 0/3] Add and use seprintf() instead of less ergonomic APIs

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Message-ID: <87a55fw5aq.fsf@prevas.dk>
Date: Tue, 08 Jul 2025 08:43:57 +0200
From: Rasmus Villemoes <linux@...musvillemoes.dk>
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>
Subject: Re: [RFC v1 0/3] Add and use seprintf() instead of less ergonomic APIs

On Sat, Jul 05 2025, Alejandro Colomar <alx@...nel.org> wrote:

> On top of that, I have a question about the functions I'm adding,
> and the existing kernel snprintf(3): The standard snprintf(3)
> can fail (return -1), but the kernel one doesn't seem to return <0 ever.
> Should I assume that snprintf(3) doesn't fail here?

Yes. Just because the standard says it may return an error, as a QoI
thing the kernel's implementation never fails. That also means that we
do not ever do memory allocation or similar in the guts of vsnsprintf
(that would anyway be a mine field of locking bugs).

If we hit some invalid or unsupported format specifier (i.e. a bug in
the caller), we return early, but still report what we wrote until
hitting that.

Rasmus