linux-kernel - Re: [PATCH 1/1] lib/vsprintf: Implement ssprintf() to catch truncated strings

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Message-ID: <202401301351.83A809993@keescook>
Date: Tue, 30 Jan 2024 13:55:39 -0800
From: Kees Cook <keescook@...omium.org>
To: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc: Lee Jones <lee@...nel.org>, David Laight <David.Laight@...lab.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Crutcher Dunnavant <crutcher+kernel@...astacks.com>,
	Juergen Quade <quade@...r.de>
Subject: Re: [PATCH 1/1] lib/vsprintf: Implement ssprintf() to catch
 truncated strings

On Tue, Jan 30, 2024 at 04:18:42PM +0100, Rasmus Villemoes wrote:
> So here scnprint() would have returned 1, leaving size at 1. scnprintf()
> has the invariant that, for non-zero size, the return value is strictly
> less than that size, so when passed a size of 1, all subsequent calls
> return 0 (corresponding to the fact that all it could do was to write
> the '\0' terminator).
> 
> This pattern already exists, and is really the reason scnprint exists.
> Yes, scnprintf() cannot distinguish overflow from
> it-just-exactly-fitted. Maybe it would have been better to make it work
> like this, but I don't think there's a real use - and we do have
> seq_buf() if one really wants an interface that can build a string
> piece-meal while keeping track of whether it ever caused overflow.

Yeah, I think we can take the handful of places that really need to know
about the overflow and can't reliably use scnprintf() and migrate them
to the seq_buf API. It should be much easier to use now[1] too.

That way we won't add a new string API, and we can continue to remove
snprintf.

-Kees

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/include/linux/seq_buf.h?id=dcc4e5728eeaeda84878ca0018758cff1abfca21
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/include/linux/seq_buf.h?id=7a8e9cdf9405819105ae7405cd91e482bf574b01

-- 
Kees Cook