linux-kernel - Re: [PATCH] lib: string_helpers: fix potential snprintf() output truncation

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Message-ID: <fae2751f6881bfc402a074e909be6a67ef9467df.camel@HansenPartnership.com>
Date: Tue, 22 Oct 2024 10:30:18 -0400
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Rasmus Villemoes <ravi@...vas.dk>, Andy Shevchenko
	 <andy.shevchenko@...il.com>
Cc: Jiri Slaby <jirislaby@...nel.org>, Bartosz Golaszewski <brgl@...ev.pl>, 
 Kees Cook <kees@...nel.org>, Andy Shevchenko <andy@...nel.org>, Andrew
 Morton <akpm@...ux-foundation.org>, Greg KH <gregkh@...uxfoundation.org>, 
 linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org, Bartosz
 Golaszewski <bartosz.golaszewski@...aro.org>, stable@...r.kernel.org
Subject: Re: [PATCH] lib: string_helpers: fix potential snprintf() output
 truncation

On Tue, 2024-10-22 at 15:46 +0200, Rasmus Villemoes wrote:
[...]
> -       snprintf(buf, len, "%u%s %s", (u32)size,
> -                tmp, unit);
> +       if (j)
> +               snprintf(buf, len, "%u.%03u %s", (u32)size,
> remainder, unit);
> +       else
> +               snprintf(buf, len, "%u %s", (u32)size, unit);
>  }
>  EXPORT_SYMBOL(string_get_size);

Where are you getting this from?  The current statement you'd be
replacing is

	return snprintf(buf, len, "%u%s%s%s%s", (u32)size, tmp,
			(units & STRING_UNITS_NO_SPACE) ? "" : " ",
			unit,
			(units & STRING_UNITS_NO_BYTES) ? "" : "B");

The use of a tmp buffer prevents that having to be repeated.

If we had a way of specifying "ignore argument" I suppose we could
switch the format string on J and then only have a single statement.

Regards,

James