linux-kernel - Re: [PATCH] lib/vsprintf.c: fix the incorrect return value of vsnprintf()

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Date:	18 Jul 2013 02:42:38 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	andrei.emeltchenko@...el.com, andriy.shevchenko@...ux.intel.com,
	gang.chen@...anux.com, jkosina@...e.cz, linux@...izon.com
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf.c: fix the incorrect return value of vsnprintf()

> When "str >= end", necessary to reset 'str' to "end - 1", or the return
> value will be larger than the real one, the callers which depend on the
> return value, may cause memory overflow.

NAK.  This is the documented (by both the function itself and the
ANSI/ISO C standard) and desired return value: the number of bytes that
*would* have been in the output string if the buffer were large enough.
In particular, it is common to do:

size = vsnprintf(NULL, 0, fmt, args) + 1;
p = malloc(size, GFP_KERNEL);
vsnprintf(p, size, fmt, args);

You want vscnprintf.  If you have a caller that needs the *actual* number of
bytes written, use that.
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