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Date: Wed, 7 Mar 2018 15:40:19 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: "Tobin C. Harding" <me@...in.cc>, Jonathan Corbet <corbet@....net>,
Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
"Steven Rostedt (VMware)" <rostedt@...dmis.org>,
kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org,
"Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH] vsprintf: Remove accidental VLA usage
On Wed, 7 Mar 2018 15:07:14 -0800 Kees Cook <keescook@...omium.org> wrote:
> The "sym" calculation is actually a fixed size, but since the max()
> macro uses some extensive tricks for safety, it ends up looking like a
> variable size. This replaces max() with a simple max macro which is
> sufficient for the calculation of the array size.
>
> Seen with -Wvla. Fixed as part of the directive to remove all VLAs from
> the kernel: https://lkml.org/lkml/2018/3/7/621
>
> ...
>
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -744,8 +744,9 @@ char *resource_string(char *buf, char *end, struct resource *res,
> #define FLAG_BUF_SIZE (2 * sizeof(res->flags))
> #define DECODED_BUF_SIZE sizeof("[mem - 64bit pref window disabled]")
> #define RAW_BUF_SIZE sizeof("[mem - flags 0x]")
> - char sym[max(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE,
> - 2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)];
> +#define SIMPLE_MAX(x, y) ((x) > (y) ? (x) : (y))
> + char sym[SIMPLE_MAX(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE,
> + 2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)];
>
> char *p = sym, *pend = sym + sizeof(sym);
> int decode = (fmt[0] == 'R') ? 1 : 0;
A year from now I'll be receiving an email titled
[patch] lib/vsprintf.c: use standard max() macro
won't I?
--- a/lib/vsprintf.c~vsprintf-remove-accidental-vla-usage-fix
+++ a/lib/vsprintf.c
@@ -754,6 +754,7 @@ char *resource_string(char *buf, char *e
#define FLAG_BUF_SIZE (2 * sizeof(res->flags))
#define DECODED_BUF_SIZE sizeof("[mem - 64bit pref window disabled]")
#define RAW_BUF_SIZE sizeof("[mem - flags 0x]")
+/* regular max() tricks gcc into creating a variable length array */
#define SIMPLE_MAX(x, y) ((x) > (y) ? (x) : (y))
char sym[SIMPLE_MAX(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE,
2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)];
_
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