lkml.org 
[lkml]   [2018]   [Mar]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH] vsprintf: Remove accidental VLA usage
On Wed, 7 Mar 2018 15:07:14 -0800 Kees Cook <keescook@chromium.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)];
_
\
 
 \ /
  Last update: 2018-03-08 00:41    [W:0.063 / U:44.148 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site