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SubjectRe: [PATCH] vsprintf: Remove accidental VLA usage
On Wed, Mar 7, 2018 at 3:40 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> 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?

Hopefully a year from now we'll have added -Wvla to the global build
flags and no one will be able to try that. :)

-Kees

--
Kees Cook
Pixel Security

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