Re: [PATCH] kexec_file: Adjust type of kexec_purgatory

[Kees Cook]

On Tue, May 9, 2017 at 4:13 PM, Daniel Micay <danielmicay@xxxxxxxxx> wrote:
> On Tue, 2017-05-09 at 16:06 -0700, Kees Cook wrote:
>> Defining kexec_purgatory as a zero-length char array upsets compile
>> time size checking. Since this is entirely runtime sized, switch
>> this to void *. This silences the warning generated by the future
>> CONFIG_FORTIFY_SOURCE, which did not like the memcmp() of a "0 byte"
>> array.
>>
>> Cc: Daniel Micay <danielmicay@xxxxxxxxx>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>  kernel/kexec_file.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
>> index b118735fea9d..bc86f85f1329 100644
>> --- a/kernel/kexec_file.c
>> +++ b/kernel/kexec_file.c
>> @@ -30,7 +30,7 @@
>>   * Declare these symbols weak so that if architecture provides a
>> purgatory,
>>   * these will be overridden.
>>   */
>> -char __weak kexec_purgatory[0];
>> +void * __weak kexec_purgatory;
>>  size_t __weak kexec_purgatory_size = 0;
>>
>>  static int kexec_calculate_store_digests(struct kimage *image);
>> --
>> 2.7.4
>
> It seems more correct to use char `char __weak kexec_purgatory[]`,
> otherwise isn't __builtin_object_size ending up as 8, which is still
> wrong?

I tried [], that was my instinct, too, but since this is a __weak and
not an extern, that doesn't work:

kernel/kexec_file.c:33:13: warning: array âkexec_purgatoryâ assumed to
have one element
 char __weak kexec_purgatory[];
             ^~~~~~~~~~~~~~~

-Kees

-- 
Kees Cook
Pixel Security