linux-kernel - Re: [PATCH] kexec_file: Adjust type of kexec

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Date:   Tue, 9 May 2017 16:22:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Daniel Micay <danielmicay@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kexec Mailing List <kexec@...ts.infradead.org>
Subject: Re: [PATCH] kexec_file: Adjust type of kexec_purgatory

On Tue, May 9, 2017 at 4:13 PM, Daniel Micay <danielmicay@...il.com> 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@...il.com>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  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 ‘kexec_purgatory’ assumed to
have one element
 char __weak kexec_purgatory[];
             ^~~~~~~~~~~~~~~

-Kees

-- 
Kees Cook
Pixel Security