Re: [Qemu-devel] Re: [PATCH 07/17] block/vvfat.c: fix warnings with _FORTIFY_SOURCE

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Jan 20, 2010 at 2:15 PM, Markus Armbruster <armbru@redhat.com> wrote:
> Kevin Wolf <kwolf@redhat.com> writes:
>
>> Am 20.01.2010 12:09, schrieb Kirill A. Shutemov:
>>> On Wed, Jan 20, 2010 at 12:33 PM, Daniel P. Berrange
>>> <berrange@redhat.com> wrote:
>>>> On Wed, Jan 20, 2010 at 08:19:26AM +0200, Kirill A. Shutemov wrote:
>>>>> On Wed, Jan 20, 2010 at 1:56 AM, Juan Quintela <quintela@redhat.com> wrote:
> [...]
>>>>>> diff --git a/block/vvfat.c b/block/vvfat.c
>>>>>> index 063f731..df957e5 100644
>>>>>> --- a/block/vvfat.c
>>>>>> +++ b/block/vvfat.c
>>>>>> @@ -868,7 +868,8 @@ static int init_directories(BDRVVVFATState* s,
>>>>>>     {
>>>>>>        direntry_t* entry=array_get_next(&(s->directory));
>>>>>>        entry->attributes=0x28; /* archive | volume label */
>>>>>> -       snprintf((char*)entry->name,11,"QEMU VVFAT");
>>>>>> +       memcpy(entry->name,"QEMU VVF",8);
>>>>>> +       memcpy(entry->extension,"AT ",3);
>>>>>>     }
>>>>>
>>>>> Better to use
>>>>>
>>>>> memcpy(entry->name, "QEMU VVFAT", 11);
>>>>>
>>>>> memcpy() doesn't check bounds.
>
> No, this is evil, and may well be flagged by static analysis tools.

If so, the tool is stupid.

>>>> It doesn't *currently* check bounds.
>>>
>>> No. memcpy() will never check bounds. It's totaly different from strcpy,
>>> http://gcc.gnu.org/ml/gcc-patches/2009-06/msg00419.html
>>
>> Regardless if deliberately overflowing the buffer works or doesn't
>> making it explicit is better. Someone might reorder the struct or add
>> new fields in between (okay, unlikely in this case, but still) and
>> you'll overflow into fields you never wanted to touch.
>
> Moreover, compilers are free to put padding between members name and
> extension.

No, compiler can't add anything between. 'char' is always byte-aligned.