Re: [PATCH] MdePkg/BaseMemoryLibOptDxe: check for zero length in ZeroMem ()

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 3 November 2016 at 17:38, Laszlo Ersek <lersek@redhat.com> wrote:
> On 11/03/16 18:31, Ard Biesheuvel wrote:
>> Unlike other string functions in this library, ZeroMem () does not
>> return early when the length of the input buffer is 0. So add the
>> same to ZeroMem () as well.
>>
>> Contributed-under: TianoCore Contribution Agreement 1.0
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>>  MdePkg/Library/BaseMemoryLibOptDxe/ZeroMemWrapper.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/MdePkg/Library/BaseMemoryLibOptDxe/ZeroMemWrapper.c b/MdePkg/Library/BaseMemoryLibOptDxe/ZeroMemWrapper.c
>> index 2a0a038fd6c5..fbc2f5742c8c 100644
>> --- a/MdePkg/Library/BaseMemoryLibOptDxe/ZeroMemWrapper.c
>> +++ b/MdePkg/Library/BaseMemoryLibOptDxe/ZeroMemWrapper.c
>> @@ -46,6 +46,10 @@ ZeroMem (
>>    IN UINTN  Length
>>    )
>>  {
>> +  if (Length == 0) {
>> +    return Buffer;
>> +  }
>> +
>>    ASSERT (!(Buffer == NULL && Length > 0));
>>    ASSERT (Length <= (MAX_ADDRESS - (UINTN)Buffer + 1));
>>    return InternalMemZeroMem (Buffer, Length);
>>
>
> 1. Why is this necessary?
>

The 32-bit accelerated ARM code writes at least one byte, and given
that the other string functions take the same shortcut, this seemed
the most appropriate way to fix that.

> 2. After the new check, Length is guaranteed to be positive. The first
> ASSERT() should be updated (simplified), I think:
>
>   ASSERT (Buffer != NULL);
>

Good point. I will change that