Re: [PATCH 4/6] MdeModulePkg/DxeNetLib: Fix negative value left shift

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/09/2017 13:43, Hao Wu wrote:
> REF: https://bugzilla.tianocore.org/show_bug.cgi?id=698
> 
> Within function NetRandomInitSeed(), left shift a negative value is used
> in:
> "~Time.Hour << 24"
> 
> which involves undefined behavior.
> 
> Since Time.Hour is of type UINT8 (range from 0 to 23), hence ~Time.Hour
> will be a negative value (of type int, signed).
> 
> According to the C11 spec, Section 6.5.7:
>> 4 The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
>>   bits are filled with zeros. If E1 has an unsigned type, the value
>>   of the result is E1 * 2^E2 , reduced modulo one more than the
>>   maximum value representable in the result type. If E1 has a signed
>>   type and nonnegative value, and E1 * 2^E2 is representable in the
>>   result type, then that is the resulting value; otherwise, the
>>   behavior is undefined.
> 
> This commit explicitly cast 'Time.Hour' with UINT32 to resolve this issue.
> 
> Cc: Steven Shi <steven.shi@intel.com>
> Cc: Fu Siyuan <siyuan.fu@intel.com>
> Cc: Ye Ting <ting.ye@intel.com>
> Cc: Wu Jiaxin <jiaxin.wu@intel.com>
> Cc: Star Zeng <star.zeng@intel.com>
> Cc: Eric Dong <eric.dong@intel.com>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Hao Wu <hao.a.wu@intel.com>
> ---
>  MdeModulePkg/Library/DxeNetLib/DxeNetLib.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/MdeModulePkg/Library/DxeNetLib/DxeNetLib.c b/MdeModulePkg/Library/DxeNetLib/DxeNetLib.c
> index 7cd7e3aca0..ca5413edcc 100644
> --- a/MdeModulePkg/Library/DxeNetLib/DxeNetLib.c
> +++ b/MdeModulePkg/Library/DxeNetLib/DxeNetLib.c
> @@ -872,7 +872,7 @@ NetRandomInitSeed (
>    UINT64                    MonotonicCount;
>  
>    gRT->GetTime (&Time, NULL);
> -  Seed = (~Time.Hour << 24 | Time.Day << 16 | Time.Minute << 8 | Time.Second);
> +  Seed = (~(UINT32)Time.Hour << 24 | Time.Day << 16 | Time.Minute << 8 | Time.Second);
>    Seed ^= Time.Nanosecond;
>    Seed ^= Time.Year << 7;
Is there any reason why Seed must be XORed with 0xFF000000 (that's what
the ~ does)?  Would it make sense to either write it explicitly as a
XOR, or perhaps the ~ can be removed altogether?

Paolo