Re: [PATCH v2 4/6] MdeModulePkg/DxeNetLib: Fix negative value left shift

["Zeng, Star" <star.zeng@intel.com>]

Leave the review to module experts.

Thanks,
Star
-----Original Message-----
From: Wu, Hao A 
Sent: Thursday, September 21, 2017 2:46 PM
To: edk2-devel@lists.01.org
Cc: Wu, Hao A <hao.a.wu@intel.com>; Shi, Steven <steven.shi@intel.com>; Fu, Siyuan <siyuan.fu@intel.com>; Ye, Ting <ting.ye@intel.com>; Wu, Jiaxin <jiaxin.wu@intel.com>; Long, Qin <qin.long@intel.com>; Zeng, Star <star.zeng@intel.com>; Dong, Eric <eric.dong@intel.com>; Paolo Bonzini <pbonzini@redhat.com>
Subject: [PATCH v2 4/6] MdeModulePkg/DxeNetLib: Fix negative value left shift

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 will remove the '~' operator before 'Time.Hour', since it seems like an implementation choice for generating the seed.

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: Qin Long <qin.long@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.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..1ee77fe036 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 = (Time.Hour << 24 | Time.Day << 16 | Time.Minute << 8 | 
+ Time.Second);
   Seed ^= Time.Nanosecond;
   Seed ^= Time.Year << 7;
 
--
2.12.0.windows.1