Re: [edk2] [PATCH 4/6] MdeModulePkg/DxeNetLib: Fix negative value left shift

[Wu, Jiaxin]

Reviewed-by: Wu Jiaxin 


> -Original Message-
> From: Wu, Hao A
> Sent: Tuesday, September 19, 2017 7:44 PM
> To: edk2-devel@lists.01.org
> Cc: Wu, Hao A ; Shi, Steven ;
> Fu, Siyuan ; Ye, Ting ; Wu, Jiaxin
> ; Zeng, Star ; Dong, Eric
> 
> Subject: [PATCH 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 explicitly cast 'Time.Hour' with UINT32 to resolve this issue.
> 
> Cc: Steven Shi 
> Cc: Fu Siyuan 
> Cc: Ye Ting 
> Cc: Wu Jiaxin 
> Cc: Star Zeng 
> Cc: Eric Dong 
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Hao Wu 
> ---
>  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 (
>UINT64MonotonicCount;
> 
>gRT->GetTime (, 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;
> 
> --
> 2.12.0.windows.1

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Re: [edk2] [PATCH 4/6] MdeModulePkg/DxeNetLib: Fix negative value left shift

[Paolo Bonzini]

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 
> Cc: Fu Siyuan 
> Cc: Ye Ting 
> Cc: Wu Jiaxin 
> Cc: Star Zeng 
> Cc: Eric Dong 
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Hao Wu 
> ---
>  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 (
>UINT64MonotonicCount;
>  
>gRT->GetTime (, 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 0xFF00 (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
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[edk2] [PATCH 4/6] MdeModulePkg/DxeNetLib: Fix negative value left shift

[Hao Wu]

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 
Cc: Fu Siyuan 
Cc: Ye Ting 
Cc: Wu Jiaxin 
Cc: Star Zeng 
Cc: Eric Dong 
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu 
---
 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 (
   UINT64MonotonicCount;
 
   gRT->GetTime (, 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;
 
-- 
2.12.0.windows.1

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