Re: [edk2-devel] [PATCH v2 1/2] MdePkg/BaseLib: Fix unaligned API prototypes

["Andrew Fish" <afish@apple.com>]

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> On Aug 9, 2021, at 9:15 AM, Michael D Kinney <michael.d.kinney@intel.com> wrote:
> 
> Hi Marvin,
> 
> Can you provide an example of which C compiler is flagging this as
> an error and what error message is generated.
> 
> Please enter a BZ with this background information and add link to the
> BZ in the commit message.
> 
> This is a change to the BaseLib class, so we need to make sure there
> are no impacts to any existing code.  I looks like a safe change
> because changing from a pointer to a fixed size type to VOID * 
> should be compatible.  Please add that analysis to the background
> in the BZ as well.
> 

MIke,

I want to say we had a discussion about this years ago? I don’t remember the outcome. 

Dereferencing a misaligned pointer is UB (Undefined Behavior) in C [1], but historically x86 compilers have let it slide.

I think the situation we are in is the BaseLib functions don’t contain UB, but it is UB for the caller to use the returned pointer directly. 

Here is a simple example with clang UndefinedBehaviorSanitizer (UBSan) . 

~/work/Compiler>cat ub.c
#include <stdlib.h>

#define EFIAPI
#define IN
#define OUT

typedef unsigned char 	UINT8;
typedef unsigned short 	UINT16;

UINT16
EFIAPI
WriteUnaligned16 (
  OUT UINT16                    *Buffer,
  IN  UINT16                    Value
  )
{
  // ASSERT (Buffer != NULL);

  ((volatile UINT8*)Buffer)[0] = (UINT8)Value;
  ((volatile UINT8*)Buffer)[1] = (UINT8)(Value >> 8);

  return Value;
}


int main()
{
	UINT8 *buffer = malloc(64);
	UINT16 *pointer = (UINT16 *)(buffer + 1);
	
	WriteUnaligned16 (pointer, 42);
	
	// *pointer = 42; // Error: misaligned integer pointer assignment
	return *pointer;
}
~/work/Compiler>clang -fsanitize=undefined  ub.c
~/work/Compiler>./a.out
ub.c:34:9: runtime error: load of misaligned address 0x7feac6405aa1 for type 'UINT16' (aka 'unsigned short'), which requires 2 byte alignment
0x7feac6405aa1: note: pointer points here
 00 00 00  64 2a 00 79 6d 28 52 54  4c 44 5f 44 45 46 41 55  4c 54 2c 20 73 77 69 66  74 5f 64 65 6d
              ^ 
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ub.c:34:9 in 

FYI line 39 is `return *pointer`and 42 is 0x2A. So reading an writing to *pointer is UB. 


As you can see in [1] the general advice is to take code that looks like:
int8_t *buffer = malloc(64);
int32_t *pointer = (int32_t *)(buffer + 1);
*pointer = 42; // Error: misaligned integer pointer assignment
And replace it with;
int8_t *buffer = malloc(64);
int32_t value = 42;
memcpy(buffer + 1, &value, sizeof(int32_t)); // Correct

But in these cases the result is in a byte aligned buffer….

[1] https://developer.apple.com/documentation/xcode/misaligned-pointer

Thanks,

Andrew Fish

> Thanks,
> 
> Mike
> 
> 
>> -----Original Message-----
>> From: Marvin Häuser <mhaeuser@posteo.de <mailto:mhaeuser@posteo.de>>
>> Sent: Monday, August 9, 2021 2:51 AM
>> To: devel@edk2.groups.io <mailto:devel@edk2.groups.io>
>> Cc: Kinney, Michael D <michael.d.kinney@intel.com <mailto:michael.d.kinney@intel.com>>; Liming Gao <gaoliming@byosoft.com.cn <mailto:gaoliming@byosoft.com.cn>>; Liu, Zhiguang
>> <zhiguang.liu@intel.com <mailto:zhiguang.liu@intel.com>>; Vitaly Cheptsov <vit9696@protonmail.com <mailto:vit9696@protonmail.com>>
>> Subject: [PATCH v2 1/2] MdePkg/BaseLib: Fix unaligned API prototypes
>> 
>> C prohibits not only dereferencing but also casting to unaligned
>> pointers. Thus, the current set of unaligned APIs cannot be called
>> safely. Update their prototypes to take VOID * pointers, which must
>> be able to represent any valid pointer.
>> 
>> Cc: Michael D Kinney <michael.d.kinney@intel.com>
>> Cc: Liming Gao <gaoliming@byosoft.com.cn>
>> Cc: Zhiguang Liu <zhiguang.liu@intel.com>
>> Cc: Vitaly Cheptsov <vit9696@protonmail.com>
>> Signed-off-by: Marvin Häuser <mhaeuser@posteo.de>
>> ---
>> MdePkg/Library/BaseLib/Arm/Unaligned.c | 14 ++++-----
>> MdePkg/Library/BaseLib/Unaligned.c     | 32 ++++++++++----------
>> MdePkg/Include/Library/BaseLib.h       | 16 +++++-----
>> 3 files changed, 31 insertions(+), 31 deletions(-)
>> 
>> diff --git a/MdePkg/Library/BaseLib/Arm/Unaligned.c b/MdePkg/Library/BaseLib/Arm/Unaligned.c
>> index e9934e7003cb..57f19fc44e0b 100644
>> --- a/MdePkg/Library/BaseLib/Arm/Unaligned.c
>> +++ b/MdePkg/Library/BaseLib/Arm/Unaligned.c
>> @@ -59,7 +59,7 @@ ReadUnaligned16 (
>> UINT16
>> 
>> EFIAPI
>> 
>> WriteUnaligned16 (
>> 
>> -  OUT UINT16                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT16                    Value
>> 
>>   )
>> 
>> {
>> 
>> @@ -87,7 +87,7 @@ WriteUnaligned16 (
>> UINT32
>> 
>> EFIAPI
>> 
>> ReadUnaligned24 (
>> 
>> -  IN CONST UINT32              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   )
>> 
>> {
>> 
>>   ASSERT (Buffer != NULL);
>> 
>> @@ -116,7 +116,7 @@ ReadUnaligned24 (
>> UINT32
>> 
>> EFIAPI
>> 
>> WriteUnaligned24 (
>> 
>> -  OUT UINT32                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT32                    Value
>> 
>>   )
>> 
>> {
>> 
>> @@ -143,7 +143,7 @@ WriteUnaligned24 (
>> UINT32
>> 
>> EFIAPI
>> 
>> ReadUnaligned32 (
>> 
>> -  IN CONST UINT32              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   )
>> 
>> {
>> 
>>   UINT16  LowerBytes;
>> 
>> @@ -175,7 +175,7 @@ ReadUnaligned32 (
>> UINT32
>> 
>> EFIAPI
>> 
>> WriteUnaligned32 (
>> 
>> -  OUT UINT32                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT32                    Value
>> 
>>   )
>> 
>> {
>> 
>> @@ -202,7 +202,7 @@ WriteUnaligned32 (
>> UINT64
>> 
>> EFIAPI
>> 
>> ReadUnaligned64 (
>> 
>> -  IN CONST UINT64              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   )
>> 
>> {
>> 
>>   UINT32  LowerBytes;
>> 
>> @@ -234,7 +234,7 @@ ReadUnaligned64 (
>> UINT64
>> 
>> EFIAPI
>> 
>> WriteUnaligned64 (
>> 
>> -  OUT UINT64                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT64                    Value
>> 
>>   )
>> 
>> {
>> 
>> diff --git a/MdePkg/Library/BaseLib/Unaligned.c b/MdePkg/Library/BaseLib/Unaligned.c
>> index a419cb85e53c..3041adcde606 100644
>> --- a/MdePkg/Library/BaseLib/Unaligned.c
>> +++ b/MdePkg/Library/BaseLib/Unaligned.c
>> @@ -26,12 +26,12 @@
>> UINT16
>> 
>> EFIAPI
>> 
>> ReadUnaligned16 (
>> 
>> -  IN CONST UINT16              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   )
>> 
>> {
>> 
>>   ASSERT (Buffer != NULL);
>> 
>> 
>> 
>> -  return *Buffer;
>> 
>> +  return *(CONST UINT16 *) Buffer;
>> 
>> }
>> 
>> 
>> 
>> /**
>> 
>> @@ -52,13 +52,13 @@ ReadUnaligned16 (
>> UINT16
>> 
>> EFIAPI
>> 
>> WriteUnaligned16 (
>> 
>> -  OUT UINT16                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT16                    Value
>> 
>>   )
>> 
>> {
>> 
>>   ASSERT (Buffer != NULL);
>> 
>> 
>> 
>> -  return *Buffer = Value;
>> 
>> +  return *(UINT16 *) Buffer = Value;
>> 
>> }
>> 
>> 
>> 
>> /**
>> 
>> @@ -77,12 +77,12 @@ WriteUnaligned16 (
>> UINT32
>> 
>> EFIAPI
>> 
>> ReadUnaligned24 (
>> 
>> -  IN CONST UINT32              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   )
>> 
>> {
>> 
>>   ASSERT (Buffer != NULL);
>> 
>> 
>> 
>> -  return *Buffer & 0xffffff;
>> 
>> +  return *(CONST UINT32 *) Buffer & 0xffffff;
>> 
>> }
>> 
>> 
>> 
>> /**
>> 
>> @@ -103,13 +103,13 @@ ReadUnaligned24 (
>> UINT32
>> 
>> EFIAPI
>> 
>> WriteUnaligned24 (
>> 
>> -  OUT UINT32                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT32                    Value
>> 
>>   )
>> 
>> {
>> 
>>   ASSERT (Buffer != NULL);
>> 
>> 
>> 
>> -  *Buffer = BitFieldWrite32 (*Buffer, 0, 23, Value);
>> 
>> +  *(UINT32 *) Buffer = BitFieldWrite32 (*(CONST UINT32 *) Buffer, 0, 23, Value);
>> 
>>   return Value;
>> 
>> }
>> 
>> 
>> 
>> @@ -129,12 +129,12 @@ WriteUnaligned24 (
>> UINT32
>> 
>> EFIAPI
>> 
>> ReadUnaligned32 (
>> 
>> -  IN CONST UINT32              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   )
>> 
>> {
>> 
>>   ASSERT (Buffer != NULL);
>> 
>> 
>> 
>> -  return *Buffer;
>> 
>> +  return *(CONST UINT32 *) Buffer;
>> 
>> }
>> 
>> 
>> 
>> /**
>> 
>> @@ -155,13 +155,13 @@ ReadUnaligned32 (
>> UINT32
>> 
>> EFIAPI
>> 
>> WriteUnaligned32 (
>> 
>> -  OUT UINT32                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT32                    Value
>> 
>>   )
>> 
>> {
>> 
>>   ASSERT (Buffer != NULL);
>> 
>> 
>> 
>> -  return *Buffer = Value;
>> 
>> +  return *(UINT32 *) Buffer = Value;
>> 
>> }
>> 
>> 
>> 
>> /**
>> 
>> @@ -180,12 +180,12 @@ WriteUnaligned32 (
>> UINT64
>> 
>> EFIAPI
>> 
>> ReadUnaligned64 (
>> 
>> -  IN CONST UINT64              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   )
>> 
>> {
>> 
>>   ASSERT (Buffer != NULL);
>> 
>> 
>> 
>> -  return *Buffer;
>> 
>> +  return *(CONST UINT64 *) Buffer;
>> 
>> }
>> 
>> 
>> 
>> /**
>> 
>> @@ -206,11 +206,11 @@ ReadUnaligned64 (
>> UINT64
>> 
>> EFIAPI
>> 
>> WriteUnaligned64 (
>> 
>> -  OUT UINT64                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT64                    Value
>> 
>>   )
>> 
>> {
>> 
>>   ASSERT (Buffer != NULL);
>> 
>> 
>> 
>> -  return *Buffer = Value;
>> 
>> +  return *(UINT64 *) Buffer = Value;
>> 
>> }
>> 
>> diff --git a/MdePkg/Include/Library/BaseLib.h b/MdePkg/Include/Library/BaseLib.h
>> index 2452c1d92e51..4d30f0539c6b 100644
>> --- a/MdePkg/Include/Library/BaseLib.h
>> +++ b/MdePkg/Include/Library/BaseLib.h
>> @@ -3420,7 +3420,7 @@ DivS64x64Remainder (
>> UINT16
>> 
>> EFIAPI
>> 
>> ReadUnaligned16 (
>> 
>> -  IN CONST UINT16              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   );
>> 
>> 
>> 
>> 
>> 
>> @@ -3442,7 +3442,7 @@ ReadUnaligned16 (
>> UINT16
>> 
>> EFIAPI
>> 
>> WriteUnaligned16 (
>> 
>> -  OUT UINT16                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT16                    Value
>> 
>>   );
>> 
>> 
>> 
>> @@ -3463,7 +3463,7 @@ WriteUnaligned16 (
>> UINT32
>> 
>> EFIAPI
>> 
>> ReadUnaligned24 (
>> 
>> -  IN CONST UINT32              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   );
>> 
>> 
>> 
>> 
>> 
>> @@ -3485,7 +3485,7 @@ ReadUnaligned24 (
>> UINT32
>> 
>> EFIAPI
>> 
>> WriteUnaligned24 (
>> 
>> -  OUT UINT32                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT32                    Value
>> 
>>   );
>> 
>> 
>> 
>> @@ -3506,7 +3506,7 @@ WriteUnaligned24 (
>> UINT32
>> 
>> EFIAPI
>> 
>> ReadUnaligned32 (
>> 
>> -  IN CONST UINT32              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   );
>> 
>> 
>> 
>> 
>> 
>> @@ -3528,7 +3528,7 @@ ReadUnaligned32 (
>> UINT32
>> 
>> EFIAPI
>> 
>> WriteUnaligned32 (
>> 
>> -  OUT UINT32                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT32                    Value
>> 
>>   );
>> 
>> 
>> 
>> @@ -3549,7 +3549,7 @@ WriteUnaligned32 (
>> UINT64
>> 
>> EFIAPI
>> 
>> ReadUnaligned64 (
>> 
>> -  IN CONST UINT64              *Buffer
>> 
>> +  IN CONST VOID                *Buffer
>> 
>>   );
>> 
>> 
>> 
>> 
>> 
>> @@ -3571,7 +3571,7 @@ ReadUnaligned64 (
>> UINT64
>> 
>> EFIAPI
>> 
>> WriteUnaligned64 (
>> 
>> -  OUT UINT64                    *Buffer,
>> 
>> +  OUT VOID                      *Buffer,
>> 
>>   IN  UINT64                    Value
>> 
>>   );
>> 
>> 
>> 
>> --
>> 2.31.1
> 
> 
> 
> 


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