In function ReadCapacity(), the following expression:
MediaInfo->LastBlock = (Data.LastLba3 << 24) |
(Data.LastLba2 << 16) |
(Data.LastLba1 << 8) |
Data.LastLba0;
(There is also a similar case in this function.)
will involve undefined behavior in signed left shift operations.
Since Data.LastLbaX is of type UINT8, and MediaInfo->LastBlock is of type
UINTN. Therefore, Data.LastLbaX will be promoted to int (32 bits, signed)
first, and then perform the left shift operation.
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.
So if bit 7 of Data.LastLba3 is 1, (Data.LastLba3 << 24) will be out of
the range within int type. The undefined behavior of the signed left shift
will lead to a potential of setting the high 32 bits of
MediaInfo->LastBlock to 1 during the cast from type int to type UINT64
for X64 builds.
This commit will add an explicit UINT32 type cast for Data.LastLba3 to
resolve this issue.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
There are cases that the operands of an expression are all with rank less
than UINT64/INT64 and the result of the expression is explicitly cast to
UINT64/INT64 to fit the target size.
An example will be:
UINT32 a,b;
// a and b can be any unsigned int type with rank less than UINT64, like
// UINT8, UINT16, etc.
UINT64 c;
c = (UINT64) (a + b);
Some static code checkers may warn that the expression result might
overflow within the rank of "int" (integer promotions) and the result is
then cast to a bigger size.
The commit refines codes by the following rules:
1). When the expression is possible to overflow the range of unsigned int/
int:
c = (UINT64)a + b;
2). When the expression will not overflow within the rank of "int", remove
the explicit type casts:
c = a + b;
3). When the expression will be cast to pointer of possible greater size:
UINT32 a,b;
VOID *c;
c = (VOID *)(UINTN)(a + b); --> c = (VOID *)((UINTN)a + b);
4). When one side of a comparison expression contains only operands with
rank less than UINT32:
UINT8 a;
UINT16 b;
UINTN c;
if ((UINTN)(a + b) > c) {...} --> if (((UINT32)a + b) > c) {...}
For rule 4), if we remove the 'UINTN' type cast like:
if (a + b > c) {...}
The VS compiler will complain with warning C4018 (signed/unsigned
mismatch, level 3 warning) due to promoting 'a + b' to type 'int'.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
Hao Wu
Feng Tian
Eric Dong
jljusten