Prior to this change the alignment of the SEC module would be 4 bytes.
This is inconsistent with the expectations of the compiler and can lead
to unexpected behavior.
For example a modern version of clang with size optimizations enabled
(-Oz) can break the ALIGN_POINTER macro in the SEC module.
Here is a minimal example:
https://godbolt.org/z/fzW67ndTY
```
unsigned char mArray[112];
#define ALIGN_VALUE(Value, Alignment) ((Value) + (((Alignment) - (Value)) & ((Alignment) - 1)))
#define ALIGN_POINTER(Pointer, Alignment) ((void *) (ALIGN_VALUE ((uint64_t)(Pointer), (Alignment))))
int main() {
printf("0x%llx\n", mArray);
printf("0x%llx\n", ALIGN_POINTER(mArray, 64));
return 0;
}
```
The above code compiles down to:
```
main: # @main
push r14
push rbx
push rax
lea rbx, [rip + .L.str]
lea r14, [rip + mArray]
mov rdi, rbx
mov rsi, r14
xor eax, eax
call printf@PLT
push 64
pop rsi
sub esi, r14d
and esi, 48
add rsi, r14
mov rdi, rbx
xor eax, eax
call printf@PLT
xor eax, eax
add rsp, 8
pop rbx
pop r14
ret
mArray:
.zero 112
.L.str:
.asciz "0x%llx\n"
```
Note that the `value & 63` in the ALIGN_VALUE implementation gets
transformed into `and esi, 48` in the assembly. The compiler knows
the array address is already aligned so it believes there is no need to
clear the last 4 bits of the address.
However by mapping the data section that contains mArray onto a
4-byte-aligned base address we violate the compiler's expectations. The
last 4 bits of the mArray address are no longer zeroes leading to an
ALIGN_POINTER macro that doesn't work.
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Signed-off-by: Glenn Griffin <ggriffiniii@gmail.com>
---
OvmfPkg/OvmfPkgX64.fdf | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/OvmfPkg/OvmfPkgX64.fdf b/OvmfPkg/OvmfPkgX64.fdf
index eb3fb90cb8b6..8b60355de40b 100644
--- a/OvmfPkg/OvmfPkgX64.fdf
+++ b/OvmfPkg/OvmfPkgX64.fdf
@@ -434,7 +434,7 @@ [FV.FVMAIN_COMPACT]
[Rule.Common.SEC]
FILE SEC = $(NAMED_GUID) {
- PE32 PE32 $(INF_OUTPUT)/$(MODULE_NAME).efi
+ PE32 PE32 Align=Auto $(INF_OUTPUT)/$(MODULE_NAME).efi
UI STRING ="$(MODULE_NAME)" Optional
VERSION STRING ="$(INF_VERSION)" Optional BUILD_NUM=$(BUILD_NUMBER)
}
--
2.44.0.396.g6e790dbe36-goog