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+tianoc...@kernel.org> Signed-off-by: Glenn Griffin <ggriffin...@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 -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#117139): https://edk2.groups.io/g/devel/message/117139 Mute This Topic: https://groups.io/mt/105165329/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
