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[v7,07/22] x86/decompressor: Use proper sequence to take the address of the GOT

Message ID 20230728090916.1538550-8-ardb@kernel.org
State New
Headers show
Series efi/x86: Avoid bare metal decompressor during EFI boot | expand

Commit Message

Ard Biesheuvel July 28, 2023, 9:09 a.m. UTC 
The 32-bit decompressor does not actually use a global offset table
(GOT), but as is common for 32-bit position independent code, it uses
the magic symbol _GLOBAL_OFFSET_TABLE_ as an anchor from which to derive
the actual runtime addresses of other symbols, using special @GOTOFF
symbol references that are resolved at link time, and populated with the
distance between the address of the magic _GLOBAL_OFFSET_TABLE_ anchor
and the address of the symbol in question.

This means _GLOBAL_OFFSET_TABLE_ is the only symbol whose actual runtime
address needs to be determined explicitly, which is one of the first
things that happens in startup_32. However, it does so by taking the
absolute address via the immediate field of an ADD instruction (plus a
small offset), which seems to defeat the point.

Fortunately, the assembler knows that _GLOBAL_OFFSET_TABLE_ is magic,
and emits a special relative R_386_GOTPC relocation instead, and so the
resulting code works as expected. However, this is not obvious for
someone reading the code, and the use of LEA with an explicit relative
addend is more idiomatic so use that instead.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/compressed/head_32.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
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Patch

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 8876ffe30e9a4819..3530465b5b85ccf3 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -58,7 +58,7 @@  SYM_FUNC_START(startup_32)
 	leal	(BP_scratch+4)(%esi), %esp
 	call	1f
 1:	popl	%edx
-	addl	$_GLOBAL_OFFSET_TABLE_+(.-1b), %edx
+	leal	(_GLOBAL_OFFSET_TABLE_ - 1b)(%edx), %edx
 
 	/* Load new GDT */
 	leal	gdt@GOTOFF(%edx), %eax