linux-kernel - [PATCH v5 05/20] x86/decompressor: Use proper sequence to take the address of the GOT

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Message-Id: <20230607072342.4054036-6-ardb@kernel.org>
Date:   Wed,  7 Jun 2023 09:23:27 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-efi@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Evgeniy Baskov <baskov@...ras.ru>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Mario Limonciello <mario.limonciello@....com>,
        Kees Cook <keescook@...omium.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>
Subject: [PATCH v5 05/20] x86/decompressor: Use proper sequence to take the address of the GOT

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 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@...nel.org>
---
 arch/x86/boot/compressed/head_32.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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@...OFF(%edx), %eax
-- 
2.39.2