linux-kernel - [RFC PATCH 1/9] x86/purgatory: Drop function entry padding from purgatory

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Date: Wed, 24 Apr 2024 17:53:11 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Eric Biederman <ebiederm@...ssion.com>, kexec@...ts.infradead.org, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Kees Cook <keescook@...omium.org>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Masahiro Yamada <masahiroy@...nel.org>
Subject: [RFC PATCH 1/9] x86/purgatory: Drop function entry padding from purgatory

From: Ard Biesheuvel <ardb@...nel.org>

The purgatory is a completely separate ELF executable carried inside the
kernel as an opaque binary blob. This means that function entry padding
and the associated ELF metadata are not exposed to the branch tracking
and code patching machinery, and can there be dropped from the purgatory
binary.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/purgatory/Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index a18591f6e6d9..2df4a4b70ff5 100644
--- a/arch/x86/purgatory/Makefile
+++ b/arch/x86/purgatory/Makefile
@@ -23,6 +23,9 @@ KBUILD_CFLAGS := $(filter-out -fprofile-sample-use=% -fprofile-use=%,$(KBUILD_CF
 # by kexec. Remove -flto=* flags.
 KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS))
 
+# Drop the function entry padding, which is not needed here
+KBUILD_CFLAGS := $(filter-out $(PADDING_CFLAGS),$(KBUILD_CFLAGS))
+
 # When linking purgatory.ro with -r unresolved symbols are not checked,
 # also link a purgatory.chk binary without -r to check for unresolved symbols.
 PURGATORY_LDFLAGS := -e purgatory_start -z nodefaultlib
-- 
2.44.0.769.g3c40516874-goog