linux-kernel - [PATCH v3] x86/purgatory: Remove LTO flags

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Message-ID: <20230914170138.995606-1-song@kernel.org>
Date:   Thu, 14 Sep 2023 10:01:38 -0700
From:   Song Liu <song@...nel.org>
To:     <linux-kernel@...r.kernel.org>
CC:     <ndesaulniers@...gle.com>, Song Liu <song@...nel.org>,
        Ricardo Ribalda <ribalda@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        <kexec@...ts.infradead.org>, <x86@...nel.org>,
        <llvm@...ts.linux.dev>
Subject: [PATCH v3] x86/purgatory: Remove LTO flags

-flto* implies -ffunction-sections. With LTO enabled, ld.lld generates
multiple .text sections for purgatory.ro:

$ readelf -S purgatory.ro  | grep " .text"
  [ 1] .text             PROGBITS         0000000000000000  00000040
  [ 7] .text.purgatory   PROGBITS         0000000000000000  000020e0
  [ 9] .text.warn        PROGBITS         0000000000000000  000021c0
  [13] .text.sha256_upda PROGBITS         0000000000000000  000022f0
  [15] .text.sha224_upda PROGBITS         0000000000000000  00002be0
  [17] .text.sha256_fina PROGBITS         0000000000000000  00002bf0
  [19] .text.sha224_fina PROGBITS         0000000000000000  00002cc0

This cause WARNING from kexec_purgatory_setup_sechdrs():

WARNING: CPU: 26 PID: 110894 at kernel/kexec_file.c:919
kexec_load_purgatory+0x37f/0x390

Fix this by disabling LTO for purgatory.

We could also fix this with an explicit linker script to rejoin .text.*
sections back into .text. However, given the benefit of LTOing pugatory
is small, simply disable the production of more .text.* sections for now.

Fixes: b33fff07e3e3 ("x86, build: allow LTO to be selected")
Cc: Ricardo Ribalda <ribalda@...omium.org>
Cc: Sami Tolvanen <samitolvanen@...gle.com>
Cc: kexec@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org
Cc: x86@...nel.org
Cc: llvm@...ts.linux.dev
Signed-off-by: Song Liu <song@...nel.org>

---
AFAICT, x86 is the only arch that supports LTO and purgatory.

Changes in v3:
1. Fix "Fixes" tag. (Nick Desaulniers)
2. Add description of an alternative fix, with linker script.
   (Nick Desaulniers)

Changes in v2:
1. Use CC_FLAGS_LTO instead of hardcode -flto. (Nick Desaulniers)
---
 arch/x86/purgatory/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index c2a29be35c01..08aa0f25f12a 100644
--- a/arch/x86/purgatory/Makefile
+++ b/arch/x86/purgatory/Makefile
@@ -19,6 +19,10 @@ CFLAGS_sha256.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
 # optimization flags.
 KBUILD_CFLAGS := $(filter-out -fprofile-sample-use=% -fprofile-use=%,$(KBUILD_CFLAGS))
 
+# When LTO is enabled, llvm emits many text sections, which is not supported
+# by kexec. Remove -flto=* flags.
+KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO),$(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
-- 
2.34.1