linux-kernel - [tip: x86/urgent] x86/purgatory: Remove LTO flags

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Message-ID: <169493728660.27769.18183881403388976059.tip-bot2@tip-bot2>
Date:   Sun, 17 Sep 2023 07:54:46 -0000
From:   "tip-bot2 for Song Liu" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Song Liu <song@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/purgatory: Remove LTO flags

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     75b2f7e4c9e0fd750a5a27ca9736d1daa7a3762a
Gitweb:        https://git.kernel.org/tip/75b2f7e4c9e0fd750a5a27ca9736d1daa7a3762a
Author:        Song Liu <song@...nel.org>
AuthorDate:    Thu, 14 Sep 2023 10:01:38 -07:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Sun, 17 Sep 2023 09:49:03 +02:00

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 causes 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.

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

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

Fixes: b33fff07e3e3 ("x86, build: allow LTO to be selected")
Signed-off-by: Song Liu <song@...nel.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>
Link: https://lore.kernel.org/r/20230914170138.995606-1-song@kernel.org
---
 arch/x86/purgatory/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index c2a29be..08aa0f2 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