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

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Message-ID: <ZPunWKU/TlrN9xiH@google.com>
Date:   Fri, 8 Sep 2023 15:59:36 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Song Liu <song@...nel.org>
Cc:     linux-kernel@...r.kernel.org, songliubraving@...com,
        Ricardo Ribalda <ribalda@...omium.org>,
        kexec@...ts.infradead.org, x86@...nel.org, samitolvanen@...gle.com,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] x86/purgatory: Remove LTO flags

On Fri, Sep 08, 2023 at 03:53:53PM -0700, Song Liu wrote:
> 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.
> 
> Fixes: 8652d44f466a ("kexec: support purgatories with .text.hot sections")
> Cc: Ricardo Ribalda <ribalda@...omium.org>
> Cc: kexec@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
> Cc: x86@...nel.org
> Signed-off-by: Song Liu <song@...nel.org>
> 
> ---
> AFAICT, x86 is the only arch that supports LTO and purgatory.
> ---
>  arch/x86/purgatory/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index c2a29be35c01..9c0bff4992ea 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 := $(filter-out -flto=%,$(KBUILD_CFLAGS))

Thanks for the patch!

Please prefer something akin to:
commit 6e20f18579c5 ("efi/libstub: disable LTO")

Please also CC Sami and llvm@...ts.linux.dev for v2.

Pretty sure we use a linker script to rejoin .text.* elsewhere in the
kernel, but it doesn't look like x86 purgatory has one.

> +
>  # 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
>