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

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Message-ID: <CAPhsuW5ZovvWG=KyDR80tEDbKda0=sGsVoKzLJDfBz1n4A7BPg@mail.gmail.com>
Date:   Fri, 8 Sep 2023 16:10:44 -0700
From:   Song Liu <song@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
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 8, 2023 at 3:59 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> 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")

Thanks for the pointer! This does look better.

Thanks,
Song

>
> 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
> >