On Thu, Sep 14, 2023 at 10:02 AM Song Liu <song@xxxxxxxxxx> wrote:
>
> -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@xxxxxxxxxxxx>
> Cc: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Cc: kexec@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: x86@xxxxxxxxxx
> Cc: llvm@xxxxxxxxxxxxxxx
> Signed-off-by: Song Liu <song@xxxxxxxxxx>
>
> ---
> 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
LGTM, thanks for the patch!
Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Sami
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