Re: [PATCH v3] x86/purgatory: Remove LTO flags

[Sami Tolvanen]

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