2023-09-09 10:01:18

by Song Liu

Subject: [PATCH v2] x86/purgatory: Remove LTO flags

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 <[email protected]>
Cc: Sami Tolvanen <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Song Liu <[email protected]>

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

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
--
2.34.1

2023-09-14 16:33:28

by Song Liu

Subject: Re: [PATCH v2] x86/purgatory: Remove LTO flags

Hi folks,

On Fri, Sep 8, 2023 at 4:13 PM Song Liu <[email protected]> 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 <[email protected]>
> Cc: Sami Tolvanen <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Signed-off-by: Song Liu <[email protected]>

What would be the next step for this work? We hope to back port it
to our production kernel soon.

Thanks,
Song

>
> ---
> AFAICT, x86 is the only arch that supports LTO and purgatory.
>
> 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
> --
> 2.34.1
>

2023-09-14 17:05:33

by Song Liu

Subject: Re: [PATCH v2] x86/purgatory: Remove LTO flags

On Thu, Sep 14, 2023 at 9:34 AM Nick Desaulniers
<[email protected]> wrote:
>
> On Thu, Sep 14, 2023 at 9:31 AM Song Liu <[email protected]> wrote:
> >
> > What would be the next step for this work? We hope to back port it
> > to our production kernel soon.
>
> Please send a v3 with the fixes tag updated. I wouldn't mind if you
> added a comment to the commit message to the effect of:
>
> We could also add the use of an explicit linker script to rejoin
> .text.* sections back into .text. Simply disable the production of
> more .text.* sections for now; -flto* implies -ffunction-sections.

Got it. I just sent v3 with these changes.

Thanks,
Song

2023-09-14 18:01:57

by Nick Desaulniers

Subject: Re: [PATCH v2] x86/purgatory: Remove LTO flags

On Thu, Sep 14, 2023 at 9:31 AM Song Liu <[email protected]> wrote:
>
> What would be the next step for this work? We hope to back port it
> to our production kernel soon.

Please send a v3 with the fixes tag updated. I wouldn't mind if you
added a comment to the commit message to the effect of:

We could also add the use of an explicit linker script to rejoin
.text.* sections back into .text. Simply disable the production of
more .text.* sections for now; -flto* implies -ffunction-sections.

--
Thanks,
~Nick Desaulniers