[PATCH] kbuild: merge module sections under CONFIG_LD_DEAD_CODE_DATA_ELIMINATION too

[PATCH] kbuild: merge module sections under CONFIG_LD_DEAD_CODE_DATA_ELIMINATION too

[Alexander Lobakin]

When building with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
-fdata-sections and -ffunction-sections are being enabled by the
top-level Makefile, and module section merging is also needed.
Expand the ifdef (and the comment block) to cover that case too.

Fixes: 6a3193cdd5e5 ("kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 scripts/module.lds.S | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index 2c52535f9b56..d6bbdfc55e08 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -20,11 +20,14 @@ SECTIONS {

 	__patchable_function_entries : { *(__patchable_function_entries) }

-#ifdef CONFIG_LTO_CLANG
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
 	/*
 	 * With CONFIG_LTO_CLANG, LLD always enables -fdata-sections and
-	 * -ffunction-sections, which increases the size of the final module.
-	 * Merge the split sections in the final binary.
+	 * -ffunction-sections. With CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
+	 * -fdata-sections and -ffunction-sections are being enabled by
+	 * the top-level Makefile.
+	 * This increases the size of the final module. Merge the split
+	 * sections in the final binary.
 	 */
 	.bss : {
 		*(.bss .bss.[0-9a-zA-Z_]*)
--
2.31.1

Re: [PATCH] kbuild: merge module sections under CONFIG_LD_DEAD_CODE_DATA_ELIMINATION too

[Sami Tolvanen]

On Fri, Apr 2, 2021 at 5:40 AM Alexander Lobakin <alobakin@pm.me> wrote:
>
> When building with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
> -fdata-sections and -ffunction-sections are being enabled by the
> top-level Makefile, and module section merging is also needed.
> Expand the ifdef (and the comment block) to cover that case too.
>
> Fixes: 6a3193cdd5e5 ("kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled")

Wouldn't this trigger the ld.bfd bug described in the commit message
when LD_DEAD_CODE_DATA_ELIMINATION is enabled? LTO_CLANG always uses
LLD, so it won't have this issue.

Sami

Re: [PATCH] kbuild: merge module sections under CONFIG_LD_DEAD_CODE_DATA_ELIMINATION too

[Alexander Lobakin]

On Friday, 2 April 2021, 18:09, Sami
Tolvanen <samitolvanen@google.com> wrote:

> On Fri, Apr 2, 2021 at 5:40 AM Alexander Lobakin alobakin@pm.me wrote:
>
> > When building with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
> > -fdata-sections and -ffunction-sections are being enabled by the
> > top-level Makefile, and module section merging is also needed.
> > Expand the ifdef (and the comment block) to cover that case too.
> > Fixes: 6a3193cdd5e5 ("kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled")
>
> Wouldn't this trigger the ld.bfd bug described in the commit message
> when LD_DEAD_CODE_DATA_ELIMINATION is enabled? LTO_CLANG always uses
> LLD, so it won't have this issue.

LD_DEAD_CODE_DATA_ELIMINATION is marked
“EXPERIMENTAL“ in the config prompt, and
arches have to opt-in
HAS_LD_DEAD_CODE_DATA_ELIMINATION to give
an access to it (only a few does). This
should be relatively safe.

> Sami

Thanks,
Al

Re: [PATCH] kbuild: merge module sections under CONFIG_LD_DEAD_CODE_DATA_ELIMINATION too

[Masahiro Yamada]

On Tue, Apr 6, 2021 at 11:42 PM Alexander Lobakin <alobakin@pm.me> wrote:
>
> On Friday, 2 April 2021, 18:09, Sami
> Tolvanen <samitolvanen@google.com> wrote:
>
> > On Fri, Apr 2, 2021 at 5:40 AM Alexander Lobakin alobakin@pm.me wrote:
> >
> > > When building with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
> > > -fdata-sections and -ffunction-sections are being enabled by the
> > > top-level Makefile, and module section merging is also needed.
> > > Expand the ifdef (and the comment block) to cover that case too.
> > > Fixes: 6a3193cdd5e5 ("kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled")


Did you test this patch before submission?


See the top Makefile closely:

ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
LDFLAGS_vmlinux += --gc-sections
endif


-ffunction-sections -fdata-sections are passed to only
built-in objects, but not to module objects in the
first place.

KBUILD_CFLAGS_KERNEL is only passed to built-in objects.


The situation you claimed never happens.







> > Wouldn't this trigger the ld.bfd bug described in the commit message
> > when LD_DEAD_CODE_DATA_ELIMINATION is enabled? LTO_CLANG always uses
> > LLD, so it won't have this issue.
>
> LD_DEAD_CODE_DATA_ELIMINATION is marked
> “EXPERIMENTAL“ in the config prompt, and
> arches have to opt-in
> HAS_LD_DEAD_CODE_DATA_ELIMINATION to give
> an access to it (only a few does). This
> should be relatively safe.
>
> > Sami
>
> Thanks,
> Al



-- 
Best Regards
Masahiro Yamada