Re: [PATCH] kbuild: keep .modinfo when vmlinux is linked with --gc-sections

[Lorenzo Stoakes (ARM)]

On Sun, Sep 27, 2026 at 12:12:27PM -0400, Sasha Levin wrote:
> Builds with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y, such as arm
> allmodconfig, now fail while extracting modules.builtin.modinfo:
>
>   arm-linux-gnueabihf-objcopy: vmlinux.unstripped: can't dump section
>   '.modinfo' - it does not exist: file format not recognized
>   /bin/sh: 1: cannot open modules.builtin.modinfo: No such file
>
> or, with LLVM:
>
>   llvm-objcopy: error: 'vmlinux.unstripped': section '.modinfo' not
>   found
>
> Nothing references the MODULE_INFO() strings, so --gc-sections
> discards every .modinfo input section and vmlinux.unstripped ends up
> with no .modinfo data. This is not new: ever since
> modules.builtin.modinfo started being extracted from
> vmlinux.unstripped, these builds have silently produced an empty
> file, losing the modinfo of every built-in module. An arm
> multi_v7_defconfig build with LD_DEAD_CODE_DATA_ELIMINATION on current
> mainline yields a 0 byte modules.builtin.modinfo. The switch to
> --dump-section only turned that into a build error.

Yikes, so an existing bug but now exposed by my series.

>
> Wrap the input section in KEEP() so that the linker retains it. The
> output section is still (INFO), so nothing is allocated for it. With
> the same arm config, modules.builtin.modinfo now has 14119 entries,
> and on x86_64 defconfig, where nothing is garbage collected, it is
> byte-identical before and after this change.

Sounds reasonable.

>
> Found by KernelCI builds of the linus-next tree.
>
> Fixes: 39cfd5b12160 ("kbuild: extract modules.builtin.modinfo from vmlinux.unstripped")
> Fixes: 5fe4dd596641 ("kbuild: do not allocate .modinfo in vmlinux")
> Assisted-by: LLM
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

Acked-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

Nathan - do you want me to fold this into the 1st patch of my series on respin
or fine to treat separately?

> ---
>  include/asm-generic/vmlinux.lds.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index a6730d34e8c67..7a3e9f00d6283 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -855,7 +855,7 @@
>  		KLP_SYMID
>
>  #define MODINFO								\
> -		.modinfo (INFO) : { *(.modinfo) }
> +		.modinfo (INFO) : { KEEP(*(.modinfo)) }
>
>  #ifdef CONFIG_GENERIC_BUG
>  #define BUG_TABLE							\
> --
> 2.53.0
>

--
Cheers, Lorenzo