Re: [PATCH 09/23] kbuild: do not allocate .modinfo in vmlinux

[Lorenzo Stoakes (ARM)]

On Wed, Sep 09, 2026 at 09:19:12PM -0700, Nathan Chancellor wrote:
> > Commit 3e86e4d74c04 ("kbuild: keep .modinfo section in vmlinux.unstripped")
> > placed MODULE_INFO() strings in the .modinfo section in vmlinux.unstripped,
> > however it left the section allocatable, so it gets assigned an address and
> > on arm64, arm and riscv, the section is tagged as PT_LOAD.
> >
> > This is useless as the data is ultimately stripped anyway.
> >
> > Doing this results in a lot of unnecessary work - each pass copies the
> > whole file, 450 MIB with relocations for an x86 allmodconfig build and 250
> > MiB for an arm64 allmodconfig build.
> >
> > This adds ~0.7s on the serial tail of every build for arm64 and ~0.2s for
> > x86 (the tail is single-threaded work done after parallel work has
> > finished).
> >
> > Nothing requires .modinfo to exist at an address, so fix this by using
> > --dump-section which prevents the allocation.
> >
> > Image, bzImage, System.map and modules.builtin.modinfo are unchanged and
> > the stripped vmlinux differs only in its program headers.
> >
> > The objcopy passes are on the serial tail of every build that links
> > vmlinux, no-op builds are unchanged.
> >
> > Whole build, 128-thread Threadripper 9980X, best of N runs:
> >
> >                                          before   after     delta
> >                                          -------------------------------
> >   x86 defconfig, touch mm/vma.c, gcc        8.9s     8.8s    -0.16s (-2%)
> >   x86 defconfig, touch mm/vma.c, clang      8.1s     7.9s    -0.20s (-2%)
> >   x86 defconfig, clean, gcc                27.6s    27.5s    -0.13s (0%)
> >   x86 defconfig, clean, clang              27.2s    27.0s    -0.18s (-1%)
> >   x86 allmodconfig, touch mm/vma.c, gcc    42.0s    41.6s    -0.40s (-1%)
> >
> > Assisted-by: LLM
> > Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index b2988aa12f66..7be9e0321867 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -855,7 +855,7 @@
> >  		KLP_SYMID
> >
> >  #define MODINFO								\
> > -		.modinfo : { *(.modinfo) . = ALIGN(8); }
> > +		.modinfo (INFO) : { *(.modinfo) . = ALIGN(8); }
> >
> >  #ifdef CONFIG_GENERIC_BUG
> >  #define BUG_TABLE							\
> > diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux
> > index 4b54aaeca65d..1fc7a0ca8eaf 100644
> > --- a/scripts/Makefile.vmlinux
> > +++ b/scripts/Makefile.vmlinux
> > @@ -90,11 +90,9 @@ remove-section-$(CONFIG_ARCH_VMLINUX_NEEDS_RELOCS) += '.rel.*'
> >
> >  remove-symbols := -w --strip-unneeded-symbol='__mod_device_table__*'
> >
> > -# To avoid warnings: "empty loadable segment detected at ..." from GNU objcopy,
> > -# it is necessary to remove the PT_LOAD flag from the segment.
> > +# none of the removed sections is allocated, so no segment is left empty
>
> I think we could just drop this comment, I don't think it is
> particularly helpful

Ack!

>
> >  quiet_cmd_strip_relocs = OBJCOPY $@
> > -      cmd_strip_relocs = $(OBJCOPY) $(patsubst %,--set-section-flags %=noload,$(remove-section-y)) $< $@; \
> > -                         $(OBJCOPY) $(addprefix --remove-section=,$(remove-section-y)) $(remove-symbols) $@
> > +      cmd_strip_relocs = $(OBJCOPY) $(addprefix --remove-section=,$(remove-section-y)) $(remove-symbols) $< $@
> >
> >  targets += vmlinux
> >  vmlinux: vmlinux.unstripped FORCE
> > @@ -103,21 +101,11 @@ vmlinux: vmlinux.unstripped FORCE
> >  # modules.builtin.modinfo
> >  # ---------------------------------------------------------------------------
> >
> > -# .modinfo in vmlinux.unstripped is aligned to 8 bytes for compatibility with
> > -# tools that expect vmlinux to have sufficiently aligned sections but the
> > -# additional bytes used for padding .modinfo to satisfy this requirement break
> > -# certain versions of kmod with
> > -#
> > -#   depmod: ERROR: kmod_builtin_iter_next: unexpected string without modname prefix
> > -#
> > -# Strip the trailing padding bytes after extracting .modinfo to comply with
> > -# what kmod expects to parse.
> > +# use --dump-section to include non-allocated sections
>
> Same thing here, I am not sure anyone will question '--dump-section'
> here. Otherwise, this seems reasonable. I was curious about the $@.tmp
> until I read the objcopy manual but that probably is not worth a
> comment.

Ack too.

>
> I can probably take this one faster than the rest if you want to put it
> early in the stack since it is pretty low hanging fruit and earlier it
> gets exposure to -next, the better.

Sure sounds good, the sooner we get these in -next the better.

I was planning to go through feedback/sashiko (+ give a bit more time for people
to respond) before doing a v2, but I guess you could put this to -next as-is,
given v2 will just be stripping comments here (as it stands anyway!)

>
> --
> Cheers,
> Nathan
>

--
Cheers, Lorenzo