linux-kernel - Re: [PATCH] kbuild: Use --strip-unneeded with INSTALL_MOD

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Message-ID: <CAK7LNAR=1sNs+hOW8gL=7xOs=gHLToTAnAUTF1SizroYoui8sg@mail.gmail.com>
Date: Sat, 1 Feb 2025 00:10:02 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH] kbuild: Use --strip-unneeded with INSTALL_MOD_STRIP

On Fri, Jan 31, 2025 at 3:54 PM Charlie Jenkins <charlie@...osinc.com> wrote:
>
> On Thu, Jan 30, 2025 at 08:52:45PM -0700, Nathan Chancellor wrote:
> > On Wed, Jan 22, 2025 at 07:17:26PM -0800, Charlie Jenkins wrote:
> > > On riscv, kernel modules end up with a significant number of local
> > > symbols. This becomes apparent when compiling modules with debug symbols
> > > enabled. Using amdgpu.ko as an example of a large module, on riscv the
> > > size is 754MB (no stripping), 53MB (--strip-debug), and 21MB
> > > (--strip-unneeded). ON x86, amdgpu.ko is 482MB (no stripping), 21MB
> > > (--strip-debug), and 20MB (--strip-unneeded).
> > >
> > > Use --strip-unneeded instead of --strip-debug to strip modules so
> > > decrease the size of the resulting modules. This is particularly
> > > relevant for riscv, but also marginally aids other architectures.
> > >
> > > Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> >
> > Is there any sort of regression risk with this patch? If so, another
> > option may be to give another level to INSTALL_MOD_STRIP like 2 so that
> > INSTALL_MOD_STRIP=1 continues to behave as before but people can easily
> > opt into this option. No strong opinion because I am not sure but was
> > not sure if it was considered.
>
> I do not believe this would cause regressions. The description on gnu
> strip is:
>
> "Remove all symbols that are not needed for relocation processing in
> addition to debugging symbols and sections stripped by --strip-debug."
>
> The description on llvm-strip is:
>
> "Remove from the output all local or undefined symbols that are not
> required by relocations. Also remove all debug sections."
>
> gnu strip --strip-unneeded strips slightly more aggressively but it does
> not appear this causes any issues.
>
> >
> > Regardless:
> >
> > Reviewed-by: Nathan Chancellor <nathan@...nel.org>
>
> Thanks!
>


It is true --strip-unneeded drops a lot of compiler-generated symbols, but
it also drops real symbols that originate in the source code.

So, this would give user-visible changes for kallsyms at least.


$ riscv64-linux-gnu-nm  -n
/tmp/strip-unneeded/lib/modules/6.13.0-09760-g69e858e0b8b2/kernel/drivers/gpu/drm/amd/amdgpu/amdgpu.ko
 > /tmp/symbol-with-strip-unneeded
$ riscv64-linux-gnu-nm  -n
/tmp/strip-debug/lib/modules/6.13.0-09760-g69e858e0b8b2/kernel/drivers/gpu/drm/amd/amdgpu/amdgpu.ko
 > /tmp/symbol-with-strip-debug

$  diff -u /tmp/symbol-with-strip-debug  /tmp/symbol-with-strip-unneeded
 [ snip ]
 00000000001676cc t uvd_v6_0_ring_test_ring
 0000000000167802 t uvd_v6_0_ring_emit_pipeline_sync
 0000000000167a02 t uvd_v6_0_ring_emit_fence
-0000000000167b58 r CSWTCH.2
-0000000000167b68 r abm_settings
-0000000000167b80 r abm_config
-0000000000167b90 r min_reduction_table_v_2_2
-0000000000167ba0 r max_reduction_table_v_2_2
-0000000000167bb0 r min_reduction_table
-0000000000167bc0 r max_reduction_table
-0000000000167bd0 r custom_backlight_curve0
 0000000000167c38 r abm_settings_config2
 0000000000167c70 r abm_settings_config1
 0000000000167ca8 r abm_settings_config0




--
Best Regards
Masahiro Yamada