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

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Message-ID: <CAK7LNAS5RVtm078rLFJNxQwPf+1VH+zf12dQJ1Xh-Wc4_qFGTw@mail.gmail.com>
Date: Thu, 6 Feb 2025 00:00:40 +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 Wed, Feb 5, 2025 at 3:29 AM Charlie Jenkins <charlie@...osinc.com> wrote:
>
> On Tue, Feb 04, 2025 at 01:04:26PM +0900, Masahiro Yamada wrote:
> > On Sat, Feb 1, 2025 at 6:33 AM Charlie Jenkins <charlie@...osinc.com> wrote:
> > >
> > > On Sat, Feb 01, 2025 at 12:10:02AM +0900, Masahiro Yamada wrote:
> > > > 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.
> > >
> > > Adding INSTALL_MOD_STRIP="--strip-unneeded" would be sufficient for
> > > riscv. However, this has the downside that riscv will require different
> > > flags than other architectures to get reasonably sized modules.
> >
> > You can use INSTALL_MOD_STRIP=--strip-unneeded for all architecture if you like.
> >
> > I assume this is a riscv issue. Specifically, riscv gcc.
> > With LLVM=1, I see much smaller riscv modules using INSTALL_MOD_STRIP=1.
> >
> > --strip-unneeded is needlessly aggressive for other architectures,
> > and I do not see a good reason to change the default.
>
> Yes it is primarily an issue with riscv GCC. I was hoping for something
> more standardized so that other people using riscv GCC wouldn't
> encounter this. Would it be reasonable to add this flag by default only
> for the riscv architecture, or do you think it's just better to leave it
> up to the user's choice?

The latter.

INSTALL_MOD_STRIP=1 passes --strip-debug.
This is clearly documented in Documentation/kbuild/makefiles.rst
and it has worked like that for many years, with no exception.

If users want it to work differently, the flexibility is already there.

If INSTALL_MOD_STRIP=1 worked differently, silently only for riscv,
I would regard it as insane behavior.


-- 
Best Regards
Masahiro Yamada