linux-kernel - Re: [PATCH v4 0/3] kbuild: Avoid weak external linkage where possible

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Message-ID: <CAMj1kXFt5kbZ8yFgO-jU5ZP3-WZi5ZZJKKTCpEYRdUYFRj9CYQ@mail.gmail.com>
Date: Sat, 20 Apr 2024 15:59:53 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: patchwork-bot+netdevbpf@...nel.org, Ard Biesheuvel <ardb+git@...gle.com>, 
	linux-kernel@...r.kernel.org, arnd@...db.de, martin.lau@...ux.dev, 
	linux-arch@...r.kernel.org, linux-kbuild@...r.kernel.org, bpf@...r.kernel.org, 
	andrii@...nel.org, olsajiri@...il.com
Subject: Re: [PATCH v4 0/3] kbuild: Avoid weak external linkage where possible

On Sat, 20 Apr 2024 at 15:56, Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Sat, 20 Apr 2024 at 15:42, Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > On Sat, Apr 20, 2024 at 9:35 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> > >
> > > On Sat, 20 Apr 2024 at 14:32, Masahiro Yamada <masahiroy@...nel.org> wrote:
> > > >
> > > > On Fri, Apr 19, 2024 at 4:57 PM Ard Biesheuvel <ardb@...nelorg> wrote:
> > > > >
> > > > > On Tue, 16 Apr 2024 at 16:40, <patchwork-bot+netdevbpf@...nel.org> wrote:
> > > > > >
> > > > > > Hello:
> > > > > >
> > > > > > This series was applied to bpf/bpf-next.git (master)
> > > > > > by Daniel Borkmann <daniel@...earbox.net>:
> > > > > >
> > > > > > On Mon, 15 Apr 2024 18:20:42 +0200 you wrote:
> > > > > > > From: Ard Biesheuvel <ardb@...nel.org>
> > > > > > >
> > > > > > > Weak external linkage is intended for cases where a symbol reference
> > > > > > > can remain unsatisfied in the final link. Taking the address of such a
> > > > > > > symbol should yield NULL if the reference was not satisfied.
> > > > > > >
> > > > > > > Given that ordinary RIP or PC relative references cannot produce NULL,
> > > > > > > some kind of indirection is always needed in such cases, and in position
> > > > > > > independent code, this results in a GOT entry. In ordinary code, it is
> > > > > > > arch specific but amounts to the same thing.
> > > > > > >
> > > > > > > [...]
> > > > > >
> > > > > > Here is the summary with links:
> > > > > >   - [v4,1/3] kallsyms: Avoid weak references for kallsyms symbols
> > > > > >     (no matching commit)
> > > > > >   - [v4,2/3] vmlinux: Avoid weak reference to notes section
> > > > > >     (no matching commit)
> > > > > >   - [v4,3/3] btf: Avoid weak external references
> > > > > >     https://git.kernel.org/bpf/bpf-next/c/fc5eb4a84e4c
> > > > > >
> > > > >
> > > > >
> > > > > Thanks.
> > > > >
> > > > > Masahiro, could you pick up patches #1 and #2 please?
> > > > >
> > > >
> > > >
> > > > I do not like PROVIDE() because it potentially shifts
> > > > a build error (i.e. link error) into
> > > > a run-time error, which is usually more difficult to debug
> > > > than build error.
> > > >
> > > > If someone references the kallsyms_* symbols
> > > > when CONFIG_KALLSYMS=n, it is likely a mistake.
> > > > In general, it should be reported as a link error.
> > > >
> > >
> > > OK, so the PROVIDE() should be conditional on CONFIG_KALLSYM=y. I can fix that.
> >
> >
> > You may need to take care of the dependency
> > between CONFIG_KALLSYMS and CONFIG_VMCORE_INFO
> > because kernel/vmcore_info.c has references
> > to the kallsyms_* symbols.
> >
> > (I am still not a big fan of PROVIDE() though)
> >
>
>
> OK, how about we use weak definitions (as opposed to weak references)
> in kernel/kallsyms.c, which will get superseded by the actual ones in
> the second linker pass.
>
> The only difference is that we will use some space in the binary for
> the weak definitions that are never used in the final build.

Btw those references in kernel/vmcore_info.c are guarded by #ifdef
CONFIG_KALLSYMS=y too.