linux-kernel - Re: [PATCH 2/3] kbuild: remove PROVIDE() for kallsyms symbols

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Date: Thu, 23 May 2024 11:31:49 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, linux-arch@...r.kernel.org
Subject: Re: [PATCH 2/3] kbuild: remove PROVIDE() for kallsyms symbols

On Wed, 22 May 2024 at 13:48, Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> This reimplements commit 951bcae6c5a0 ("kallsyms: Avoid weak references
> for kallsyms symbols").
>
> I am not a big fan of PROVIDE() because it always satisfies the linker
> even in situations that should result in a link error. In other words,
> it can potentially shift a compile-time error into a run-time error.
>

I don't disagree. However, I did realize that, in this particular
case, we could at least make the preliminary symbol definitions
conditional on CONFIG_KALLSYMS rather than always providing them.

This approach is also fine with me, though.


> Duplicating kallsyms_* in vmlinux.lds.h also reduces maintainability.
>
> As an alternative solution, this commit prepends one more kallsyms step.
>
>     KSYMS   .tmp_vmlinux.kallsyms0.S          # added
>     AS      .tmp_vmlinux.kallsyms0.o          # added
>     LD      .tmp_vmlinux.btf
>     BTF     .btf.vmlinux.bin.o
>     LD      .tmp_vmlinux.kallsyms1
>     NM      .tmp_vmlinux.kallsyms1.syms
>     KSYMS   .tmp_vmlinux.kallsyms1.S
>     AS      .tmp_vmlinux.kallsyms1.o
>     LD      .tmp_vmlinux.kallsyms2
>     NM      .tmp_vmlinux.kallsyms2.syms
>     KSYMS   .tmp_vmlinux.kallsyms2.S
>     AS      .tmp_vmlinux.kallsyms2.o
>     LD      vmlinux
>
> Step 0 takes /dev/null as input, and generates .tmp_vmlinux.kallsyms0.o,
> which has a valid kallsyms format with the empty symbol list, and can be
> linked to vmlinux. Since it is really small, the added compile-time cost
> is negligible.
>

OK, so the number of linker invocations is the same, right? The
difference is that the kallsyms symbol references are satisfied by a
dummy object?

That seems reasonable to me.

For the series,

Acked-by: Ard Biesheuvel <ardb@...nel.org>