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Date: Tue, 09 Apr 2024 17:04:36 +0200
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Ard Biesheuvel" <ardb+git@google.com>, linux-kernel@vger.kernel.org
Cc: "Ard Biesheuvel" <ardb@kernel.org>,
 "Masahiro Yamada" <masahiroy@kernel.org>,
 "Martin KaFai Lau" <martin.lau@linux.dev>,
 Linux-Arch <linux-arch@vger.kernel.org>, linux-kbuild@vger.kernel.org,
 bpf@vger.kernel.org, "Andrii Nakryiko" <andrii@kernel.org>,
 "Nick Desaulniers" <ndesaulniers@google.com>,
 "Kees Cook" <keescook@chromium.org>
Subject: Re: [PATCH v2 1/3] kallsyms: Avoid weak references for kallsyms symbols
Content-Type: text/plain

On Tue, Apr 9, 2024, at 17:01, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
>
> kallsyms is a directory of all the symbols in the vmlinux binary, and so
> creating it is somewhat of a chicken-and-egg problem, as its non-zero
> size affects the layout of the binary, and therefore the values of the
> symbols.
>
> For this reason, the kernel is linked more than once, and the first pass
> does not include any kallsyms data at all. For the linker to accept
> this, the symbol declarations describing the kallsyms metadata are
> emitted as having weak linkage, so they can remain unsatisfied. During
> the subsequent passes, the weak references are satisfied by the kallsyms
> metadata that was constructed based on information gathered from the
> preceding passes.
>
> Weak references lead to somewhat worse codegen, because taking their
> address may need to produce NULL (if the reference was unsatisfied), and
> this is not usually supported by RIP or PC relative symbol references.
>
> Given that these references are ultimately always satisfied in the final
> link, let's drop the weak annotation, and instead, provide fallback
> definitions in the linker script that are only emitted if an unsatisfied
> reference exists.
>
> While at it, drop the FRV specific annotation that these symbols reside
> in .rodata - FRV is long gone.
>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com> # Boot
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Link: https://lkml.kernel.org/r/20230504174320.3930345-1-ardb%40kernel.org
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Acked-by: Arnd Bergmann <arnd@arndb.de>