linux-kernel - Re: [PATCH v5] kallsyms: Avoid weak references for kallsyms symbols

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Date: Tue, 23 Apr 2024 18:01:44 +0200
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Masahiro Yamada <masahiroy@...nel.org>,
 Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
 linux-kbuild@...r.kernel.org, Nick Desaulniers <ndesaulniers@...gle.com>,
 Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v5] kallsyms: Avoid weak references for kallsyms symbols



On 4/22/24 18:02, Masahiro Yamada wrote:
> On Sat, Apr 20, 2024 at 11:53 PM Ard Biesheuvel <ardb+git@...gle.com> wrote:
>>
>> From: Ard Biesheuvel <ardb@...nel.org>
>>
>> kallsyms is a directory of all the symbols in the vmlinux binary, and so
>> creating it poses 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 on the declarations, and instead,
>> provide fallback definitions with weak linkage. This informs the
>> compiler that ultimately, the reference will always be satisfied.
>>
>> While at it, drop the FRV specific annotation that these symbols reside
>> in .rodata - FRV is long gone.
>>
>> Cc: Masahiro Yamada <masahiroy@...nel.org>
>> Cc: linux-kbuild@...r.kernel.org
>> Acked-by: Nick Desaulniers <ndesaulniers@...gle.com>
>> Acked-by: Kees Cook <keescook@...omium.org>
>> Acked-by: Arnd Bergmann <arnd@...db.de>
>> Link: https://lore.kernel.org/all/20240415075837.2349766-5-ardb+git@google.com
>> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
>> ---
>> v5: - avoid PROVIDE() in the linker script, use weak definitions instead
>>      - drop tested-by, replace reviewed-by with acked-by
>>
> 
> Applied to linux-kbuild. Thanks.

Hi, this commit seems to break call traces, resulting in output like:

[    2.777006] Call trace:
[    2.777007]  _text+0x89e7e8/0x39e0000
[    2.777008]  _text+0x89e82c/0x39e0000
[    2.777009]  _text+0x2b940cc/0x2bd2a90
[    2.777011]  _text+0x2b941a4/0x2bd2a90
[    2.777012]  _text+0x145dc/0x39e0000
[    2.777014]  _text+0x2b51184/0x2bd2a90
[    2.777016]  _text+0x18fc6a4/0x39e0000
[    2.777018]  _text+0x15644/0x39e0000
[    2.777019] ---[ end trace 0000000000000000 ]---

Konrad