Date: Tue, 23 Apr 2024 18:24:05 +0200
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>,
 Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.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 4/23/24 18:22, Ard Biesheuvel wrote:
> On Tue, 23 Apr 2024 at 18:01, Konrad Dybcio <konrad.dybcio@...aro.org> wrote:
>>
>>
>>
>> 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 ]---
>>
> 
> This patch triggers an issue in the compiler, which appears to perform
> constant propagation on variables defined as weak, and this is
> definitely a compiler bug. (A weak variable can be superseded by
> another instance from a different object at link time, so the compiler
> cannot make assumptions based on the version of the variable it
> observes at compile time)

Sounds like fun..

> 
> It has already been dropped from the kbuild tree.

Thanks!

Konrad

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