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SubjectRe: [PATCH v5] kallsyms: Avoid weak references for kallsyms symbols
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On 4/22/24 18:02, Masahiro Yamada wrote:
> On Sat, Apr 20, 2024 at 11:53 PM Ard Biesheuvel <ardb+git@google.com> wrote:
>>
>> From: Ard Biesheuvel <ardb@kernel.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@kernel.org>
>> Cc: linux-kbuild@vger.kernel.org
>> Acked-by: Nick Desaulniers <ndesaulniers@google.com>
>> Acked-by: Kees Cook <keescook@chromium.org>
>> Acked-by: Arnd Bergmann <arnd@arndb.de>
>> Link: https://lore.kernel.org/all/20240415075837.2349766-5-ardb+git@google.com
>> Signed-off-by: Ard Biesheuvel <ardb@kernel.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

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