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

[Konrad Dybcio]

On 4/22/24 18:02, Masahiro Yamada wrote:
> On Sat, Apr 20, 2024 at 11:53 PM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
> > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Cc: linux-kbuild@xxxxxxxxxxxxxxx
> > Acked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> > Link: https://lore.kernel.org/all/20240415075837.2349766-5-ardb+git@xxxxxxxxxx
> > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > ---
> > 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