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

[Konrad Dybcio]

On 4/23/24 18:22, Ard Biesheuvel wrote:
> On Tue, 23 Apr 2024 at 18:01, Konrad Dybcio <konrad.dybcio@xxxxxxxxxx> wrote:
> > 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 ]---
>
> 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