Re: [PATCH] kexec_file: Drop weak attribute from arch_kexec_apply_relocations[_add]

[Naveen N. Rao]

Baoquan He wrote:
> Hi Eric,
>
> On 05/18/22 at 04:59pm, Eric W. Biederman wrote:
> > "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> writes:
> >
> > > Since commit d1bcae833b32f1 ("ELF: Don't generate unused section
> > > symbols") [1], binutils (v2.36+) started dropping section symbols that
> > > it thought were unused.  This isn't an issue in general, but with
> > > kexec_file.c, gcc is placing kexec_arch_apply_relocations[_add] into a
> > > separate .text.unlikely section and the section symbol ".text.unlikely"
> > > is being dropped. Due to this, recordmcount is unable to find a non-weak
> > > symbol in .text.unlikely to generate a relocation record against.
> > >
> > > Address this by dropping the weak attribute from these functions:
> > > - arch_kexec_apply_relocations() is not overridden by any architecture
> > >   today, so just drop the weak attribute.
> > > - arch_kexec_apply_relocations_add() is only overridden by x86 and s390.
> > >   Retain the function prototype for those and move the weak
> > >   implementation into the header as a static inline for other
> > >   architectures.
> > >
> > > [1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1
> >
> > Any chance you can also get machine_kexec_post_load,
> > crash_free_reserved_phys_range, arch_kexec_protect_protect_crashkres,
> > arch_kexec_unprotect_crashkres, arch_kexec_kernel_image_probe,
> > arch_kexec_kernel_image_probe, arch_kimage_file_post_load_cleanup,
> > arch_kexec_kernel_verify_sig, and arch_kexec_locate_mem_hole as well.

I've posted a v2 that uses the approach suggested by Michael, and 
something that was in use in kexec already. If you are ok with that 
approach, I will take a stab at converting the rest of the functions 
that are marked __weak.

> > That is everything in kexec that uses a __weak symbol.  If we can't
> > count on them working we might as well just get rid of the rest
> > preemptively.
>
> Is there a new rule that __weak is not suggested in kernel any more?
> Please help provide a pointer if yes, so that I can learn that.

I'm not aware of a move away from __weak in the kernel, in general. 
Steven doesn't prefer it for ftrace, and other maintainers may have a 
preference.

> In my mind, __weak is very simple and clear as a mechanism to add
> ARCH related functionality.

Notwithstanding the ftrace issue, the other caveat with __weak functions 
are that they still make it into the final vmlinux even if they are 
overridden. That is, you will have instructions from both the __weak 
variant as well as from the overridden variant in the final vmlinux, 
which can add up if the weak variants are non-trivial. 


- Naveen