linux-kernel - Re: [PATCH] kexec_file: Drop weak attribute from arch_kexec_apply_relocations[

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Message-Id: <1652951723.o9i6ngwfda.naveen@linux.ibm.com>
Date:   Thu, 19 May 2022 14:58:20 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Baoquan He <bhe@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH] kexec_file: Drop weak attribute from
 arch_kexec_apply_relocations[_add]

Baoquan He wrote:
> Hi Eric,
> 
> On 05/18/22 at 04:59pm, Eric W. Biederman wrote:
>> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> 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