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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Baoquan He <bhe@redhat.com>
Cc:     "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        kexec@lists.infradead.org,
        Andrew Morton <akpm@linux-foundation.org>
References: <20220518181828.645877-1-naveen.n.rao@linux.vnet.ibm.com>
        <87ee0q7b92.fsf@email.froward.int.ebiederm.org>
        <YoWySwbszfdZS9LU@MiWiFi-R3L-srv>
Date:   Thu, 19 May 2022 12:59:17 -0500
In-Reply-To: <YoWySwbszfdZS9LU@MiWiFi-R3L-srv> (Baoquan He's message of "Thu,
        19 May 2022 10:58:19 +0800")
Message-ID: <87bkvt4d56.fsf@email.froward.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)
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Subject: Re: [PATCH] kexec_file: Drop weak attribute from
 arch_kexec_apply_relocations[_add]
Precedence: bulk

Baoquan He <bhe@redhat.com> writes:

> Hi Eric,
>
> On 05/18/22 at 04:59pm, Eric W. Biederman wrote:
>> "Naveen N. Rao" <naveen.n.rao@linux.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.
>> 
>> 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.
>
> In my mind, __weak is very simple and clear as a mechanism to add
> ARCH related functionality.

You should be able to trace the conversation back for all of the details
but if you can't here is the summary.

There is a tool that some architectures use called recordmcount.  The
recordmcount looks for a symbol in a section, and ignores all weak
symbols.  In certain cases sections become so simple there are only weak
symbols.  At which point recordmcount fails.

Which means in practice __weak symbols are unreliable and don't work
to add ARCH related functionality.

Given that __weak symbols fail randomly I would much rather have simpler
code that doesn't fail.  It has never been the case that __weak symbols
have been very common in the kernel.  I expect they are something like
bool that have been gaining traction.  Still given that __weak symbols
don't work.  I don't want them.

Eric