linux-kernel - Re: [PATCH 0/8] unwind, arm64: add sframe unwinder for kernel

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Message-ID: <mb61py0yaz3qq.fsf@kernel.org>
Date: Thu, 13 Feb 2025 07:46:21 +0000
From: Puranjay Mohan <puranjay@...nel.org>
To: Song Liu <song@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Weinan Liu <wnliu@...gle.com>, Steven Rostedt <rostedt@...dmis.org>,
 Indu Bhagat <indu.bhagat@...cle.com>, Peter Zijlstra
 <peterz@...radead.org>, Mark Rutland <mark.rutland@....com>,
 roman.gushchin@...ux.dev, Will Deacon <will@...nel.org>, Ian Rogers
 <irogers@...gle.com>, linux-toolchains@...r.kernel.org,
 linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
 joe.lawrence@...hat.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/8] unwind, arm64: add sframe unwinder for kernel

Song Liu <song@...nel.org> writes:

> On Wed, Feb 12, 2025 at 6:45 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>>
>> On Wed, Feb 12, 2025 at 06:36:04PM -0800, Song Liu wrote:
>> > > > [   81.261748]  copy_process+0xfdc/0xfd58 [livepatch_special_static]
>> > >
>> > > Does that copy_process+0xfdc/0xfd58 resolve to this line in
>> > > copy_process()?
>> > >
>> > >                         refcount_inc(&current->signal->sigcnt);
>> > >
>> > > Maybe the klp rela reference to 'current' is bogus, or resolving to the
>> > > wrong address somehow?
>> >
>> > It resolves the following line.
>> >
>> > p->signal->tty = tty_kref_get(current->signal->tty);
>> >
>> > I am not quite sure how 'current' should be resolved.
>>
>> Hm, on arm64 it looks like the value of 'current' is stored in the
>> SP_EL0 register.  So I guess that shouldn't need any relocations.
>>
>> > The size of copy_process (0xfd58) is wrong. It is only about
>> > 5.5kB in size. Also, the copy_process function in the .ko file
>> > looks very broken. I will try a few more things.
>
> When I try each step of kpatch-build, the copy_process function
> looks reasonable (according to gdb-disassemble) in fork.o and
> output.o. However, copy_process looks weird in livepatch-special-static.o,
> which is generated by ld:
>
> ld -EL  -maarch64linux -z norelro -z noexecstack
> --no-warn-rwx-segments -T ././kpatch.lds  -r -o
> livepatch-special-static.o ./patch-hook.o ./output.o
>
> I have attached these files to the email. I am not sure whether
> the email server will let them through.

I think, I am missing something here,

I did :

objdump -Dr livepatch-special-static.o | less

and

objdump -Dr output.o | less

and the disassembly of copy_process() looks exactly same.

> Indu, does this look like an issue with ld?
>
> Thanks,
> Song

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