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

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Message-ID: <CAPhsuW4iDuTBfZowJRhxLFyK=g=s+-pK2Eq4+SNj9uL99eNkmw@mail.gmail.com>
Date: Wed, 12 Feb 2025 23:25:37 -0800
From: Song Liu <song@...nel.org>
To: 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, 
	Puranjay Mohan <puranjay@...nel.org>
Subject: Re: [PATCH 0/8] unwind, arm64: add sframe unwinder for kernel

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.

Indu, does this look like an issue with ld?

Thanks,
Song

Download attachment "patch-hook.o" of type "application/octet-stream" (204128 bytes)

Download attachment "output.o" of type "application/octet-stream" (1214808 bytes)

Download attachment "livepatch-special-static.o" of type "application/octet-stream" (1417232 bytes)