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

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Message-ID: <CAPhsuW7dV7UR3PsGVx_DLBH5-95DAmLMGTPuY0NfUwWLZMSTrQ@mail.gmail.com>
Date: Thu, 13 Feb 2025 11:40:43 -0800
From: Song Liu <song@...nel.org>
To: Puranjay Mohan <puranjay@...nel.org>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, 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

On Wed, Feb 12, 2025 at 11:46 PM Puranjay Mohan <puranjay@...nel.org> wrote:
>
> 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.

Yeah, objdump does show the same disassembly. However, if
I open the file with gdb, and do "disassemble copy_process",
the one in livepatch-special-static.o looks very weird.

Thanks,
Song