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

Song Liu
2025-02-13T19:40:55Z

On Wed, Feb 12, 2025 at 11:46 PM Puranjay Mohan <puranjay@kernel.org> wrote:
>
> Song Liu <song@kernel.org> writes:
>
> > On Wed, Feb 12, 2025 at 6:45 PM Josh Poimboeuf <jpoimboe@kernel.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