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

Indu Bhagat
2025-02-14T17:40:10Z

On 2/13/25 11:57 PM, Puranjay Mohan wrote:
> Indu Bhagat <indu.bhagat@oracle.com> writes:
>
>> On 2/12/25 11:25 PM, Song Liu wrote:
>>> 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.
>>>
>>> Indu, does this look like an issue with ld?
>>>
>>
>> Sorry for the delay.
>>
>> Looks like there has been progress since and issue may be elsewhere, but:
>>
>> FWIW, I looked at the .sframe and .rela.sframe sections here, the data
>> does look OK. I noted that there is no .sframe for copy_process () in
>> output.o... I will take a look into it.
>
> Hi Indu,
>
> I saw another issue in my kernel build with sframes enabled (-Wa,--gsframe):
>
> ld: warning: orphan section `.init.sframe' from `arch/arm64/kernel/pi/lib-fdt.pi.o' being placed in section `.init.sframe'
> [... Many more similar warnings (.init.sframe) ...]
>
> So, this orphan sections is generated in the build process.
>
> I am using GNU ld version 2.41-50 and gcc (GCC) 11.4.1
>
> Is this section needed for sframes to work? or can we discard

No this should not be discarded. This looks like a wrongly-named but
valid SFrame section.

Once correctly named as .sframe, the linker should do the right thing.
Let me check whats going on..

> .init.sframe section with a patch like following to the linker script:
>
> -- 8< --
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 6a437bd08..8e704c0a6 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -1044,9 +1044,16 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
> # define SANITIZER_DISCARDS
> #endif
>
> +#if defined(CONFIG_SFRAME_UNWIND_TABLE)
> +#define DISCARD_INIT_SFRAME *(.init.sframe)
> +#else
> +#define DISCARD_INIT_SFRAME
> +#endif
> +
> #define COMMON_DISCARDS \
> SANITIZER_DISCARDS \
> PATCHABLE_DISCARDS \
> + DISCARD_INIT_SFRAME \
> *(.discard) \
> *(.discard.*) \
> *(.export_symbol) \
>
> -- >8 --
>
> Thanks,
> Puranjay