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

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Message-ID: <f8d93a1b-3ad0-4e19-846f-c08d9cb19f48@oracle.com>
Date: Fri, 14 Feb 2025 09:39:35 -0800
From: Indu Bhagat <indu.bhagat@...cle.com>
To: Puranjay Mohan <puranjay@...nel.org>, Song Liu <song@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Weinan Liu <wnliu@...gle.com>, Steven Rostedt <rostedt@...dmis.org>,
        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 2/13/25 11:57 PM, Puranjay Mohan wrote:
> Indu Bhagat <indu.bhagat@...cle.com> writes:
> 
>> On 2/12/25 11:25 PM, Song Liu wrote:
>>> 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?
>>>
>>
>> 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