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

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Message-ID: <d91eba9a-dbd1-488f-8e1b-bc5121c30cd1@oracle.com>
Date: Fri, 14 Feb 2025 10:41:13 -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 2/14/25 9:39 AM, Indu Bhagat wrote:
> 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.
> 

Not wrongly named. --prefix-alloc-sections=.init is expected to do that 
as .sframe is an allocated 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
>