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

Indu Bhagat
2025-03-09T14:44:25Z

On 2/27/25 10:47 PM, Indu Bhagat wrote:
> On 2/27/25 1:38 AM, Puranjay Mohan wrote:
>> Indu Bhagat <indu.bhagat@oracle.com> writes:
>>
>>> On 2/26/25 2:23 AM, Puranjay Mohan wrote:
>>>> Indu Bhagat <indu.bhagat@oracle.com> writes:
>>>>
>>>>> On 2/25/25 3:54 PM, Weinan Liu wrote:
>>>>>> On Tue, Feb 25, 2025 at 11:38 AM Indu Bhagat
>>>>>> <indu.bhagat@oracle.com> wrote:
>>>>>>>
>>>>>>> On Mon, Feb 10, 2025 at 12:30 AM Weinan Liu <wnliu@google.com>
>>>>>>> wrote:
>>>>>>>>> I already have a WIP patch to add sframe support to the kernel
>>>>>>>>> module.
>>>>>>>>> However, it is not yet working. I had trouble unwinding frames
>>>>>>>>> for the
>>>>>>>>> kernel module using the current algorithm.
>>>>>>>>>
>>>>>>>>> Indu has likely identified the issue and will be addressing it
>>>>>>>>> from the
>>>>>>>>> toolchain side.
>>>>>>>>>
>>>>>>>>> https://sourceware.org/bugzilla/show_bug.cgi?id=32666
>>>>>>>>
>>>>>>>> I have a working in progress patch that adds sframe support for
>>>>>>>> kernel
>>>>>>>> module.
>>>>>>>> https://github.com/heuza/linux/tree/sframe_unwinder.rfc
>>>>>>>>
>>>>>>>> According to the sframe table values I got during runtime
>>>>>>>> testing, looks
>>>>>>>> like the offsets are not correct .
>>>>>>>>
>>>>>>>
>>>>>>> I hope to sanitize the fix for 32666 and post upstream soon (I
>>>>>>> had to
>>>>>>> address other related issues).  Unless fixed, relocating .sframe
>>>>>>> sections using the .rela.sframe is expected to generate incorrect
>>>>>>> output.
>>>>>>>
>>>>>>>> When unwind symbols init_module(0xffff80007b155048) from the kernel
>>>>>>>> module(livepatch-sample.ko), the start_address of the FDE
>>>>>>>> entries in the
>>>>>>>> sframe table of the kernel modules appear incorrect.
>>>>>>>
>>>>>>> init_module will apply the relocations on the .sframe section,
>>>>>>> isnt it ?
>>>>>>>
>>>>>>>> For instance, the first FDE's start_addr is reported as -20564.
>>>>>>>> Adding
>>>>>>>> this offset to the module's sframe section address
>>>>>>>> (0xffff80007b15a040)
>>>>>>>> yields 0xffff80007b154fec, which is not within the livepatch-
>>>>>>>> sample.ko
>>>>>>>> memory region(It should be larger than 0xffff80007b155000).
>>>>>>>>
>>>>>>>
>>>>>>> Hmm..something seems off here.  Having tested a potential fix for
>>>>>>> 32666
>>>>>>> locally, I do not expect the first FDE to show this symptom.
>>>>>>>
>>>>>>
>>>>
>>>> Hi,
>>>>
>>>> Sorry for not responding in the past few days.  I was on PTO and was
>>>> trying to improve my snowboarding technique, I am back now!!
>>>>
>>>> I think what we are seeing is expected behaviour:
>>>>
>>>>    | For instance, the first FDE's start_addr is reported as -20564.
>>>> Adding
>>>>    | this offset to the module's sframe section address
>>>> (0xffff80007b15a040)
>>>>    | yields 0xffff80007b154fec, which is not within the livepatch-
>>>> sample.ko
>>>>    | memory region(It should be larger than 0xffff80007b155000).
>>>>
>>>>
>>>> Let me explain using a __dummy__ example.
>>>>
>>>> Assume Memory layout before relocation:
>>>>
>>>>    | Address | Element                                 | Relocation
>>>>    |  ....   | ....                                    |
>>>>    |   60    | init_module (start address)             |
>>>>    |   72    | init_module (end address)               |
>>>>    |  ....   | .....                                   |
>>>>    |   100   | Sframe section header start address     |
>>>>    |   128   | First FDE's start address               |
>>>> RELOC_OP_PREL -> Put init_module address (60) - current address (128)
>>>>
>>>> So, after relocation First FDE's start address has value 60 - 128 = -68
>>>>
>>>
>>> For SFrame FDE function start address is :
>>>
>>> "Signed 32-bit integral field denoting the virtual memory address of the
>>> described function, for which the SFrame FDE applies.  The value encoded
>>> in the ‘sfde_func_start_address’ field is the offset in bytes of the
>>> function’s start address, from the SFrame section."
>>>
>>> So, in your case, after applying the relocations, you will get:
>>> S + A - P = 60 - 128 = -68
>>>
>>> This is the distance of the function start address (60) from the current
>>> location in SFrame section (128)
>>>
>>> But what we intend to store is the distance of the function start
>>> address from the start of the SFrame section.  So we need to do an
>>> additional step for SFrame FDE:  Value += r_offset
>>
>> Thanks for the explaination, now it makes sense.
>>
>> But I couldn't find a relocation type in AARCH64 that does this extra +=
>> r_offset along with PREL32.
>>
>> The kernel's module loader is only doing the R_AARCH64_PREL32 which is
>> why we see this issue.
>>
>> How is this working even for the kernel itself? or for that matter, any
>> other binary compiled with sframe?
>>
>
> For the usual executables or shared objects, the calculations are
> applied by ld.bfd at this time.  Hence, the issue manifests in
> relocatable files.
>
>>  From my limited undestanding, the way to fix this would be to hack the
>> relocator to do this additional step while relocating .sframe sections.
>> Or the 'addend' values in .rela.sframe should already have the +r_offset
>> added to it, then no change to the relocator would be needed.
>>
>
> Of the two, adjusting the addend values in .rela.sframe may be a
> reasonable way to go about it.  Let me try it out in GAS and ld.bfd.
>

A fix for this is in the works (being discussed on the
binutils@sourceware list). I will keep you posted.

Thanks
Indu