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

Indu Bhagat
2025-02-26T17:41:33Z

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

-68 + 28 = -40
Where 28 is the r_offset in the RELA.

So we really expect a -40 in the relocated SFrame section instead of -68
above. IOW, the RELAs of SFrame section will need an additional step
after relocation.

> Now, while doing unwinding we Try to add this value to the sframe
> section header's start address which is in this example 100,
>
> so 100 + (-68) = 32
>
> So, 32 is not within [60, 72], i.e. within init_module.
>
> You can see that it is possible for this value to be less than the start
> address of the module's memory region when this function's address is
> very close to the start of the memory region.
>
> The crux is that the offset in the FDE's start address is calculated
> based on the address of the FDE's start_address and not based on the
> address of the sframe section.
>
>
> Thanks,
> Puranjay