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

Indu Bhagat
2025-02-26T00:22:35Z

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.
>>
>
> Yes, I think init_module will apply the relocation as well.
> To further investigate, here's the relevant relocation and symbol table
> information for the kernel module:
>
> Relocation section '.rela.sframe' at offset 0x28350 contains 3 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 00000000001c  000100000105 R_AARCH64_PREL32  0000000000000000 .text + 8
> 000000000030  000100000105 R_AARCH64_PREL32  0000000000000000 .text + 28
> 000000000044  000100000105 R_AARCH64_PREL32  0000000000000000 .text + 68
>

The offsets look OK..

> Symbol table '.symtab' contains 68 entries:
>    Num:    Value          Size Type    Bind   Vis      Ndx Name
>      0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
>      1: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 .text
> ...
>     32: 0000000000000008    12 FUNC    LOCAL  DEFAULT    1 livepatch_exit
>     33: 0000000000000008     0 NOTYPE  LOCAL  DEFAULT    3 $d
>     34: 0000000000000028    44 FUNC    LOCAL  DEFAULT    1 livepatch_init
>     35: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    9 $d
>     36: 0000000000000010     0 NOTYPE  LOCAL  DEFAULT    3 $d
>     37: 0000000000000068    56 FUNC    LOCAL  DEFAULT    1 livepatch_cmdlin[...]
> ...
>     63: 0000000000000008    12 FUNC    GLOBAL DEFAULT    1 cleanup_module
>     64: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND klp_enable_patch
>     65: 0000000000000028    44 FUNC    GLOBAL DEFAULT    1 init_module