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

[Puranjay Mohan]

Indu Bhagat <indu.bhagat@xxxxxxxxxx> writes:

> On 2/12/25 11:25 PM, Song Liu wrote:
>> On Wed, Feb 12, 2025 at 6:45 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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
.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

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