> On 10 Feb 2021, at 01:02, Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> On Tue, Feb 09, 2021 at 12:09:31PM -0800, Nick Desaulniers wrote:
>> On Tue, Feb 9, 2021 at 11:06 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>>>
>>> On Tue, Feb 09, 2021 at 05:13:42PM +0100, Jiri Olsa wrote:
>>>> On Tue, Feb 09, 2021 at 04:09:36PM +0100, Jiri Olsa wrote:
>>>>
>>>> SNIP
>>>>
>>>>>>>>> DW_AT_prototyped (true)
>>>>>>>>> DW_AT_type (0x01cfdfe4 "long int")
>>>>>>>>> DW_AT_external (true)
>>>>>>>>>
>>>>>>>>
>>>>>>>> Ok, the problem appears to be not in DWARF, but in mcount_loc data.
>>>>>>>> vfs_truncate's address is not recorded as ftrace-attachable, and thus
>>>>>>>> pahole ignores it. I don't know why this happens and it's quite
>>>>>>>> strange, given vfs_truncate is just a normal global function.
>>>>>>
>>>>>> right, I can't see it in mcount adresses.. but it begins with instructions
>>>>>> that appears to be nops, which would suggest it's traceable
>>>>>>
>>>>>> ffff80001031f430 <vfs_truncate>:
>>>>>> ffff80001031f430: 5f 24 03 d5 hint #34
>>>>>> ffff80001031f434: 1f 20 03 d5 nop
>>>>>> ffff80001031f438: 1f 20 03 d5 nop
>>>>>> ffff80001031f43c: 3f 23 03 d5 hint #25
>>>>>>
>>>>>>>>
>>>>>>>> I'd like to understand this issue before we try to fix it, but there
>>>>>>>> is at least one improvement we can make: pahole should check ftrace
>>>>>>>> addresses only for static functions, not the global ones (global ones
>>>>>>>> should be always attachable, unless they are special, e.g., notrace
>>>>>>>> and stuff). We can easily check that by looking at the corresponding
>>>>>>>> symbol. But I'd like to verify that vfs_truncate is ftrace-attachable
>>>>>
>>>>> I'm still trying to build the kernel.. however ;-)
>>>>
>>>> I finally reproduced.. however arm's not using mcount_loc
>>>> but some other special section.. so it's new mess for me
>>>
>>> so ftrace data actualy has vfs_truncate address but with extra 4 bytes:
>>>
>>> ffff80001031f434
>>>
>>> real vfs_truncate address:
>>>
>>> ffff80001031f430 g F .text 0000000000000168 vfs_truncate
>>>
>>> vfs_truncate disasm:
>>>
>>> ffff80001031f430 <vfs_truncate>:
>>> ffff80001031f430: 5f 24 03 d5 hint #34
>>> ffff80001031f434: 1f 20 03 d5 nop
>>> ffff80001031f438: 1f 20 03 d5 nop
>>> ffff80001031f43c: 3f 23 03 d5 hint #25
>>>
>>> thats why we don't match it in pahole.. I checked few other functions
>>> and some have the same problem and some match the function boundary
>>>
>>> those that match don't have that first hint instrucion, like:
>>>
>>> ffff800010321e40 <do_faccessat>:
>>> ffff800010321e40: 1f 20 03 d5 nop
>>> ffff800010321e44: 1f 20 03 d5 nop
>>> ffff800010321e48: 3f 23 03 d5 hint #25
>>>
>>> any hints about hint instructions? ;-)
>>
>> aarch64 makes *some* newer instructions reuse the "hint" ie "nop"
>> encoding space to make software backwards compatible on older hardware
>> that doesn't support such instructions. Is this BTI, perhaps? (The
>> function is perhaps the destination of an indirect call?)
>
> It seems like it. The issue is not reproducible when
> CONFIG_ARM64_BTI_KERNEL is not set.
>
llvm-objdump --mattr=pa —mattr=bti -d … will print new mnemonic for the hint space instructions.
It is intentional to put a landing pad (BTI) at the very beginning for the patchable functions.
ffff80001031f430 <vfs_truncate>:
ffff80001031f430: 5f 24 03 d5 hint #34 // BTI C <— landing pad for indirect calls.
ffff80001031f434: 1f 20 03 d5 nop // <— the to be patched area.
ffff80001031f438: 1f 20 03 d5 nop
ffff80001031f43c: 3f 23 03 d5 hint #25 // paciasp <— protecting the link register..
The ftrace data(ffff80001031f434) seems correct because it should point to the “nops".
HTH
Cheers,
Daniel
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