Re: [PATCH] powerpc/ftrace: fix syscall tracing on PPC64_ELF_ABI_V1

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2022-12-05 17:50, Michael Ellerman wrote:
> Michael Jeanson <mjeanson@efficios.com> writes:
>> On 2022-12-05 15:11, Michael Jeanson wrote:
>>>>>> Michael Jeanson <mjeanson@efficios.com> writes:
>>>>>>> In v5.7 the powerpc syscall entry/exit logic was rewritten in C, on
>>>>>>> PPC64_ELF_ABI_V1 this resulted in the symbols in the syscall table
>>>>>>> changing from their dot prefixed variant to the non-prefixed ones.
>>>>>>>
>>>>>>> Since ftrace prefixes a dot to the syscall names when matching them to
>>>>>>> build its syscall event list, this resulted in no syscall events being
>>>>>>> available.
>>>>>>>
>>>>>>> Remove the PPC64_ELF_ABI_V1 specific version of
>>>>>>> arch_syscall_match_sym_name to have the same behavior across all powerpc
>>>>>>> variants.
>>>>>>
>>>>>> This doesn't seem to work for me.
>>>>>>
>>>>>> Event with it applied I still don't see anything in
>>>>>> /sys/kernel/debug/tracing/events/syscalls
>>>>>>
>>>>>> Did we break it in some other way recently?
>>>>>>
>>>>>> cheers
>>
>> I did some further testing, my config also enabled KALLSYMS_ALL, when I remove
>> it there is indeed no syscall events.
> 
> Aha, OK that explains it I guess.
> 
> I was using ppc64_guest_defconfig which has ABI_V1 and FTRACE_SYSCALLS,
> but does not have KALLSYMS_ALL. So I guess there's some other bug
> lurking in there.

I don't have the setup handy to validate it, but I suspect it is caused 
by the way scripts/kallsyms.c:symbol_valid() checks whether a symbol 
entry needs to be integrated into the assembler output when 
--all-symbols is not specified. It only keeps symbols which addresses 
are in the text range. On PPC64_ELF_ABI_V1, this means only the 
dot-prefixed symbols will be kept (those point to the function begin), 
leaving out the non-dot-prefixed symbols (those point to the function 
descriptors).

So I see two possible solutions there: either we ensure that 
FTRACE_SYSCALLS selects KALLSYMS_ALL on PPC64_ELF_ABI_V1, or we modify 
scripts/kallsyms.c:symbol_valid() to also include function descriptor 
symbols. This would mean accepting symbols pointing into the .opd ELF 
section.

IMHO the second option would be better because it does not increase the 
kernel image size as much as KALLSYMS_ALL.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com