Re: [PATCH dwarves 3/3] dwarf_loader: Check DW_OP_[GNU_]entry_value for possible parameter matching

[Yonghong Song <yonghong.song@linux.dev>]

On 11/11/24 1:54 AM, Jiri Olsa wrote:
> On Fri, Nov 08, 2024 at 10:05:24AM -0800, Yonghong Song wrote:
>> Song Liu reported that a kernel func (perf_event_read()) cannot be traced
>> in certain situations since the func is not in vmlinux bTF. This happens
>> in kernels 6.4, 6.9 and 6.11 and the kernel is built with pahole 1.27.
>>
>> The perf_event_read() signature in kernel (kernel/events/core.c):
>>     static int perf_event_read(struct perf_event *event, bool group)
>>
>> Adding '-V' to pahole command line, and the following error msg can be found:
>>     skipping addition of 'perf_event_read'(perf_event_read) due to unexpected register used for parameter
>>
>> Eventually the error message is attributed to the setting
>> (parm->unexpected_reg = 1) in parameter__new() function.
>>
>> The following is the dwarf representation for perf_event_read():
>>      0x0334c034:   DW_TAG_subprogram
>>                  DW_AT_low_pc    (0xffffffff812c6110)
>>                  DW_AT_high_pc   (0xffffffff812c640a)
>>                  DW_AT_frame_base        (DW_OP_reg7 RSP)
>>                  DW_AT_GNU_all_call_sites        (true)
>>                  DW_AT_name      ("perf_event_read")
>>                  DW_AT_decl_file ("/rw/compile/kernel/events/core.c")
>>                  DW_AT_decl_line (4641)
>>                  DW_AT_prototyped        (true)
>>                  DW_AT_type      (0x03324f6a "int")
>>      0x0334c04e:     DW_TAG_formal_parameter
>>                    DW_AT_location        (0x007de9fd:
>>                       [0xffffffff812c6115, 0xffffffff812c6141): DW_OP_reg5 RDI
>>                       [0xffffffff812c6141, 0xffffffff812c6323): DW_OP_reg14 R14
>>                       [0xffffffff812c6323, 0xffffffff812c63fe): DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
>>                       [0xffffffff812c63fe, 0xffffffff812c6405): DW_OP_reg14 R14
>>                       [0xffffffff812c6405, 0xffffffff812c640a): DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value)
>>                    DW_AT_name    ("event")
>>                    DW_AT_decl_file       ("/rw/compile/kernel/events/core.c")
>>                    DW_AT_decl_line       (4641)
>>                    DW_AT_type    (0x0333aac2 "perf_event *")
>>      0x0334c05e:     DW_TAG_formal_parameter
>>                    DW_AT_location        (0x007dea82:
>>                       [0xffffffff812c6137, 0xffffffff812c63f2): DW_OP_reg12 R12
>>                       [0xffffffff812c63f2, 0xffffffff812c63fe): DW_OP_GNU_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
>>                       [0xffffffff812c63fe, 0xffffffff812c640a): DW_OP_reg12 R12)
>>                    DW_AT_name    ("group")
>>                    DW_AT_decl_file       ("/rw/compile/kernel/events/core.c")
>>                    DW_AT_decl_line       (4641)
>>                    DW_AT_type    (0x03327059 "bool")
> hi,
> I don't see that on gcc compiled kernel, is that related to clang?

Yes, the case I tried to fix is from clang compiled kernel.

>
>
>   <1><318d475>: Abbrev Number: 74 (DW_TAG_subprogram)
>      <318d476>   DW_AT_name        : (indirect string, offset: 0xf5776): perf_event_read
>      <318d47a>   DW_AT_decl_file   : 1
>      <318d47a>   DW_AT_decl_line   : 4746
>      <318d47c>   DW_AT_decl_column : 12
>      <318d47d>   DW_AT_prototyped  : 1
>      <318d47d>   DW_AT_type        : <0x3135e35>
>      <318d481>   DW_AT_low_pc      : 0xffffffff8135be90
>      <318d489>   DW_AT_high_pc     : 0x196
>      <318d491>   DW_AT_frame_base  : 1 byte block: 9c    (DW_OP_call_frame_cfa)
>      <318d493>   DW_AT_call_all_calls: 1
>      <318d493>   DW_AT_sibling     : <0x318d900>
>   <2><318d497>: Abbrev Number: 30 (DW_TAG_formal_parameter)
>      <318d498>   DW_AT_name        : (indirect string, offset: 0x491590): event
>      <318d49c>   DW_AT_decl_file   : 1
>      <318d49c>   DW_AT_decl_line   : 4746
>      <318d49e>   DW_AT_decl_column : 47
>      <318d49f>   DW_AT_type        : <0x313a680>
>      <318d4a3>   DW_AT_location    : 0x70c118 (location list)
>      <318d4a7>   DW_AT_GNU_locviews: 0x70c110
>   <2><318d4ab>: Abbrev Number: 30 (DW_TAG_formal_parameter)
>      <318d4ac>   DW_AT_name        : (indirect string, offset: 0x51a865): group
>      <318d4b0>   DW_AT_decl_file   : 1
>      <318d4b0>   DW_AT_decl_line   : 4746
>      <318d4b2>   DW_AT_decl_column : 59
>      <318d4b3>   DW_AT_type        : <0x3136055>
>      <318d4b7>   DW_AT_location    : 0x70c144 (location list)
>      <318d4bb>   DW_AT_GNU_locviews: 0x70c13e
>
> locations:
>      0070c144 ffffffff8135be90 (base address)
>      0070c14d v000000000000000 v000000000000000 views at 0070c13e for:
>               ffffffff8135be90 ffffffff8135bed2 (DW_OP_reg4 (rsi))
>      0070c152 v000000000000000 v000000000000000 views at 0070c140 for:
>               ffffffff8135bed2 ffffffff8135bf17 (DW_OP_reg14 (r14))
>      0070c158 v000000000000000 v000000000000000 views at 0070c142 for:
>               ffffffff8135bf17 ffffffff8135c026 (DW_OP_entry_value: (DW_OP_reg4 (rsi)); DW_OP_stack_value)
>      0070c162 <End of list>
>
>
> other than that lgtm and I like the change Eduard suggested
>
> thanks,
> jirka
>
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