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

Yonghong Song
2024-11-12T17:07:47Z

On 11/12/24 8:56 AM, Alan Maguire wrote:
> On 12/11/2024 01:51, Yonghong Song wrote:
>>
>>
>> On 11/11/24 7:39 AM, Alan Maguire wrote:
>>> On 08/11/2024 18:05, 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")
>>>>
>>>> By inspecting the binary, the second argument ("bool group") is used
>>>> in the function. The following are the disasm code:
>>>>      ffffffff812c6110 <perf_event_read>:
>>>>      ffffffff812c6110: 0f 1f 44 00 00        nopl    (%rax,%rax)
>>>>      ffffffff812c6115: 55                    pushq   %rbp
>>>>      ffffffff812c6116: 41 57                 pushq   %r15
>>>>      ffffffff812c6118: 41 56                 pushq   %r14
>>>>      ffffffff812c611a: 41 55                 pushq   %r13
>>>>      ffffffff812c611c: 41 54                 pushq   %r12
>>>>      ffffffff812c611e: 53                    pushq   %rbx
>>>>      ffffffff812c611f: 48 83 ec 18           subq    $24, %rsp
>>>>      ffffffff812c6123: 41 89 f4              movl    %esi, %r12d
>>>>      <=========== NOTE that here '%esi' is used and moved to '%r12d'.
>>>>      ffffffff812c6126: 49 89 fe              movq    %rdi, %r14
>>>>      ffffffff812c6129: 65 48 8b 04 25 28 00 00 00    movq    %gs:40,
>>>> %rax
>>>>      ffffffff812c6132: 48 89 44 24 10        movq    %rax, 16(%rsp)
>>>>      ffffffff812c6137: 8b af a8 00 00 00     movl    168(%rdi), %ebp
>>>>      ffffffff812c613d: 85 ed                 testl   %ebp, %ebp
>>>>      ffffffff812c613f: 75 3f                 jne
>>>> 0xffffffff812c6180 <perf_event_read+0x70>
>>>>      ffffffff812c6141: 66 2e 0f 1f 84 00 00 00 00 00 nopw    %cs:
>>>> (%rax,%rax)
>>>>      ffffffff812c614b: 0f 1f 44 00 00        nopl    (%rax,%rax)
>>>>      ffffffff812c6150: 49 8b 9e 28 02 00 00  movq    552(%r14), %rbx
>>>>      ffffffff812c6157: 48 89 df              movq    %rbx, %rdi
>>>>      ffffffff812c615a: e8 c1 a0 d7 00        callq
>>>> 0xffffffff82040220 <_raw_spin_lock_irqsave>
>>>>      ffffffff812c615f: 49 89 c7              movq    %rax, %r15
>>>>      ffffffff812c6162: 41 8b ae a8 00 00 00  movl    168(%r14), %ebp
>>>>      ffffffff812c6169: 85 ed                 testl   %ebp, %ebp
>>>>      ffffffff812c616b: 0f 84 9a 00 00 00     je
>>>> 0xffffffff812c620b <perf_event_read+0xfb>
>>>>      ffffffff812c6171: 48 89 df              movq    %rbx, %rdi
>>>>      ffffffff812c6174: 4c 89 fe              movq    %r15, %rsi
>>>>      <=========== NOTE: %rsi is overwritten
>>>>      ......
>>>>      ffffffff812c63f0: 41 5c                 popq    %r12
>>>>      <============ POP r12
>>>>      ffffffff812c63f2: 41 5d                 popq    %r13
>>>>      ffffffff812c63f4: 41 5e                 popq    %r14
>>>>      ffffffff812c63f6: 41 5f                 popq    %r15
>>>>      ffffffff812c63f8: 5d                    popq    %rbp
>>>>      ffffffff812c63f9: e9 e2 a8 d7 00        jmp
>>>> 0xffffffff82040ce0 <__x86_return_thunk>
>>>>      ffffffff812c63fe: 31 c0                 xorl    %eax, %eax
>>>>      ffffffff812c6400: e9 be fe ff ff        jmp
>>>> 0xffffffff812c62c3 <perf_event_read+0x1b3>
>>>>
>>>> It is not clear why dwarf didn't encode %rsi in locations. But
>>>> DW_OP_GNU_entry_value(DW_OP_reg4 RSI) tells us that RSI is live at
>>>> the entry of perf_event_read(). So this patch tries to search
>>>> DW_OP_GNU_entry_value/DW_OP_entry_value location/expression so if
>>>> the expected parameter register matchs the register in
>>>> DW_OP_GNU_entry_value/DW_OP_entry_value, then the original parameter
>>>> is not optimized.
>>>>
>>>> For one of internal 6.11 kernel, there are 62498 functions in BTF and
>>>> perf_event_read() is not there. With this patch, there are 61552
>>>> functions
>>>> in BTF and perf_event_read() is included.
>>>>
>>> hi Yonghong,
>>>
>>> I'm confused by these numbers. I would have thought your changes would
>>> have led to a net increase of functions encoded in vmlinux BTF since we
>>> are now likely catching more cases where registers are expected.  When I
>>> ran your patches against an LLVM-built kernel, that's what I saw; 70
>>> additional functions were recognized as having expected parameters, and
>>> thus were encoded in BTF. In your case it looks like we lost nearly 1000
>>> functions. Any idea what's going on there? If you can share your config,
>>> LLVM version I can dig into this from my side too. Thanks!
>> Attached is my config (based on one of meta internal configs). I tried
>> with master branch with head:
>>
>> 7b6e5bfa2541380b478ea1532880210ea3e39e11 (HEAD -> master, origin/master,
>> origin/HEAD) Merge branch 'refactor-lock-management'
>> ae6e3a273f590a2b64f14a9fab3546c3a8f44ed4 bpf: Drop special callback
>> reference handling
>> f6b9a69a9e56b2083aca8a925fc1a28eb698e3ed bpf: Refactor active lock
>> management
>>
>> I am using pahole v1.27.
>>
>> I am using an llvm built from upstream. The following is llvm-project head:
>> beb12f92c71981670e07e47275efc6b5647011c1 (HEAD -> main) [RISCV] Add
>> +optimized-nfN-segment-load-store (#114414)
>> 6bad4514c938b3b48c0c719b8dd98b3906f2c290 [AArch64] Extend vector mull
>> test coverage. NFC
>> 915b910d800d7fab6a692294ff1d7075d8cba824 [libc] Fix typos in proxy type
>> headers (#114717)
>> 98ea1a81a28a6dd36941456c8ab4ce46f665f57a [IPO] Remove unused includes
>> (NFC) (#114716)
>>
>> With the above setup, when to do
>>
>> pahole -JV --
>> btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs vmlinux >& log.pahole
>>
>> You will find the below info in the log:
>>   skipping addition of 'perf_event_read'(perf_event_read) due to
>> unexpected register used for paramet
>>
>> In the dwarf:
>>
>> 0x02122746:   DW_TAG_subprogram
>>                 DW_AT_low_pc    (0xffffffff81299740)
>>                 DW_AT_high_pc   (0xffffffff812999f7)
>>                 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 ("/home/yhs/work/bpf-next/kernel/events/
>> core.c")
>>                 DW_AT_decl_line (4746)
>>                 DW_AT_prototyped        (true)
>>                 DW_AT_type      (0x020f95f5 "int")
>>
>> 0x02122760:     DW_TAG_formal_parameter
>>                   DW_AT_location        (0x00769b72:
>>                      [0xffffffff81299745, 0xffffffff81299764):
>> DW_OP_reg5 RDI
>>                      [0xffffffff81299764, 0xffffffff81299937):
>> DW_OP_reg3 RBX
>>                      [0xffffffff81299937, 0xffffffff812999f0):
>> DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
>>                      [0xffffffff812999f0, 0xffffffff812999f7):
>> DW_OP_reg3 RBX)
>>                   DW_AT_name    ("event")
>>                   DW_AT_decl_file       ("/home/yhs/work/bpf-next/
>> kernel/events/core.c")
>>                   DW_AT_decl_line       (4746)
>>                   DW_AT_type    (0x0210f654 "perf_event *")
>>                     0x02122770:     DW_TAG_formal_parameter
>>                   DW_AT_location        (0x00769c61:
>>                      [0xffffffff81299758, 0xffffffff81299926):
>> DW_OP_reg6 RBP
>>                      [0xffffffff81299926, 0xffffffff812999f0):
>> DW_OP_GNU_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
>>                      [0xffffffff812999f0, 0xffffffff812999f7):
>> DW_OP_reg6 RBP)
>>                   DW_AT_name    ("group")
>>                   DW_AT_decl_file       ("/home/yhs/work/bpf-next/
>> kernel/events/core.c")
>>
>> The above is slightly different from our production kernel where Song
>> reported. But essence is the same.
>> The second parameter needs to check DW_OP_GNU_entry_value(DW_OP_reg4
>> RSI) to ensure the second
>> argument is available.
>>
>> My patch is supposed to only make improvement. I am curiously why you
>> get less functions encoded in BTF.
>>
> Thanks for the config etc! When I build bpf-next using master branch
> llvm and this config, I see
>
> with baseline (master branch pahole): 62371 functions, no perf_event_read
> your series on top of master branch pahole: 62433 functions,
> perf_event_read present
>
> So that's consistent with what I've seen with other configs; more
> functions are present in vmlinux BTF since we are now seeing more cases
> where parameters are in fact consistent. The part that confuses me
> though is the numbers you initially reported above
>
> "for one of internal 6.11 kernel, there are 62498 functions in BTF and
> perf_event_read() is not there. With this patch, there are 61552
> functions in BTF and perf_event_read() is included."
>
> These numbers suggest you lost nearly 1000 functions when building
> vmlinux BTF with pahole using this series. That's the part I don't
> understand - we should just see a gain in numbers of functions in
> vmlinux BTF, right? Did you mean 62552 functions rather than 61552 perhaps?

Sorry, really embarrassing. it is typo. Indeed it should be 62552 functions
in BTF instead.

>
> Alan
>
>>> Alan
>>>
>>>> Reported-by: Song Liu <song@kernel.org>
>>>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>>>> ---
>>>>   dwarf_loader.c | 81 +++++++++++++++++++++++++++++++++++---------------
>>>>   1 file changed, 57 insertions(+), 24 deletions(-)
>>>>
>> …