Re: [PATCH dwarves v3 1/9] dwarf_loader: Reduce parameter checking with clang DW_AT_calling_convention attr

Yonghong Song
2026-03-22T17:36:15Z

On 3/21/26 4:10 PM, Jiri Olsa wrote:
> On Fri, Mar 20, 2026 at 12:09:22PM -0700, Yonghong Song wrote:
>> Currently every function is checked for its parameters to identify whether
>> the signature changed or not. If signature indeed changed, pahole may do
>> some adjustment for parameters for true signatures.
>>
>> In clang, any function with the following attribute
>> DW_AT_calling_convention (DW_CC_nocall)
>> indicates this function having signature changed.
>> pahole can take advantage of this to avoid parameter checking if
>> DW_AT_calling_convention is not DW_CC_nocall.
>>
>> But more importantly, DW_CC_nocall can identify signature-changed functions
>> and parameters can be checked one-after-another to create the true
>> signatures. Otherwise, it takes more effort to identify whether a
>> function has signature changed or not. For example, for funciton
>> __bpf_kfunc static void bbr_main(struct sock *sk, u32 ack, int flag,
>> const struct rate_sample *rs) { ... }
>> and bbr_main() is a callback function in
>> .cong_control = bbr_main
>> in 'struct tcp_congestion_ops tcp_bbr_cong_ops'.
>> In the above bbr_main(...), parameter 'ack' and 'flag' are not used.
>> The following are some details:
>>
>> 0x0a713b8d: DW_TAG_formal_parameter
>> DW_AT_location (indexed (0x28) loclist = 0x0166d452:
>> [0xffffffff83e77fd9, 0xffffffff83e78016): DW_OP_reg5 RDI
>> ...
>> DW_AT_name ("sk")
>> DW_AT_type (0x0a6f5b2b "sock *")
>> ...
>>
>> 0x0a713b98: DW_TAG_formal_parameter
>> DW_AT_name ("ack")
>> DW_AT_type (0x0a6f58fd "u32")
>> ...
>>
>> 0x0a713ba2: DW_TAG_formal_parameter
>> DW_AT_name ("flag")
>> DW_AT_type (0x0a6f57d1 "int")
>> ...
>>
>> 0x0a713bac: DW_TAG_formal_parameter
>> DW_AT_location (indexed (0x29) loclist = 0x0166d4a8:
>> [0xffffffff83e77fd9, 0xffffffff83e78016): DW_OP_reg2 RCX
>> ...
>> DW_AT_name ("rs")
>> DW_AT_type (0x0a710da5 "const rate_sample *")
>> DW_AT_decl_line (1027)
>>
>> Some analysis for the above dwarf can conclude that the 'ark' and 'flag'
>> may be related to RSI and RDX, considering the last one is RCX. Basically this
>> requires all parameters are available to collectively decide whether the
>> true signature can be found or not. In such case, DW_CC_nocall can make things
>> easier as parameter can be checked one after another.
>>
>> For a clang built bpf-next kernel, in non-LTO setup, the number of kernel functions
>> is 69103 and the number of signature changed functions is 875, based on
>> DW_AT_calling_convention (DW_CC_nocall)
>> indication.
> we don't display these stats, right? would be great and probably easy
> to count different flags and aggregate them

The actual stats will be different depending on kernel code and compiler optimization.
But I think I can still count them for each commit so people will know
which change caused how much true signature.

>
>> Among 875 signature changed functions, after this patch, 495 functions
>> can have proper true signatures, mostly due to simple dead argument
>> elimination. The number of remaining functions, which cannot get the
>> true signature, is 379. They will be addressed in the subsequent commits.
> after this change I have more functions added (below) and I checked
> few and they seem ok (w/o DW_CC_nocall)

Most global functions should be okay. But some global functions
esp. if argument is more than 8 byte (struct type), they may be
considered as signature change with the current implementation.

>
> jirka
>
>
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