Re: [PATCH dwarves v7 0/5] pahole: Encode true signatures in kernel BTF

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On 23/06/2026 13:28, Jiri Olsa wrote:
> On Mon, Jun 22, 2026 at 09:07:04PM -0700, Yonghong Song wrote:
>> Current vmlinux BTF encoding is based on the source level signatures.
>> But the compiler may do some optimization and changed the signature.
>> If the user tried with source level signature, their initial implementation
>> may have wrong results and then the user need to check what is the
>> problem and work around it, e.g. through kprobe since kprobe does not
>> need vmlinux BTF.
>>
>> Majority of changed signatures are due to dead argument elimination.
>> The following is a more complex one. The original source signature:
>>   typedef struct {
>>         union {
>>                 void            *kernel;
>>                 void __user     *user;
>>         };
>>         bool            is_kernel : 1;
>>   } sockptr_t;
>>   typedef sockptr_t bpfptr_t;
>>   static int map_create(union bpf_attr *attr, bpfptr_t uattr) { ... }
>> After compiler optimization, the signature becomes:
>>   static int map_create(union bpf_attr *attr, bool uattr__is_kernel) { ... }
>> In the above, uattr__is_kernel corresponds to 'is_kernel' field in sockptr_t.
>> This makes it easier for developers to understand what changed.
>>
>> The new signature needs to properly follow ABI specification based on
>> locations. Otherwise, that signature should be discarded. For example,
>>
>>     0x0242f1f7:   DW_TAG_subprogram
>>                     DW_AT_name      ("memblock_find_in_range")
>>                     DW_AT_calling_convention        (DW_CC_nocall)
>>                     DW_AT_type      (0x0242decc "phys_addr_t")
>>                     ...
>>     0x0242f22e:     DW_TAG_formal_parameter
>>                       DW_AT_location        (indexed (0x14a) loclist = 0x005595bc:
>>                          [0xffffffff87a000f9, 0xffffffff87a00178): DW_OP_reg5 RDI
>>                          [0xffffffff87a00178, 0xffffffff87a001be): DW_OP_reg14 R14
>>                          [0xffffffff87a001be, 0xffffffff87a001c7): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
>>                          [0xffffffff87a001c7, 0xffffffff87a00214): DW_OP_reg14 R14)
>>                       DW_AT_name    ("start")
>>                       DW_AT_type    (0x0242decc "phys_addr_t")
>>                       ...
>>     0x0242f239:     DW_TAG_formal_parameter
>>                       DW_AT_location        (indexed (0x14b) loclist = 0x005595e6:
>>                          [0xffffffff87a000f9, 0xffffffff87a00175): DW_OP_reg4 RSI
>>                          [0xffffffff87a00175, 0xffffffff87a001b8): DW_OP_reg3 RBX
>>                          [0xffffffff87a001b8, 0xffffffff87a001c7): DW_OP_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
>>                          [0xffffffff87a001c7, 0xffffffff87a00214): DW_OP_reg3 RBX)
>>                       DW_AT_name    ("end")
>>                       DW_AT_type    (0x0242decc "phys_addr_t")
>>                       ...
>>     0x0242f245:     DW_TAG_formal_parameter
>>                       DW_AT_location        (indexed (0x14c) loclist = 0x00559610:
>>                          [0xffffffff87a001e3, 0xffffffff87a001ef): DW_OP_breg4 RSI+0)
>>                       DW_AT_name    ("size")
>>                       DW_AT_type    (0x0242decc "phys_addr_t")
>>                       ...
>>     0x0242f250:     DW_TAG_formal_parameter
>>                       DW_AT_const_value     (4096)
>>                       DW_AT_name    ("align")
>>                       DW_AT_type    (0x0242decc "phys_addr_t")
>>                       ...
>>
>> The third argument should correspond to RDX for x86_64. But the location suggests that
>> the parameter value is stored in the address with 'RSI + 0'. It is not clear whether
>> the parameter value is stored in RDX or not. So we have to discard this funciton in
>> vmlinux BTF to avoid incorrect true signatures.
>>
>> For llvm, any function having
>>   DW_AT_calling_convention        (DW_CC_nocall)
>> in dwarf DW_TAG_subprogram will indicate that this function has signature changed.
>> I did experiment with latest bpf-next. For x86_64, there are 69103 kernel functions
>> and 875 kernel functions having signature changed. A series of patches are intended
>> to ensure true signatures are properly represented. Eventually, only 20 functions
>> cannot have true signatures due to locations.
> 
> hi,
> I tried to get the numbers from my setup and noticed that some new
> functions were included in BTF compared to the current version
> (functions diff attached below)
> 
> like for "arp_process" function the current pahole gives me:
> 
>   arp_process : skipping BTF encoding of function due to unexpected register usage for parameter
> 
> but it's included in BTF generated with the new pahole.
> 
> in addition to your explanation above also one of the commit says:
> 
>      - a parameter with no location, a constant value, or (for non-clang) no
>        register found is marked optimized out
> 
> please check below, it seems like 2nd argument of arp_process has no location,
> so iiuc it should not be included in BTF, right?
> 
> thanks,
> jirka
> 
>

thanks for catching this; it looks like we return a bit early before detecting
missing locations in the non-true-signature code. If you get a chance, would you 
mind trying the attached patch to see if it fixes the problem?

If the fix works and Yonghong is happy with it we can add it as a followup
and land the true signature series to save another round.
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