Re: [PATCH dwarves v10 0/5] pahole: Encode true signatures in kernel BTF

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

On 6/25/26 6:22 AM, Alan Maguire wrote:
> On 25/06/2026 11:59, Jiri Olsa wrote:
>> On Wed, Jun 24, 2026 at 07:01:48PM -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.
>>> But for non DW_CC_nocall functions, it is possible that true signature still not
>>> available due to locations. So every functions will be checked.
>>>
>>> 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.
>>>
>>> For arm64, there are 863 kernel functions having signature changed, and
>>> 108 functions cannot have true signatures due to locations. I checked those
>>> functions and look like llvm arm64 backend more relaxed to compute parameter
>>> values.
>>>
>>> For full testing, I enabled true signature support in kernel scripts/Makefile.btf like below:
>>>    -pahole-flags-$(call test-ge, $(pahole-ver), 131) += --btf_features=attributes
>>>    +pahole-flags-$(call test-ge, $(pahole-ver), 131) += --btf_features=attributes --btf_features=+true_signature
>>>
>>> See individual patches for details.
>> hi,
>> I tried this version and had to choose another function, because "arp_process"
>> suddenly started to show args registers in proper order ;-)
>>
>> now I checked "stop_cpus" function which is added to btf after this change,
>> I attached the dwarf dump below
>>
>> "stop_cpus" has DW_CC_nocall set, so it has signature_changed=true and I see the
>> function ends up in the btf_encoder with optimized_parms=1
>>
>> but we do not skip such functions now, it seems like we should?
>>
>> I tried the attached change below and it removes all the extra functions
>> that were added in the btf with this change
>>
>> btw I can't see any other use for optimized_parms flag, it seems to be
>> just set and never used.. so I wonder I'm missing something
>>
> Thanks for looking at this!
>
> Yonghong can correct me if I have this wrong, but I think optimized parameters
> are okay, but only if true signature is enabled, so omitting functions with
> optimized parameters should probably only happen if !true_signature. so for
> the change below, the modification to btf_encoder__add_saved_funcs() is right
> but I don't think the modification to btf_encoder__add_true_signature() is needed.

I agree with Alan in the above.

There is no need to make change in btf_encoder__add_true_signature().
This is for true_signature, optimized parameters have been removed:

+               /* No location info/optimized + reordered means optimized out. */
+               if (ftype->reordered_parm && (!param->has_loc || param->optimized)) {
+                       state->nr_parms--;
+                       continue;
+               }

so we should not add true_state->optimized_parms checking in btf_encoder__add_true_signature().

For adding state->optimized_parms in btf_encoder__add_saved_funcs(), with true_signature disabled,
we already have a failure message:
    stop_cpus : skipping BTF encoding of function due to unexpected register usage for parameter
The expectation is for three parameters, the register usage should be
    cpumask: RDI, fn: RSI, arg: RDX
but since 'fn' is optimized away, the expected reg usage does not match the actual reg usage.

We could add:
   +		if (state->optimized_parms)
   +			skip_reason = "optimized parms\n";
to have another message to indicate possible reason is due to optimized parms.
But it is not critical as we already have one message.

I can have a follow up for this.

>   
>> jirka
>>
>>
>> dwarfdump:
>>
>> 0x016f4b1b:   DW_TAG_subprogram
>>                  DW_AT_low_pc    (0xffffffff813e0fc0)
>>                  DW_AT_high_pc   (0xffffffff813e1133)
>>                  DW_AT_frame_base        (DW_OP_reg7 RSP)
>>                  DW_AT_GNU_all_call_sites        (true)
>>                  DW_AT_name      ("stop_cpus")
>>                  DW_AT_decl_file ("/home/jolsa/kernel/linux-qemu-2/kernel/stop_machine.c")
>>                  DW_AT_decl_line (464)
>>                  DW_AT_prototyped        (true)
>>                  DW_AT_calling_convention        (DW_CC_nocall)
>>                  DW_AT_type      (0x016e94c5 "int")
>>
>> 0x016f4b36:     DW_TAG_formal_parameter
>>                    DW_AT_location        (0x0065ad9e:
>>                       [0xffffffff813e0fc5, 0xffffffff813e0fed): DW_OP_reg5 RDI
>>                       [0xffffffff813e0fed, 0xffffffff813e111e): DW_OP_reg14 R14
>>                       [0xffffffff813e111e, 0xffffffff813e1127): DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
>>                       [0xffffffff813e1127, 0xffffffff813e1133): DW_OP_reg14 R14)
>>                    DW_AT_name    ("cpumask")
>>                    DW_AT_decl_file       ("/home/jolsa/kernel/linux-qemu-2/kernel/stop_machine.c")
>>                    DW_AT_decl_line       (464)
>>                    DW_AT_type    (0x016f3242 "const cpumask *")
>>
>> 0x016f4b46:     DW_TAG_formal_parameter
>>                    DW_AT_name    ("fn")
>>                    DW_AT_decl_file       ("/home/jolsa/kernel/linux-qemu-2/kernel/stop_machine.c")
>>                    DW_AT_decl_line       (464)
>>                    DW_AT_type    (0x016f2ca5 "cpu_stop_fn_t")
>>
>> 0x016f4b52:     DW_TAG_formal_parameter
>>                    DW_AT_location        (0x0065ae0d:
>>                       [0xffffffff813e0fc5, 0xffffffff813e0ff2): DW_OP_reg4 RSI
>>                       [0xffffffff813e0ff2, 0xffffffff813e10e4): DW_OP_reg3 RBX
>>                       [0xffffffff813e10e4, 0xffffffff813e1127): DW_OP_GNU_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
>>                       [0xffffffff813e1127, 0xffffffff813e112e): DW_OP_reg3 RBX
>>                       [0xffffffff813e112e, 0xffffffff813e1133): DW_OP_GNU_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value)
>>                    DW_AT_name    ("arg")
>>                    DW_AT_decl_file       ("/home/jolsa/kernel/linux-qemu-2/kernel/stop_machine.c")
>>                    DW_AT_decl_line       (464)
>>                    DW_AT_type    (0x016e858f "void *")
>>
>>
>>
>>
>> ---
>> diff --git a/btf_encoder.c b/btf_encoder.c
>> index 38455a4c6b6b..cbc5063a6280 100644
>> --- a/btf_encoder.c
>> +++ b/btf_encoder.c
>> @@ -1564,7 +1564,8 @@ static int btf_encoder__add_true_signature(struct btf_encoder *encoder,
>>   		 */
>>   		if (true_state->unexpected_reg ||
>>   		    true_state->uncertain_parm_loc ||
>> -		    true_state->ambiguous_addr)
>> +		    true_state->ambiguous_addr ||
>> +		    true_state->optimized_parms)
>>   			continue;
>>   		err = btf_encoder__add_func(encoder, true_state);
>>   		if (err < 0)
>> @@ -1656,6 +1657,8 @@ static int btf_encoder__add_saved_funcs(struct btf_encoder *encoder, bool skip_e
>>   			skip_reason = "reordered parameters\n";
>>   		if (state->elf->ambiguous_addr)
>>   			skip_reason = "ambiguous address\n";
>> +		if (state->optimized_parms)
>> +			skip_reason = "optimized parms\n";
>>   
>>   		if (skip_reason) {
>>   			btf_encoder__log_func_skip(encoder, saved_fns[i].elf,