Re: [PATCH v2 dwarves 1/2] dwarf_loader: Check DW_OP_[GNU_]entry_value for possible parameter matching

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

On 11/14/24 10:21 AM, Eduard Zingerman wrote:
> On Thu, 2024-11-14 at 08:51 -0800, Yonghong Song wrote:
>
> [...]
>
>>> +		/* match DW_OP_entry_value(DW_OP_regXX) at any location */
>>> +		case DW_OP_entry_value:
>>> +		case DW_OP_GNU_entry_value:
>>> +			if (dwarf_getlocation_attr(attr, expr, &entry_attr) == 0 &&
>>> +			    dwarf_getlocation(&entry_attr, &entry_ops, &entry_len) == 0 &&
>>> +			    entry_len == 1) {
>>> +				ret = entry_ops->atom;
>> Could we have more than one DW_OP_entry_value? What if the second one
>> matches execpted_reg? From dwarf5 documentation, there is no say about
>> whether we could have more than one DW_OP_entry_value or not.
>>
>> If we have evidence that only one DW_OP_entry_value will appear in parameter
>> locations, a comment will be needed in the above.
>>
>> Otherwise, let us not do 'goto out' here. Rather, let us compare
>> entry_ops->atom with expected_reg. Do 'ret = entry_ops->atom' and
>> 'goto out' only if entry_ops->atom == expected_reg. Otherwise,
>> the original 'ret' value is preserved.
> Basing on this description in lldb source:
> https://github.com/llvm/llvm-project/blob/1cd981a5f3c89058edd61cdeb1efa3232b1f71e6/lldb/source/Expression/DWARFExpression.cpp#L538
> It would be surprising if DW_OP_entry_value records had different expressions.
> However, there are 50 instances of such behaviour in my clang 18.1.8 built kernel., e.g.:
>
> 0x01f75d14:   DW_TAG_subprogram
>                  DW_AT_low_pc    (0xffffffff818c43a0)
>                  DW_AT_high_pc   (0xffffffff818c43c9)
>                  DW_AT_frame_base        (DW_OP_reg7 RSP)
>                  DW_AT_call_all_calls    (true)
>                  DW_AT_name      ("hwcache_align_show")
>                  DW_AT_decl_file ("/home/eddy/work/bpf-next/mm/slub.c")
>                  DW_AT_decl_line (6621)
>                  DW_AT_prototyped        (true)
>                  DW_AT_type      (0x01f51a9b "ssize_t")
>
> 0x01f75d26:     DW_TAG_formal_parameter
>                    DW_AT_location        (indexed (0xa0f) loclist = 0x0062c64f:
>                       [0xffffffff818c43a9, 0xffffffff818c43b5): DW_OP_reg5 RDI
>                       [0xffffffff818c43b5, 0xffffffff818c43c1): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
>                       [0xffffffff818c43c1, 0xffffffff818c43c9): DW_OP_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value)
>                    DW_AT_name    ("s")
>                    DW_AT_decl_file       ("/home/eddy/work/bpf-next/mm/slub.c")
>                    DW_AT_decl_line       (6621)
>                    DW_AT_type    (0x01f4f449 "kmem_cache *")
>
> The following change seem not to affect pahole execution time:
>
> @@ -1234,7 +1234,8 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
>                              dwarf_getlocation(&entry_attr, &entry_ops, &entry_len) == 0 &&
>                              entry_len == 1) {
>                                  ret = entry_ops->atom;
> -                               goto out;
> +                               if (expr->atom == expected_reg)
> +                                       goto out;
>                          }
>                          break;
>                  }

Should we do
			...
			dwarf_getlocation(&entry_attr, &entry_ops, &entry_len) == 0 &&
			entry_len == 1 && expr->atom == expected_reg) {
				ret = entry_ops->atom;
				goto out;
		}
		...
?

>
> This question aside, I think the changes fine.
>
> [...]
>