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

[Eduard Zingerman <eddyz87@gmail.com>]

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;
                }

This question aside, I think the changes fine.

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