[PATCH 13/16] dwarf_loader: Fix cus__merging_cu failing to detect DW_FORM_ref_addr

[Arnaldo Carvalho de Melo <acme@kernel.org>]

From: Arnaldo Carvalho de Melo <acme@redhat.com>

cus__merging_cu() scans abbreviation tables looking for DW_FORM_ref_addr
to detect binaries with inter-CU type references (like Rust CUs in
perf). When found, it triggers the merged CU loading path that can
resolve cross-CU references.

However, dwarf_getabbrevattr() can fail on certain attributes, notably
when DW_FORM_implicit_const is used (DWARF5). The function was treating
this failure as terminal, returning false immediately without scanning
the remaining abbreviations. This prevented detection of
DW_FORM_ref_addr in later CUs, causing the parallel path to be taken
instead — which cannot resolve cross-CU references.

For example, with the perf binary containing 507 CUs where 7 Rust CUs
(CU 209-215) use DW_FORM_ref_addr, the function was failing at CU 0
abbreviation 20 attribute 8 and returning false, never reaching the
Rust CUs.

Before:

  $ pahole -F dwarf ~/bin/perf 2>&1 | grep "couldn't find" | wc -l
  314
  $ diff <(pahole -F dwarf ~/bin/perf 2>/dev/null) \
         <(pahole --features=force_cu_merging -F dwarf ~/bin/perf 2>/dev/null) \
    | grep '^[<>]' | wc -l
  70

After:

  $ pahole -F dwarf ~/bin/perf 2>&1 | grep "couldn't find" | wc -l
  0
  $ diff <(pahole -F dwarf ~/bin/perf 2>/dev/null) \
         <(pahole --features=force_cu_merging -F dwarf ~/bin/perf 2>/dev/null) \
    | wc -l
  0

The fix changes dwarf_getattrcnt() failure to skip the current
abbreviation (goto next_abbrev) and dwarf_getabbrevattr() failure to
skip to the next attribute (continue), both continuing to scan for
DW_FORM_ref_addr instead of aborting the entire detection.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 dwarf_loader.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/dwarf_loader.c b/dwarf_loader.c
index 7091655588cd8b4d..1ab680a4c30634c8 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -3761,7 +3761,7 @@ static bool cus__merging_cu(Dwarf *dw, Elf *elf)
 
 			size_t attrcnt;
 			if (dwarf_getattrcnt (abbrev, &attrcnt) != 0)
-				return false;
+				goto next_abbrev;
 
 			unsigned int attr_num, attr_form;
 			Dwarf_Off aboffset;
@@ -3769,10 +3769,11 @@ static bool cus__merging_cu(Dwarf *dw, Elf *elf)
 			for (j = 0; j < attrcnt; ++j) {
 				if (dwarf_getabbrevattr (abbrev, j, &attr_num, &attr_form,
 							 &aboffset))
-					return false;
+					continue;
 				if (attr_form == DW_FORM_ref_addr)
 					return true;
 			}
+next_abbrev:
 
 			offset += length;
 		}
-- 
2.54.0