Support cross-CU DW_FORM_ref_addr in addr2line's abstract_origin

Under GCC LTO the C-level backtrace in a crash report printed only bare
hex addresses (e.g. "ruby(0x...) [0x...]") for every ruby frame, even
though gdb resolved the same binary fine.

The concrete out-of-line function DIE carries DW_AT_low_pc, so the crash
PC matches a DIE and the DWARF parse "succeeds"; but its name is reached
only through a DW_AT_abstract_origin encoded as DW_FORM_ref_addr, a
section-relative reference into another CU (LTO emits the concrete and
abstract instances in separate CUs). addr2line.c handled the CU-relative
ref forms but had DW_FORM_ref_addr as "not supported yet".

[Bug #22188]

Author: rwstauner

addr2line.c

@@ -1749,7 +1749,7 @@ ranges_inspect(DebugInfoReader *reader, ranges_t *ptr, FILE *errout)
 #endif
 
 static int
-di_read_cu(DebugInfoReader *reader, FILE *errout)
+di_read_cu_context(DebugInfoReader *reader, FILE *errout)
 {
     uint64_t unit_length;
     uint16_t version;
@@ -1784,7 +1784,6 @@ di_read_cu(DebugInfoReader *reader, FILE *errout)
 
     reader->level = 0;
     di_read_debug_abbrev_cu(reader);
-    if (di_read_debug_line_cu(reader, errout)) return -1;
 
     do {
         DIE die;
@@ -1840,12 +1839,57 @@ di_read_cu(DebugInfoReader *reader, FILE *errout)
     return 0;
 }
 
+static int
+di_read_cu(DebugInfoReader *reader, FILE *errout)
+{
+    /* Keep di_read_cu_context separate so that it can be reused by di_read_cu_at
+     * to set up arbitrary CUs without disturbing the .debug_line traversal. */
+    if (di_read_cu_context(reader, errout)) return -1;
+    if (di_read_debug_line_cu(reader, errout)) return -1;
+    return 0;
+}
+
+/* Find the .debug_info compilation unit containing the section-relative DIE
+ * offset `die_offset`, initialize `reader` with that unit's abbrev/base context,
+ * and leave reader->p pointing at the referenced DIE.  Type units, split DWARF,
+ * and supplementary debug objects are outside this parser's current scope. */
+static bool
+di_read_cu_at(DebugInfoReader *reader, uint64_t die_offset, FILE *errout)
+{
+    const uint64_t debug_info_size = reader->obj->debug_info.size;
+    if (die_offset >= debug_info_size) return false;
+
+    const char *const info = reader->obj->debug_info.ptr;
+    const char *const pend = reader->pend;
+    const char *const target = info + die_offset;
+    const char *cu = info;
+
+    while (pend - cu >= 4) {
+        const char *hp = cu;
+        uint64_t unit_length = read_uint32(&hp);
+
+        if (unit_length == 0xffffffff) {
+            if (pend - hp < 8) return false;
+            unit_length = read_uint64(&hp);
+        }
+        if (unit_length == 0 || unit_length > (uint64_t)(pend - hp)) return false;
+
+        const char *cu_end = hp + unit_length;
+        if (target >= cu && target < cu_end) {
+            reader->p = cu;
+            if (di_read_cu_context(reader, errout)) return false;
+            reader->p = target;
+            return true;
+        }
+        cu = cu_end;
+    }
+    return false;
+}
+
 static void
 read_abstract_origin(DebugInfoReader *reader, uint64_t form, uint64_t abstract_origin, line_info_t *line, FILE *errout)
 {
-    const char *p = reader->p;
-    const char *q = reader->q;
-    int level = reader->level;
+    DebugInfoReader saved = *reader; /* CU-scoped state may be rewritten below */
     DIE die;
 
     switch (form) {
@@ -1857,7 +1901,11 @@ read_abstract_origin(DebugInfoReader *reader, uint64_t form, uint64_t abstract_o
         reader->p = reader->current_cu + abstract_origin;
         break;
       case DW_FORM_ref_addr:
-        goto finish; /* not supported yet */
+        /* Section-relative; target may be in another CU.
+         * Switch to that CU's context.
+         * di_read_cu_at leaves p at the target DIE. */
+        if (!di_read_cu_at(reader, abstract_origin, errout)) goto finish;
+        break;
       case DW_FORM_ref_sig8:
         goto finish; /* not supported yet */
       case DW_FORM_ref_sup4:
@@ -1880,9 +1928,7 @@ read_abstract_origin(DebugInfoReader *reader, uint64_t form, uint64_t abstract_o
     }
 
   finish:
-    reader->p = p;
-    reader->q = q;
-    reader->level = level;
+    *reader = saved;
 }
 
 static bool