[PATCH v4 2/6] perf dwarf-aux: Add die_same_file() and die_get_type_die()

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

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

dwarf_dieoffset() is relative to the file the DIE is in, so an offset
saved by die_collect_vars()/die_collect_global_vars() is only
meaningful in that file: add die_same_file(), comparing the Dwarf each
DIE's CU belongs to, and die_get_type_die(), resolving the offset in it
with the saved tag as a sanity check.  Resolving the offset in the main
file instead parses whatever is at it when it came from the dwz common
file.

dwarf_cu_getdwarf() is new in elfutils 0.160, so the libdw feature test
probes for it and Makefile.config says 0.160.

Assisted-by: LLM
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/build/feature/test-libdw.c | 10 +++++-
 tools/perf/Makefile.config       |  2 +-
 tools/perf/util/dwarf-aux.c      | 56 ++++++++++++++++++++++++++++++--
 tools/perf/util/dwarf-aux.h      | 13 ++++++++
 4 files changed, 77 insertions(+), 4 deletions(-)

diff --git a/tools/build/feature/test-libdw.c b/tools/build/feature/test-libdw.c
index aabd63ca76b4d7e6..46f75cde59471079 100644
--- a/tools/build/feature/test-libdw.c
+++ b/tools/build/feature/test-libdw.c
@@ -49,8 +49,16 @@ int test_elfutils(void)
 	return 0;
 }
 
+/* Needs elfutils 0.160: taking its address fails the probe, not the link. */
+int test_libdw_cu_getdwarf(void)
+{
+	void *sym = (void *)dwarf_cu_getdwarf;
+
+	return sym == NULL;
+}
+
 int main(void)
 {
 	return test_libdw() + test_libdw_unwind() + test_libdw_getlocations() +
-	       test_libdw_getcfi() + test_elfutils();
+	       test_libdw_getcfi() + test_libdw_cu_getdwarf() + test_elfutils();
 }
diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
index dca62e155fb5570a..b288ae0191acf7ef 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -477,7 +477,7 @@ else
   else
     ifneq ($(feature-libdw), 1)
       ifndef NO_LIBDW
-        $(warning No libdw.h found or old libdw.h found or elfutils is older than 0.157, disables dwarf support. Please install new elfutils-devel/libdw-dev)
+        $(warning No libdw.h found or old libdw.h found or elfutils is older than 0.160, disables dwarf support. Please install new elfutils-devel/libdw-dev)
         NO_LIBDW := 1
       endif
     endif # Dwarf support
diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
index 2c135ebb1551fca4..54f8b5ec74a2fe70 100644
--- a/tools/perf/util/dwarf-aux.c
+++ b/tools/perf/util/dwarf-aux.c
@@ -1677,6 +1677,15 @@ Dwarf_Die *die_find_variable_by_addr(Dwarf_Die *sc_die, Dwarf_Addr addr,
 	return result;
 }
 
+/*
+ * Whether two DIEs live in the same debug file: dwarf_dieoffset() is
+ * relative to the file the DIE is in.
+ */
+bool die_same_file(Dwarf_Die *die_a, Dwarf_Die *die_b)
+{
+	return dwarf_cu_getdwarf(die_a->cu) == dwarf_cu_getdwarf(die_b->cu);
+}
+
 static int __die_collect_vars_cb(Dwarf_Die *die_mem, void *arg)
 {
 	struct die_var_type **var_types = arg;
@@ -1721,6 +1730,8 @@ static int __die_collect_vars_cb(Dwarf_Die *die_mem, void *arg)
 			vt->is_reg_var_addr = true;
 
 		vt->die_off = dwarf_dieoffset(&type_die);
+		vt->die_tag = dwarf_tag(&type_die);
+		vt->from_alt = !die_same_file(die_mem, &type_die);
 		vt->addr = start;
 		vt->end = end;
 		vt->has_range = (end != 0 || start != 0);
@@ -1740,7 +1751,8 @@ static int __die_collect_vars_cb(Dwarf_Die *die_mem, void *arg)
  *
  * Save all variables and parameters in the @sc_die and save them to @var_types.
  * The @var_types is a singly-linked list containing type and location info.
- * Actual type can be retrieved using dwarf_offdie() with 'die_off' later.
+ * Actual type can be retrieved using die_get_type_die() with 'die_off',
+ * 'die_tag' and 'from_alt' later.
  *
  * Callers should free @var_types.
  */
@@ -1786,6 +1798,8 @@ static int __die_collect_global_vars_cb(Dwarf_Die *die_mem, void *arg)
 		return DIE_FIND_CB_END;
 
 	vt->die_off = dwarf_dieoffset(&type_die);
+	vt->die_tag = dwarf_tag(&type_die);
+	vt->from_alt = !die_same_file(die_mem, &type_die);
 	vt->addr = ops->number;
 	vt->end = 0;
 	vt->has_range = false;
@@ -1797,6 +1811,43 @@ static int __die_collect_global_vars_cb(Dwarf_Die *die_mem, void *arg)
 	return DIE_FIND_CB_SIBLING;
 }
 
+/**
+ * die_get_type_die - Get a type DIE saved by die_collect_vars()
+ * @dbg: the main debug info
+ * @die_off: offset of the type DIE, from dwarf_dieoffset()
+ * @die_tag: tag that DIE had when the offset was saved
+ * @from_alt: whether the type DIE is in the dwz alt file
+ * @die_mem: where to store the resulting DIE
+ *
+ * Resolve @die_off in the file it was recorded as belonging to, with no
+ * fallback; @die_tag is a sanity check.
+ */
+Dwarf_Die *die_get_type_die(Dwarf *dbg, u64 die_off, int die_tag, bool from_alt,
+			    Dwarf_Die *die_mem)
+{
+	Dwarf *target = dbg;
+	Dwarf_Die die;
+
+	if (from_alt) {
+		/* No fallback: an alt offset here parses whatever is at it. */
+		target = dwarf_getalt(dbg);
+		if (target == NULL) {
+			pr_debug("DWARF: no alt (dwz) debug file to resolve the type DIE at offset 0x%lx in\n",
+				 (unsigned long)die_off);
+			return NULL;
+		}
+	}
+
+	if (dwarf_offdie(target, die_off, &die) && dwarf_tag(&die) == die_tag) {
+		*die_mem = die;
+		return die_mem;
+	}
+
+	pr_debug("DWARF: no DIE with tag %d at offset 0x%lx in the %s debug file\n",
+		 die_tag, (unsigned long)die_off, from_alt ? "alt" : "main");
+	return NULL;
+}
+
 /**
  * die_collect_global_vars - Save all global variables
  * @cu_die: a CU DIE
@@ -1804,7 +1855,8 @@ static int __die_collect_global_vars_cb(Dwarf_Die *die_mem, void *arg)
  *
  * Save all global variables in the @cu_die and save them to @var_types.
  * The @var_types is a singly-linked list containing type and location info.
- * Actual type can be retrieved using dwarf_offdie() with 'die_off' later.
+ * Actual type can be retrieved using die_get_type_die() with 'die_off',
+ * 'die_tag' and 'from_alt' later.
  *
  * Callers should free @var_types.
  */
diff --git a/tools/perf/util/dwarf-aux.h b/tools/perf/util/dwarf-aux.h
index 9b662db710220522..6f9145510adcb841 100644
--- a/tools/perf/util/dwarf-aux.h
+++ b/tools/perf/util/dwarf-aux.h
@@ -152,6 +152,8 @@ int die_get_scopes(Dwarf_Die *cu_die, Dwarf_Addr pc, Dwarf_Die **scopes);
 struct die_var_type {
 	struct die_var_type *next;
 	u64 die_off;
+	int die_tag;
+	bool from_alt;	/* die_off is relative to the alt (dwz) file */
 	u64 addr;
 	u64 end;        /* end address of location range */
 	int reg;
@@ -183,6 +185,17 @@ Dwarf_Die *die_find_variable_by_addr(Dwarf_Die *sc_die, Dwarf_Addr addr,
 /* Save all variables and parameters in this scope */
 void die_collect_vars(Dwarf_Die *sc_die, struct die_var_type **var_types);
 
+/*
+ * Get the type DIE saved by die_collect_vars()/die_collect_global_vars():
+ * @from_alt says which file the offset must be resolved in, @die_tag is
+ * a sanity check.
+ */
+Dwarf_Die *die_get_type_die(Dwarf *dbg, u64 die_off, int die_tag, bool from_alt,
+			    Dwarf_Die *die_mem);
+
+/* Whether two DIEs live in the same debug file */
+bool die_same_file(Dwarf_Die *die_a, Dwarf_Die *die_b);
+
 /* Save all global variables in this CU */
 void die_collect_global_vars(Dwarf_Die *cu_die, struct die_var_type **var_types);
 
-- 
2.53.0