[PATCH v3 1/2] perf symbol: Add dso__load_symsrc()

[Matthew Wood <thepacketgeek@gmail.com>]

dso__load() goes through the candidate files for a DSO, such as the DSO
itself, separate debuginfo found through .gnu_debuglink, the build-id
cache and /usr/lib/debug, checks their build-ids, records the one with a
symbol table in dso__symsrc_filename(), and then reads and demangles
that symbol table.

The DWARF unwinders depend on the first part. dso__symsrc_filename() is
how they find a separate debuginfo file, for instance to use its
.debug_frame, and libdw's build-id cache fallback needs the build-id
that dso__load() reads when the DSO doesn't have one yet. They get both
today as a side effect of loading the symbols of every frame they visit.

Add dso__load_symsrc(), which does the same search, inside the DSO's
mount namespace and honouring --symfs but stops before reading any
symbols. A new symsrc_loaded flag records that it ran, so the search is
done once per DSO, and the DSO's symbols stay unloaded, so dso__load()
still loads them when they are wanted. Kernel and perf map DSOs are
skipped, as the unwinders only handle user space ELF images.

No functional change on its own; the next patch uses it.

Signed-off-by: Matthew Wood <thepacketgeek@gmail.com>
---
 tools/perf/util/dso.h    | 11 +++++++++++
 tools/perf/util/symbol.c | 40 ++++++++++++++++++++++++++++++++++++----
 tools/perf/util/symbol.h |  1 +
 3 files changed, 48 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
index 55c4aaa53c38..fcf235186978 100644
--- a/tools/perf/util/dso.h
+++ b/tools/perf/util/dso.h
@@ -340,6 +340,7 @@ DECLARE_RC_STRUCT(dso) {
 	u8		 is_64_bit:1;
 	bool		 sorted_by_name;
 	bool		 loaded;
+	bool		 symsrc_loaded;
 	u8		 rel;
 	char		 name[];
 };
@@ -603,6 +604,16 @@ static inline void dso__set_loaded(struct dso *dso)
 	RC_CHK_ACCESS(dso)->loaded = true;
 }
 
+static inline bool dso__symsrc_loaded(const struct dso *dso)
+{
+	return RC_CHK_ACCESS(dso)->symsrc_loaded;
+}
+
+static inline void dso__set_symsrc_loaded(struct dso *dso)
+{
+	RC_CHK_ACCESS(dso)->symsrc_loaded = true;
+}
+
 static inline struct mutex *dso__lock(struct dso *dso)
 {
 	return &RC_CHK_ACCESS(dso)->lock;
diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index 3587ad243159..ed2f8e89b191 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -1798,7 +1798,7 @@ static int dso__find_perf_map(char *filebuf, size_t bufsz,
 	return rc;
 }
 
-int dso__load(struct dso *dso, struct map *map)
+static int __dso__load(struct dso *dso, struct map *map, bool symbols)
 {
 	char *name;
 	int ret = -1;
@@ -1828,11 +1828,17 @@ int dso__load(struct dso *dso, struct map *map)
 	nsinfo__mountns_enter(dso__nsinfo(dso), &nsc);
 
 	/* check again under the dso->lock */
-	if (dso__loaded(dso)) {
+	if (dso__loaded(dso) || (!symbols && dso__symsrc_loaded(dso))) {
 		ret = 1;
 		goto out;
 	}
 
+	/* Unwinders only need ELF images, not kernel or perf map symbols. */
+	if (!symbols && (dso__kernel(dso) || perfmap)) {
+		ret = 0;
+		goto out;
+	}
+
 	kmod = dso__is_kmod(dso);
 
 	if (dso__kernel(dso) && !kmod) {
@@ -1915,7 +1921,7 @@ int dso__load(struct dso *dso, struct map *map)
 		}
 
 #ifdef HAVE_LIBBFD_SUPPORT
-		if (is_reg)
+		if (is_reg && symbols)
 			bfdrc = dso__load_bfd_symbols(dso, name);
 #endif
 		if (is_reg && bfdrc < 0)
@@ -1978,6 +1984,11 @@ int dso__load(struct dso *dso, struct map *map)
 	if (!runtime_ss && syms_ss)
 		runtime_ss = syms_ss;
 
+	if (!symbols) {
+		ret = 0;
+		goto out_destroy;
+	}
+
 	if (syms_ss)
 		ret = dso__load_sym(dso, map, syms_ss, runtime_ss, kmod);
 	else
@@ -1991,6 +2002,7 @@ int dso__load(struct dso *dso, struct map *map)
 			ret += nr_plt;
 	}
 
+out_destroy:
 	for (; ss_pos > 0; ss_pos--)
 		symsrc__destroy(&ss_[ss_pos - 1]);
 out_free:
@@ -1998,13 +2010,33 @@ int dso__load(struct dso *dso, struct map *map)
 	if (ret < 0 && strstr(dso__name(dso), " (deleted)") != NULL)
 		ret = 0;
 out:
-	dso__set_loaded(dso);
+	if (symbols)
+		dso__set_loaded(dso);
+	dso__set_symsrc_loaded(dso);
 	mutex_unlock(dso__lock(dso));
 	nsinfo__mountns_exit(&nsc);
 
 	return ret;
 }
 
+int dso__load(struct dso *dso, struct map *map)
+{
+	return __dso__load(dso, map, /*symbols=*/true);
+}
+
+/*
+ * Discover the same debug images as symbol loading, since they may contain
+ * unwind information. Do not read symbol tables, demangle names, synthesize
+ * PLT symbols, or mark the DSO's symbols as loaded.
+ */
+int dso__load_symsrc(struct dso *dso)
+{
+	if (dso__loaded(dso) || dso__symsrc_loaded(dso))
+		return 1;
+
+	return __dso__load(dso, NULL, /*symbols=*/false);
+}
+
 /*
  * Always takes ownership of vmlinux when vmlinux_allocated == true, even if
  * it returns an error.
diff --git a/tools/perf/util/symbol.h b/tools/perf/util/symbol.h
index d0bac824c79c..7dd7924a098d 100644
--- a/tools/perf/util/symbol.h
+++ b/tools/perf/util/symbol.h
@@ -196,6 +196,7 @@ struct ref_reloc_sym {
 };
 
 int dso__load(struct dso *dso, struct map *map);
+int dso__load_symsrc(struct dso *dso);
 int dso__load_vmlinux(struct dso *dso, struct map *map,
 		      const char *vmlinux, bool vmlinux_allocated);
 int dso__load_vmlinux_path(struct dso *dso, struct map *map);
-- 
2.53.0-Meta