perf: we can now read separate debug-info files based on a build ID

Date: Mon, 7 Sep 2015 19:27:01 -0700
(Please Cc me when replying; I'm not subscribed)

Hi. Perf currently has trouble reading separate debug-info files when trying to look up symbols in a 'perf report'. According to the gdb documentation:

https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html

separate debug-info files can live in any of

/usr/lib/debug/.build-id/ab/cdef1234.debug
/usr/bin/debuglink
/usr/bin/.debug/debuglink
/usr/lib/debug/usr/bin/debuglink

Prior to this patch series, perf reads the value of the debuglink from the .gnu_debuglink section, and tries to read a file of that name in the same directory as the stripped ELF file. For an executable, this would be "/usr/bin/debuglink" above. And that's it; the other paths are not checked.

The "normal" thing to do on Debian boxes these days is

/usr/lib/debug/.build-id/ab/cdef1234.debug

This patch series makes this work. The remaining two paths are still unchecked, but I don't know if they're "standard" anywhere

Dima Kogan (2):
perf: fixed type error when reading a build-id
perf: we can now read separate debug-info files based on a build ID

 tools/perf/util/symbol-minimal.c | 2 +-
 tools/perf/util/symbol.c         | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

2.1.4

From: Dima Kogan <[email protected]>
Date: Mon, 7 Sep 2015 17:30:28 -0700
Subject: [PATCH 1/2] perf: fixed type error when reading a build-id

This was benign, but wrong. The build-id should live in a char[], not a char*[]

---
 tools/perf/util/symbol-minimal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index fd8477c..4890633 100644
@@ -337,7 +337,7 @@ int dso__load_sym(struct dso *dso, struct map *map __maybe_unused,
 		  symbol_filter_t filter __maybe_unused,
 		  int kmodule __maybe_unused)
 {
-	unsigned char *build_id[BUILD_ID_SIZE];
+	unsigned char build_id[BUILD_ID_SIZE];
 	int ret;
 
 	ret = fd__is_64_bit(ss->fd);

2.1.4

From: Dima Kogan <[email protected]>
Date: Mon, 7 Sep 2015 17:34:19 -0700
Subject: [PATCH 2/2] perf: we can now read separate debug-info files based on
a build ID

Recent GDB (at least on a vanilla Debian box) looks for debug information in

/usr/lib/debug/.build-id/nn/nnnnnnn

where nn/nnnnnn is the build-id of the stripped ELF binary. This is documented here:

https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html

This was not working in perf because we didn't read the build id until AFTER we searched for the separate debug information file. This patch reads the build ID and THEN does the search.

---
 tools/perf/util/symbol.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index 1f97ffb..05c656b 100644
@@ -1406,6 +1406,7 @@ int dso__load(struct dso *dso, struct map *map, symbol_filter_t filter)
 	struct symsrc ss_[2];
 	struct symsrc *syms_ss = NULL, *runtime_ss = NULL;
 	bool kmod;
+	unsigned char build_id[BUILD_ID_SIZE];
 
 	pthread_mutex_lock(&dso->lock);
 

@@ -1461,6 +1462,14 @@ int dso__load(struct dso *dso, struct map *map, symbol_filter_t filter)
 		dso->symtab_type == DSO_BINARY_TYPE__GUEST_KMODULE ||
 		dso->symtab_type == DSO_BINARY_TYPE__GUEST_KMODULE_COMP;
 
+
+	/*
+	 * Read the build id if possible. This is required for
+	 * DSO_BINARY_TYPE__BUILDID_DEBUGINFO to work
+	 */
+	if (filename__read_build_id(dso->name, build_id, BUILD_ID_SIZE) > 0)
+		dso__set_build_id(dso, build_id);
+
 	/*
 	 * Iterate over candidate debug images.
 	 * Keep track of "interesting" ones (those which have a symtab, dynsym,

2.1.4