debuginfod-based dwarf downloading, was Re: Wrong Perf Backtraces

["Frank Ch. Eigler" <fche@redhat.com>]

Hi -

I was reminded by the thread that debuginfo downloading for perf is
worth automating more.  Since perf uses elfutils, recently-added
facilities to elfutils 0.178 (2019-11) already allow automated
downloading from some perf code paths.  The following patch [2] builds
a little bit on that by ensuring that the downloaded debuginfo is
plopped into the $HOME/.debug/ hierarchy explicitly, and should cover
a few more code paths.  Feel free to take / adapt this code as you
like.

debuginfod can also do source code downloading on the fly, but the
perf/util/srccode.c machinery would need buildids passed, and I
couldn't quickly figure it out.  (Sorry, I also couldn't quickly
figure out the perf testsuite.)

[1] https://sourceware.org/elfutils/Debuginfod.html

[2]

Author: Frank Ch. Eigler <fche@redhat.com>
Date:   Mon Mar 30 16:15:47 2020 -0400

    perf build-ids: fall back to debuginfod query if debuginfo not found
    
    During a perf-record, use the -ldebuginfod API to query a debuginfod
    server, should the debug data not be found in the usual system
    locations.  If successful, the usual $HOME/.debug dir is populated.
    
    Signed-off-by: Frank Ch. Eigler <fche@redhat.com>

diff --git a/tools/build/Makefile.feature b/tools/build/Makefile.feature
index 574c2e0b9d20..51e051858d21 100644
--- a/tools/build/Makefile.feature
+++ b/tools/build/Makefile.feature
@@ -48,6 +48,7 @@ FEATURE_TESTS_BASIC :=                  \
         libelf-gelf_getnote             \
         libelf-getshdrstrndx            \
         libelf-mmap                     \
+        libdebuginfod                   \
         libnuma                         \
         numa_num_possible_cpus          \
         libperl                         \
@@ -114,6 +115,7 @@ FEATURE_DISPLAY ?=              \
          libbfd                 \
          libcap                 \
          libelf                 \
+         libdebuginfod          \
          libnuma                \
          numa_num_possible_cpus \
          libperl                \
diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
index 7ac0d8088565..1109f5ec96f7 100644
--- a/tools/build/feature/Makefile
+++ b/tools/build/feature/Makefile
@@ -26,6 +26,7 @@ FILES=                                          \
          test-libelf-gelf_getnote.bin           \
          test-libelf-getshdrstrndx.bin          \
          test-libelf-mmap.bin                   \
+         test-libdebuginfod.bin                 \
          test-libnuma.bin                       \
          test-numa_num_possible_cpus.bin        \
          test-libperl.bin                       \
@@ -155,6 +156,9 @@ endif
 $(OUTPUT)test-libelf-getshdrstrndx.bin:
 	$(BUILD) -lelf
 
+$(OUTPUT)test-libdebuginfod.bin:
+	$(BUILD) -ldebuginfod
+
 $(OUTPUT)test-libnuma.bin:
 	$(BUILD) -lnuma
 
diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
index 80e55e796be9..15eeecf4ff17 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -467,6 +467,11 @@ ifndef NO_LIBELF
     CFLAGS += -DHAVE_ELF_GETSHDRSTRNDX_SUPPORT
   endif
 
+  ifeq ($(feature-libdebuginfod), 1)
+    CFLAGS += -DHAVE_DEBUGINFOD_SUPPORT
+    EXTLIBS += -ldebuginfod
+  endif
+
   ifndef NO_DWARF
     ifeq ($(origin PERF_HAVE_DWARF_REGS), undefined)
       msg := $(warning DWARF register mappings have not been defined for architecture $(SRCARCH), DWARF support disabled);
diff --git a/tools/perf/util/build-id.c b/tools/perf/util/build-id.c
index c076fc7fe025..a33d00837d81 100644
--- a/tools/perf/util/build-id.c
+++ b/tools/perf/util/build-id.c
@@ -31,6 +31,10 @@
 #include "probe-file.h"
 #include "strlist.h"
 
+#if HAVE_DEBUGINFOD_SUPPORT
+#include <elfutils/debuginfod.h>
+#endif
+
 #include <linux/ctype.h>
 #include <linux/zalloc.h>
 
@@ -636,6 +640,21 @@ static char *build_id_cache__find_debug(const char *sbuild_id,
 	if (realname && access(realname, R_OK))
 		zfree(&realname);
 	nsinfo__mountns_exit(&nsc);
+
+#if HAVE_DEBUGINFOD_SUPPORT
+        if (realname == NULL) {
+                debuginfod_client* c = debuginfod_begin();
+                if (c != NULL) {
+                        int fd = debuginfod_find_debuginfo(c,
+                                                           (const unsigned char*)sbuild_id, 0,
+                                                           &realname);
+                        if (fd >= 0)
+                                close(fd); /* retaining reference by realname */
+                        debuginfod_end(c);
+                }
+        }
+#endif
+
 out:
 	free(debugfile);
 	return realname;