[PATCH] tools/perf/inject: fix dwarf support detection

[Stephane Eranian]

I ran into problems trying to use the JIT support and display
source-level information. Basically, there was no dwarf debug
info generated in the jitted-XX.so files, yet I had libdw-dev
installed.

Turns out that the feature test build for test-dwarf.bin was broken
for me. It would fail for the wrong reason:

/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/libdw.so: undefined reference to `elf_compress@xxxxxxxxxxxx'

In other words, libdw has a dependency on libelf which the Makefile
was not accounting for. Thus, perf was not compiled with HAVE_DWARF_SUPPORT
and the code in genelf.c was not compiled in.

This patch fixes the issue by adding -lelf to the build rule for test-dwarf.c.

I wasted quite some time trying to debug this, so I have added a warning
in builtin-inject.c so that the user is warned that in case dwarf support
is disabled, no source line information will be generated in the output
.so files from the jitted code.

Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
---
 tools/build/feature/Makefile | 8 +++++++-
 tools/perf/builtin-inject.c  | 3 +++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
index cff38f342283..3242e11784e0 100644
--- a/tools/build/feature/Makefile
+++ b/tools/build/feature/Makefile
@@ -104,12 +104,18 @@ __BUILDXX = $(CXX) $(CXXFLAGS) -MD -Wall -Werror -o $@ $(patsubst %.bin,%.cpp,$(
 $(OUTPUT)test-setns.bin:
 	$(BUILD)
 
-DWARFLIBS := -ldw
+#
+# there may be a dependency between libdw and libelf such as
+# /usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/libdw.so: undefined reference to `elf_compress@xxxxxxxxxxxx'
+# so force linking with libelf
+#
+DWARFLIBS := -ldw -lelf
 ifeq ($(findstring -static,${LDFLAGS}),-static)
 DWARFLIBS += -lelf -lebl -lz -llzma -lbz2
 endif
 
 $(OUTPUT)test-dwarf.bin:
+	echo DWARF=$(DWARFLIBS)
 	$(BUILD) $(DWARFLIBS)
 
 $(OUTPUT)test-dwarf_getlocations.bin:
diff --git a/tools/perf/builtin-inject.c b/tools/perf/builtin-inject.c
index 16a28547ca86..524255ed9a39 100644
--- a/tools/perf/builtin-inject.c
+++ b/tools/perf/builtin-inject.c
@@ -856,6 +856,9 @@ int cmd_inject(int argc, const char **argv)
 	}
 #ifdef HAVE_JITDUMP
 	if (inject.jit_mode) {
+#ifndef HAVE_DWARF_SUPPORT
+		pr_warning("perf tool compiled without dwarf, no source line information will be generated\n");
+#endif
 		inject.tool.mmap2	   = perf_event__jit_repipe_mmap2;
 		inject.tool.mmap	   = perf_event__jit_repipe_mmap;
 		inject.tool.ordered_events = true;
-- 
2.7.4