[PATCH v2] 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 die 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, the 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.

In v2, we removed the extraneaous echo in the make target.

Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
---
 tools/build/feature/Makefile | 7 ++++++-
 tools/perf/builtin-inject.c  | 3 +++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
index cff38f342283..be5679707aa0 100644
--- a/tools/build/feature/Makefile
+++ b/tools/build/feature/Makefile
@@ -104,7 +104,12 @@ __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
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