Date: Fri, 8 Dec 2017 12:36:26 +0100
From: Jiri Olsa <jolsa@redhat.com>
To: Stephane Eranian <eranian@google.com>
Cc: linux-kernel@vger.kernel.org, acme@redhat.com, peterz@infradead.org,
        mingo@elte.hu
Subject: Re: [PATCH] tools/perf/inject: fix dwarf support detection
Message-ID: <20171208113626.GC2799@krava>
References: <1512616084-17948-1-git-send-email-eranian@google.com>
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On Wed, Dec 06, 2017 at 07:08:04PM -0800, Stephane Eranian wrote:
> 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@ELFUTILS_1.7'
> 
> 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@google.com>
> ---
>  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:
>  	$(BUILD)
>  
> -DWARFLIBS := -ldw
> +#
> +# there may be a dependency between libdw and libelf such as
> +# /usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/libdw.so: undefined reference to `elf_compress@ELFUTILS_1.7'
> +# so force linking with libelf
> +#
> +DWARFLIBS := -ldw -lelf
>  ifeq ($(findstring -static,${LDFLAGS}),-static)
>  DWARFLIBS += -lelf -lebl -lz -llzma -lbz2

you can remove '-lelf' from above line then

>  endif
>  
>  $(OUTPUT)test-dwarf.bin:
> +	echo DWARF=$(DWARFLIBS)

whats this echo for?

thanks,
jirka

>  	$(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
>