Re: [PATCH v2] Update 'make fuzz-all' docs to reflect modern clang

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 4, 2021, 22:48 UTC
Message-ID: <xmqqlfb2cz8c.fsf@gitster.c.googlers.com>
In-Reply-To: <pull.889.v2.git.1614871707845.gitgitgadget@gmail.com>

"Andrzej Hunt via GitGitGadget" <gitgitgadget@gmail.com> writes:

> From: Andrzej Hunt <ajrhunt@google.com>

> Subject: Re: [PATCH v2] Update 'make fuzz-all' docs to reflect modern clang

I'd retitte it to

    Makefile: update 'make fuzz-all' docs to reflect modern clang

> Clang no longer produces a libFuzzer.a, instead you can include
> libFuzzer by using -fsanitize=fuzzer.

Do we see two sentences here?  IOW, s/, instead/. Instead/ is needed?

Show 6 quoted lines

> Therefore we should use
> that in the example command for building fuzzers.
>
> We also add -fsanitize=fuzzer-no-link to ensure that all the required
> instrumentation is added when compiling git [1], and remove
>  -fsanitize-coverage=trace-pc-guard as it is deprecated.

Without something like s/add/add to CFLAGS/, I found this a bit cryptic and failed to read what it wanted to do without looking at the patch text itself.

Show 6 quoted lines

> I happen to have tested with LLVM 11 - however -fsanitize=fuzzer appears to
> work in a wide range of reasonably modern clangs.
>
> (On my system: what used to be libFuzzer.a now lives under the following path,
>  which is tricky albeit not impossible for a novice such as myself to find:
> /usr/lib64/clang/11.0.0/lib/linux/libclang_rt.fuzzer-x86_64.a )

All nice things to have in the log message.

Show 22 quoted lines

>  Makefile | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index dd08b4ced01c..c7248ac6057b 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -3292,11 +3292,11 @@ cover_db_html: cover_db
>  # are not necessarily appropriate for general builds, and that vary greatly
>  # depending on the compiler version used.
>  #
> -# An example command to build against libFuzzer from LLVM 4.0.0:
> +# An example command to build against libFuzzer from LLVM 11.0.0:
>  #
>  # make CC=clang CXX=clang++ \
> -#      CFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
> -#      LIB_FUZZING_ENGINE=/usr/lib/llvm-4.0/lib/libFuzzer.a \
> +#      CFLAGS="-fsanitize=fuzzer-no-link,address" \
> +#      LIB_FUZZING_ENGINE="-fsanitize=fuzzer" \
>  #      fuzz-all
>  #
>  FUZZ_CXXFLAGS ?= $(CFLAGS)

LIB_FUZZING_ENGINE is used this way in the Makefile:

    $(FUZZ_PROGRAMS): all
            $(QUIET_LINK)$(CXX) $(FUZZ_CXXFLAGS) $(LIB_OBJS) $(BUILTIN_OBJS) \
                    $(XDIFF_OBJS) $(EXTLIBS) git.o $@.o $(LIB_FUZZING_ENGINE) -o $@

and it is somewhat annoying to see a compiler/linker option that late on the command line, where readers would expect an object file or a library archive would appear. It makes me wonder if we should instead be doing something along the following line:

 - empty LIB_FUZZING_ENGINE by default
 - add -fsanitize=fuzzer names to FUZZ_CXXFLAGS

i.e.

diff --git c/Makefile w/Makefile
index 4128b457e1..b5df76b33b 100644
--- c/Makefile
+++ w/Makefile
@@ -3306,14 +3306,15 @@ cover_db_html: cover_db
 # are not necessarily appropriate for general builds, and that vary greatly
 # depending on the compiler version used.
 #
-# An example command to build against libFuzzer from LLVM 4.0.0:
+# An example command to build against libFuzzer from LLVM 11.0.0:
 #
 # make CC=clang CXX=clang++ \
-#      CFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
-#      LIB_FUZZING_ENGINE=/usr/lib/llvm-4.0/lib/libFuzzer.a \
+#      CFLAGS="-fsanitize=fuzzer-no-link,address" \
 #      fuzz-all
 #
 FUZZ_CXXFLAGS ?= $(CFLAGS)
+FUZZ_CXXFLAGS += -fsanitize=fuzzer
+LIB_FUZZING_ENGINE =
 
 .PHONY: fuzz-all
 

In the meantime, I'll queue the version you sent as-is (modulo the
retitling).

Thanks.