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

On 2021.03.04 14:48, Junio C Hamano wrote:

"Andrzej Hunt via GitGitGadget" [off-list ref] writes:

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From: Andrzej Hunt <redacted>

quoted

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

quoted

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?

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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.

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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.

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 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.

Yes, it appears that clang has changed how the fuzzing engine is
selected, as this used to be just a library path (as you see in the
diff). We might as well move this option up with the rest of the flags.

I don't think we want to mess with FUZZ_CXXFLAGS, as oss-fuzz may be
adding conflicting -fsanitize args here. Having LIB_FUZZING_ENGINE
default to empty should be fine though.

 
 .PHONY: fuzz-all
 

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

Thanks.