On 2024.12.21 18:47, Junio C Hamano wrote:
> Junio C Hamano <gitster@xxxxxxxxx> writes:
>
> > James Mills <prologic@xxxxxxxxxxxxxxxxxxx> writes:
> >
> >> tcc: error: unsupported linker option '--allow-multiple-definition'
> >>
> >> I bisected the Git releases and traced the introduction of this new
> >> flag to v2.46.0
> >>
> >> I can't find any details of this flag really or when this was
> >> introduced in the GNU binutils and so far I haven't asked if the Tiny
> >> C devs intend to support this option (yet).
> >
> > Would
> >
> > $ make LINK_FUZZ_PROGRAMS=""
> >
> > help?
> >
> > The platform-specific tweak defined in config.mak.uname file assumes
> > that you have glibc plus gcc or clang with usual binutils niceties
> > once you claim that you are Linux. It lumps all different variants
> > of Linux into a single ball of wax and defines LINK_FUZZ_PROGRAMS
> > Makefile macro, which is a bit unfortunate.
>
> Having said that, I am not sure if the commit that introduced the
> fuzz-all linkage rules, which is 8b9a42bf (fuzz: fix fuzz test build
> rules, 2024-01-19), needed to do so in the first place. With this
> trivial patch at the end of the message applied on top of 8b9a42bf
> (or more recent 'master', for that matter), "make fuzz-all" seems to
> link things just fine.
>
> Note that "make CC=clang fuzz-all" in the older code used to fail
> due to something else from gcc+ (which recent build no longer makes
> mandatory), but in today's code with gcc/clang/ld available locally,
> the linker flag does not seem to be needed for "make fuzz-all".
>
> Josh, do you offhand know what made us add the option?
>
> Thanks.
>
> diff --git c/Makefile w/Makefile
> index c0cbed69d8..5af8935968 100644
> --- c/Makefile
> +++ w/Makefile
> @@ -3848,7 +3848,6 @@ FUZZ_CXXFLAGS ?= $(ALL_CFLAGS)
>
> $(FUZZ_PROGRAMS): %: %.o oss-fuzz/dummy-cmd-main.o $(GITLIBS) GIT-LDFLAGS
> $(QUIET_LINK)$(CXX) $(FUZZ_CXXFLAGS) -o $@ $(ALL_LDFLAGS) \
> - -Wl,--allow-multiple-definition \
> $(filter %.o,$^) $(filter %.a,$^) $(LIBS) $(LIB_FUZZING_ENGINE)
>
> fuzz-all: $(FUZZ_PROGRAMS)
Sorry for the delayed response, I'm finally back to work after holidays
/ sick leave.
Unfortunately, the --allow-multiple-definitions flag is still required.
When simply running "make fuzz-all" with no other args (with or without
the patch above), we do not actually produce a working fuzzer:
$ make clean
$ make fuzz-all
$ ./oss-fuzz/fuzz-date
BUG: oss-fuzz/dummy-cmd-main.c:12: We should not execute cmd_main() from a fuzz target
zsh: IOT instruction ./oss-fuzz/fuzz-date
This is because we're linking with common-main.o and
oss-fuzz/dummy-cmd-main.o. commain-main.o:main() calls cmd_main(), which
it finds in oss-fuzz/dummy-cmd-main.o. If we instead build with a
fuzzing engine enabled:
$ make clean
$ make CC=clang FUZZ_CXX=clang++ \
CFLAGS="-fsanitize=fuzzer-no-link,address" \
LIB_FUZZING_ENGINE="-fsanitize=fuzzer,address" \
fuzz-all
$ ./oss-fuzz/fuzz-date
INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 2884061060
INFO: Loaded 1 modules (55020 inline 8-bit counters): 55020 [0x562aff0d2840, 0x562aff0dff2c),
INFO: Loaded 1 PC tables (55020 PCs): 55020 [0x562aff0dff30,0x562aff1b6df0),
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
INFO: A corpus is not provided, starting from an empty corpus
[...]
This works because the fuzzing engine provides its own main() which
overrides ours, and so we never end up calling
dummy-cmd-main.o:cmd_main().
However, that build will fail if we remove the
--allow-multiple-definition flag:
$ make clean
$ make CC=clang FUZZ_CXX=clang++ \
CFLAGS="-fsanitize=fuzzer-no-link,address" \
LIB_FUZZING_ENGINE="-fsanitize=fuzzer,address" \
fuzz-all
[...]
LINK oss-fuzz/fuzz-commit-graph
/usr/bin/ld: common-main.o: in function `main':
common-main.c:(.text.main[main]+0x0): multiple definition of `main'; /usr/lib/llvm-16/lib/clang/16/lib/linux/libclang_rt.fuzzer-x86_64.a(FuzzerMain.cpp.o):(.text.main+0x0): first defined here
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [Makefile:3890: oss-fuzz/fuzz-commit-graph] Error 1
As Junio says, a short term fix would be to build with
LINK_FUZZ_PROGRAMS="". A better solution would be to make
config.mak.uname smarter about whether to enable this by default. I see
that we use "detect-compiler" in config.mak.dev, would it make sense to
check this in config.mak.uname as well?
