[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 1/2] Add libfuzzer target to fuzz/x86_instruction_emulator
On Tue, Jun 25, 2024 at 9:15 AM Jan Beulich <jbeulich@xxxxxxxx> wrote: > > On 25.06.2024 14:40, Tamas K Lengyel wrote: > > On Tue, Jun 25, 2024 at 7:52 AM Jan Beulich <jbeulich@xxxxxxxx> wrote: > >> > >> On 25.06.2024 13:12, Tamas K Lengyel wrote: > >>> On Tue, Jun 25, 2024 at 2:00 AM Jan Beulich <jbeulich@xxxxxxxx> wrote: > >>>> > >>>> On 24.06.2024 23:23, Tamas K Lengyel wrote: > >>>>> On Mon, Jun 24, 2024 at 11:55 AM Jan Beulich <jbeulich@xxxxxxxx> wrote: > >>>>>> > >>>>>> On 21.06.2024 21:14, Tamas K Lengyel wrote: > >>>>>>> @@ -58,6 +58,9 @@ afl-harness: afl-harness.o $(OBJS) cpuid.o > >>>>>>> wrappers.o > >>>>>>> afl-harness-cov: afl-harness-cov.o $(patsubst %.o,%-cov.o,$(OBJS)) > >>>>>>> cpuid.o wrappers.o > >>>>>>> $(CC) $(CFLAGS) $(GCOV_FLAGS) $(addprefix > >>>>>>> -Wl$(comma)--wrap=,$(WRAPPED)) $^ -o $@ > >>>>>>> > >>>>>>> +libfuzzer-harness: $(OBJS) cpuid.o > >>>>>>> + $(CC) $(CFLAGS) $(LIB_FUZZING_ENGINE) -fsanitize=fuzzer $^ -o $@ > >>>>>> > >>>>>> What is LIB_FUZZING_ENGINE? I don't think we have any use of that in > >>>>>> the > >>>>>> tree anywhere. > >>>>> > >>>>> It's used by oss-fuzz, otherwise it's not doing anything. > >>>>> > >>>>>> > >>>>>> I'm further surprised you get away here without wrappers.o. > >>>>> > >>>>> Wrappers.o was actually breaking the build for oss-fuzz at the linking > >>>>> stage. It works just fine without it. > >>>> > >>>> I'm worried here, to be honest. The wrappers serve a pretty important > >>>> role, and I'm having a hard time seeing why they shouldn't be needed > >>>> here when they're needed both for the test and afl harnesses. Could > >>>> you add some more detail on the build issues you encountered? > >>> > >>> With wrappers.o included doing the build in the oss-fuzz docker > >>> (ubuntu 20.04 base) fails with: > >>> > >>> ... > >>> clang -O1 -fno-omit-frame-pointer -gline-tables-only > >>> -Wno-error=enum-constexpr-conversion > >>> -Wno-error=incompatible-function-pointer-types > >>> -Wno-error=int-conversion -Wno-error=deprecated-declarations > >>> -Wno-error=implicit-function-declaration -Wno-error=implicit-int > >>> -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fsanitize=address > >>> -fsanitize-address-use-after-scope -fsanitize=fuzzer-no-link -m64 > >>> -DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall -Wstrict-prototypes > >>> -Wno-unused-but-set-variable -Wno-unused-local-typedefs -g3 -Werror > >>> -Og -fno-omit-frame-pointer > >>> -D__XEN_INTERFACE_VERSION__=__XEN_LATEST_INTERFACE_VERSION__ -MMD -MP > >>> -MF .libfuzzer-harness.d -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE > >>> -I/src/xen/tools/fuzz/x86_instruction_emulator/../../../tools/include > >>> -D__XEN_TOOLS__ -iquote . -fsanitize=fuzzer -fsanitize=fuzzer > >>> -Wl,--wrap=fwrite -Wl,--wrap=memcmp -Wl,--wrap=memcpy > >>> -Wl,--wrap=memset -Wl,--wrap=printf -Wl,--wrap=putchar -Wl,--wrap=puts > >>> -Wl,--wrap=snprintf -Wl,--wrap=strstr -Wl,--wrap=vprintf > >>> -Wl,--wrap=vsnprintf fuzz-emul.o x86-emulate.o x86_emulate/0f01.o > >>> x86_emulate/0fae.o x86_emulate/0fc7.o x86_emulate/decode.o > >>> x86_emulate/fpu.o cpuid.o wrappers.o -o libfuzzer-harness > >>> /usr/bin/ld: /usr/bin/ld: DWARF error: invalid or unhandled FORM value: > >>> 0x25 > >>> /usr/local/lib/clang/18/lib/x86_64-unknown-linux-gnu/libclang_rt.fuzzer.a(fuzzer.o): > >>> in function `std::__Fuzzer::__libcpp_snprintf_l(char*, unsigned long, > >>> __locale_struct*, char const*, ...)': > >>> cxa_noexception.cpp:(.text._ZNSt8__Fuzzer19__libcpp_snprintf_lEPcmP15__locale_structPKcz[_ZNSt8__Fuzzer19__libcpp_snprintf_lEPcmP15__locale_structPKcz]+0x9a): > >>> undefined reference to `__wrap_vsnprintf' > >>> clang: error: linker command failed with exit code 1 (use -v to see > >>> invocation) > >>> make: *** [Makefile:62: libfuzzer-harness] Error 1 > >>> rm x86-emulate.c wrappers.c cpuid.c > >>> make: Leaving directory '/src/xen/tools/fuzz/x86_instruction_emulator' > >>> ERROR:__main__:Building fuzzers failed. > >> > >> Hmm, yes, means we'll need an actual vsnprintf() wrapper, not just a > >> declaration thereof. > > > > I don't really get what this wrapper accomplishes > > They guard against clobbering of in-register state (SIMD registers in > particular, but going forward maybe also eGRP-s as introduced by APX) > by library functions called between emulation of individual insns (or, > especially possible for fuzzing instrumented code, I think) even from > in the middle of emulating an insn. (Something as simple as the > compiler inserting a call to memcpy() or memset() somewhere in the > translation of the emulator source code could also clobber state.) > > > and as I said, fuzzing works with oss-fuzz just fine without it. > > I'm inclined to take this as "it appears to work just fine". Fuzzed > input register state may be lost by doing a library call somewhere, > rendering the fuzzing results less useful. This would pretty > certainly stop being tolerable the moment you compared results of > native execution of a sequence of instructions with the emulated > counterpart. Yea, that may be. Any suggested way to fix the linking issue though? I'm not even sure why the problem only appears in the oss-fuzz build, when I just run make normally it seems to work. Tamas
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