Re: [PATCH 1/2] Add libfuzzer target to fuzz/x86_instruction_emulator

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.06.2024 15:40, Tamas K Lengyel wrote:
> 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?

As said before, we need to gain a real wrapper for vsnprintf(). Right
now we only have a declaration thereof, for use by the wrapper for
snprintf().

> 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.

Sounds a little odd indeed. Yet I have no idea towards the differences
between the two.

Jan