Add libfuzzer target to fuzz/x86_instruction_emulator

This target enables integration into oss-fuzz. Changing invalid input return
to -1 as values other then 0/-1 are reserved by libfuzzer. Also adding the
missing __wrap_vsnprintf wrapper which is required for successful oss-fuzz
build.

Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

tools/fuzz/x86_instruction_emulator/Makefile

@@ -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 wrappers.o
+	$(CC) $(CFLAGS) $(LIB_FUZZING_ENGINE) -fsanitize=fuzzer $(addprefix -Wl$(comma)--wrap=,$(WRAPPED)) $^ -o $@
+
 # Common targets
 .PHONY: all
 all: x86-insn-fuzz-all
@@ -67,7 +70,8 @@ distclean: clean
 
 .PHONY: clean
 clean:
-	rm -f *.a *.o $(DEPS_RM) afl-harness afl-harness-cov *.gcda *.gcno *.gcov
+	rm -f *.a *.o $(DEPS_RM) *.gcda *.gcno *.gcov
+	rm -f afl-harness afl-harness-cov libfuzzer-harness
 	rm -rf x86_emulate x86-emulate.c x86-emulate.h wrappers.c cpuid.c
 
 .PHONY: install

tools/fuzz/x86_instruction_emulator/fuzz-emul.c

@@ -906,14 +906,12 @@ int LLVMFuzzerTestOneInput(const uint8_t *data_p, size_t size)
 
     if ( size <= DATA_OFFSET )
     {
-        printf("Input too small\n");
-        return 1;
+        return -1;
     }
 
     if ( size > FUZZ_CORPUS_SIZE )
     {
-        printf("Input too large\n");
-        return 1;
+        return -1;
     }
 
     memcpy(&input, data_p, size);

tools/tests/x86_emulator/wrappers.c

@@ -91,6 +91,17 @@ int __wrap_snprintf(char *buf, size_t n, const char *fmt, ...)
     return rc;
 }
 
+int __wrap_vsnprintf(char *buf, size_t n, const char *fmt, va_list varg)
+{
+    int rc;
+
+    emul_save_fpu_state();
+    rc = __real_vsnprintf(buf, n, fmt, varg);
+    emul_restore_fpu_state();
+
+    return rc;
+}
+
 char *__wrap_strstr(const char *s1, const char *s2)
 {
     char *s;