fuzz/x86_emulate: Add 'afl-cov' target

...to generate a "normal" coverage-instrumented binary, suitable for
use with gcov or afl-cov.

This is slightly annoying because:

 - Every object file needs to have been instrumented to work
   effectively

 - You generally want to have both an afl-instrumented binary and a
   gcov-instrumented binary at the same time, but

 - gcov instrumentation and afl instrumentation are mutually exclusive

So when making the `afl-cov` target, generate a second set of object
files and a second binary with the `-cov` suffix.

While we're here, remove the redundant x86-emulate.c dependency for
x86-emulate.o.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

.gitignore

@@ -166,6 +166,7 @@ tools/fuzz/x86_instruction_emulator/asm
 tools/fuzz/x86_instruction_emulator/x86_emulate
 tools/fuzz/x86_instruction_emulator/x86-emulate.[ch]
 tools/fuzz/x86_instruction_emulator/afl-harness
+tools/fuzz/x86_instruction_emulator/afl-harness-cov
 tools/helpers/_paths.h
 tools/helpers/init-xenstore-domain
 tools/helpers/xen-init-dom0

tools/fuzz/README.afl

@@ -41,3 +41,17 @@ Use the x86 instruction emulator fuzzer as an example.
    $ $AFLPATH/afl-fuzz -t 1000 -i testcase_dir -o findings_dir -- ./afl-harness
 
 Please see AFL documentation for more information.
+
+# GENERATING COVERAGE INFORMATION
+
+To use afl-cov or gcov, you need a separate binary instrumented to
+generate coverage data.  To do this, use the target `afl-cov`:
+
+    $ make afl-cov #produces afl-harness-cov
+
+NOTE: Please also note that the coverage instrumentation hard-codes
+the absolute path for the instrumentation read and write files in the
+binary; so coverage data will always show up in the build directory no
+matter where you run the binary from.
+
+Please see afl-cov and/or gcov documentation for more information.

tools/fuzz/x86_instruction_emulator/Makefile

@@ -23,12 +23,17 @@ x86-emulate.c x86-emulate.h: %:
 
 CFLAGS += $(CFLAGS_xeninclude) -D__XEN_TOOLS__ -I.
 
+GCOV_FLAGS := --coverage
+%-cov.o: %.c
+	$(CC) -c $(CFLAGS) $(GCOV_FLAGS) $< -o $@
+
 x86.h := asm/x86-vendors.h asm/x86-defns.h asm/msr-index.h
 x86_emulate.h := x86-emulate.h x86_emulate/x86_emulate.h $(x86.h)
 
-x86-emulate.o: x86-emulate.c x86_emulate/x86_emulate.c $(x86_emulate.h)
+# x86-emulate.c will be implicit for both
+x86-emulate.o x86-emulate-cov.o: x86_emulate/x86_emulate.c $(x86_emulate.h)
 
-fuzz-emul.o: $(x86_emulate.h)
+fuzz-emul.o fuzz-emulate-cov.o: $(x86_emulate.h)
 
 x86-insn-fuzzer.a: fuzz-emul.o x86-emulate.o
 	$(AR) rc $@ $^
@@ -36,6 +41,9 @@ x86-insn-fuzzer.a: fuzz-emul.o x86-emulate.o
 afl-harness: afl-harness.o fuzz-emul.o x86-emulate.o
 	$(CC) $(CFLAGS) $^ -o $@
 
+afl-harness-cov: afl-harness-cov.o fuzz-emul-cov.o x86-emulate-cov.o
+	$(CC) $(CFLAGS) $(GCOV_FLAGS) $^ -o $@
+
 # Common targets
 .PHONY: all
 all: x86-insn-fuzz-all
@@ -46,7 +54,7 @@ distclean: clean
 
 .PHONY: clean
 clean:
-	rm -f *.a *.o .*.d afl-harness
+	rm -f *.a *.o .*.d afl-harness afl-harness-cov *.gcda *.gcno *.gcov
 
 .PHONY: install
 install: all
@@ -55,3 +63,6 @@ install: all
 
 .PHONY: afl
 afl: afl-harness
+
+.PHONY: afl-cov
+afl-cov: afl-harness-cov