fuzz: Add instructions for using generic-fuzz

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20201023150746.107063-13-alxndr@bu.edu>
Signed-off-by: Thomas Huth <thuth@redhat.com>

docs/devel/fuzzing.txt

@@ -125,6 +125,45 @@ provided by libfuzzer. Libfuzzer passes a byte array and length. Commonly the
 fuzzer loops over the byte-array interpreting it as a list of qtest commands,
 addresses, or values.
 
+== The Generic Fuzzer ==
+Writing a fuzz target can be a lot of effort (especially if a device driver has
+not be built-out within libqos). Many devices can be fuzzed to some degree,
+without any device-specific code, using the generic-fuzz target.
+
+The generic-fuzz target is capable of fuzzing devices over their PIO, MMIO,
+and DMA input-spaces. To apply the generic-fuzz to a device, we need to define
+two env-variables, at minimum:
+
+QEMU_FUZZ_ARGS= is the set of QEMU arguments used to configure a machine, with
+the device attached. For example, if we want to fuzz the virtio-net device
+attached to a pc-i440fx machine, we can specify:
+QEMU_FUZZ_ARGS="-M pc -nodefaults -netdev user,id=user0 \
+                -device virtio-net,netdev=user0"
+
+QEMU_FUZZ_OBJECTS= is a set of space-delimited strings used to identify the
+MemoryRegions that will be fuzzed. These strings are compared against
+MemoryRegion names and MemoryRegion owner names, to decide whether each
+MemoryRegion should be fuzzed. These strings support globbing. For the
+virtio-net example, we could use QEMU_FUZZ_OBJECTS=
+ * 'virtio-net'
+ * 'virtio*'
+ * 'virtio* pcspk' (Fuzz the virtio devices and the PC speaker...)
+ * '*' (Fuzz the whole machine)
+
+The "info mtree" and "info qom-tree" monitor commands can be especially useful
+for identifying the MemoryRegion and Object names used for matching.
+
+As a generic rule-of-thumb, the more MemoryRegions/Devices we match, the greater
+the input-space, and the smaller the probability of finding crashing inputs for
+individual devices. As such, it is usually a good idea to limit the fuzzer to
+only a few MemoryRegions.
+
+To ensure that these env variables have been configured correctly, we can use:
+
+./qemu-fuzz-i386 --fuzz-target=generic-fuzz -runs=0
+
+The output should contain a complete list of matched MemoryRegions.
+
 = Implementation Details =
 
 == The Fuzzer's Lifecycle ==