Adds basic libfuzzer harness

This harness simply mounts and unmounts a ramdisk
that is populated by fuzzed data. This fuzz harness
tests that littlefs is able to safely attempt to
mount a corrupted storage device.

Makefile

@@ -59,12 +59,28 @@ BENCH_PERF  := $(BENCH_RUNNER:%=%.perf)
 BENCH_TRACE := $(BENCH_RUNNER:%=%.trace)
 BENCH_CSV   := $(BENCH_RUNNER:%=%.csv)
 
-CFLAGS += -fcallgraph-info=su
 CFLAGS += -g3
 CFLAGS += -I.
 CFLAGS += -std=c99 -Wall -Wextra -pedantic
 CFLAGS += -Wmissing-prototypes
+
+ifeq ($(CC),gcc)
+CFLAGS += -fcallgraph-info=su
 CFLAGS += -ftrack-macro-expansion=0
+endif
+
+ifdef FUZZ
+CFLAGS += -fsanitize=fuzzer-no-link
+endif
+
+ifdef ASAN
+CFLAGS += -fsanitize=address
+endif
+
+ifdef UBSAN
+CFLAGS += -fsanitize=undefined
+endif
+
 ifdef DEBUG
 CFLAGS += -O0
 else
@@ -472,6 +488,14 @@ benchmarks-diff: $(BENCH_CSV)
 $(BUILDDIR)/lfs: $(OBJ)
 	$(CC) $(CFLAGS) $^ $(LFLAGS) -o $@
 
+# Libfuzzer is only supported on clang
+ifeq ($(CC),clang)
+ifdef FUZZ
+$(BUILDDIR)/fuzz_mount: $(OBJ) fuzz/fuzz_mount.c
+	$(CC) $(CFLAGS) $^ -fsanitize=fuzzer -DCUSTOM_MUTATOR $(LFLAGS)  -o $@
+endif
+endif
+
 $(BUILDDIR)/liblfs.a: $(OBJ)
 	$(AR) rcs $@ $^
 
@@ -583,3 +607,4 @@ clean:
 	rm -f $(BENCH_PERF)
 	rm -f $(BENCH_TRACE)
 	rm -f $(BENCH_CSV)
+	rm -f fuzz_mount

fuzz/fuzz_mount.c

@@ -0,0 +1,111 @@
+#include <string.h>
+#include "lfs.h"
+
+#define STORAGE_SIZE 1024*1024
+
+static uint8_t disk_buffer[STORAGE_SIZE];
+
+int min(int a, int b) {
+    if(a < b) {
+        return a;
+    }
+    return b;
+}
+
+static int read(const struct lfs_config *c, lfs_block_t block,
+            lfs_off_t off, void *buffer, lfs_size_t size) {
+    memcpy(buffer, &disk_buffer[block*c->read_size + off], size);
+    return 0;
+}
+
+static int prog(const struct lfs_config *cfg, lfs_block_t block,
+        lfs_off_t off, const void *buffer, lfs_size_t size) {
+    memcpy(&disk_buffer[block*cfg->read_size + off], buffer, size);
+    return 0;
+}
+
+static int erase(const struct lfs_config *cfg, lfs_block_t block) {
+    (void)cfg;
+    (void)block;
+    return 0;
+}
+
+static int sync(const struct lfs_config *cfg) {
+    // sync is a noop
+    (void)cfg;
+    return 0;
+}
+
+// configuration of the filesystem is provided by this struct
+const struct lfs_config filesystem_cfg = {
+    // block device operations
+    .read  = read,
+    .prog  = prog,
+    .erase = erase,
+    .sync  = sync,
+
+    // block device configuration
+    .read_size = 1024,
+    .prog_size = 1024,
+    .block_size = 1024,
+    .block_count = 1024,
+    .cache_size = 1024,
+    .lookahead_size = 1024,
+    .block_cycles = 500,
+};
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+    if(size == 0) {
+        return -1;
+    }
+    memset(disk_buffer, 0, sizeof(disk_buffer));
+
+    // variables used by the filesystem
+    lfs_t lfs;
+
+    // Copy fuzzed data into fake storage device.
+    memcpy(disk_buffer, data, min(size, STORAGE_SIZE));
+
+    // mount the filesystem
+    int err = lfs_mount(&lfs, &filesystem_cfg);
+
+    if (err) {
+      return 0;
+    }
+
+    // release any resources we were using
+    lfs_unmount(&lfs);
+
+    return 0;
+}
+
+#ifdef CUSTOM_MUTATOR
+
+// Forward-declare the libFuzzer's mutator callback.
+size_t LLVMFuzzerMutate(uint8_t *Data, size_t Size, size_t MaxSize);
+
+size_t LLVMFuzzerCustomMutator(uint8_t *data, size_t size,
+                                           size_t max_size, unsigned int seed) {
+    (void)seed;
+    memset(disk_buffer, 0, sizeof(disk_buffer));
+    // variables used by the filesystem
+    lfs_t lfs;
+
+    // Copy fuzzed data into fake storage device.
+    memcpy(disk_buffer, data, min(size, STORAGE_SIZE));
+
+    // Mount the filesystem
+    int err = lfs_mount(&lfs, &filesystem_cfg);
+
+    // Reformat if we can't mount the filesystem
+    if (err) {
+        lfs_format(&lfs, &filesystem_cfg);
+        lfs_mount(&lfs, &filesystem_cfg);
+    }
+    lfs_unmount(&lfs);
+    memcpy(data, disk_buffer, min(STORAGE_SIZE, max_size));
+
+    return LLVMFuzzerMutate(data, size, max_size);
+}
+
+#endif  // CUSTOM_MUTATOR