VFS: Early return NULL if attempting to map 0 bytes from a file

CHANGES

@@ -112,6 +112,7 @@ Misc:
  - Qt: Resume crashed game when loading a save state
  - SDL: Support exposing an axis directly as the gyro value (closes mgba.io/i/2531)
  - Windows: Attach to console if present
+ - VFS: Early return NULL if attempting to map 0 bytes from a file
  - Vita: Add bilinear filtering option (closes mgba.io/i/344)
 
 0.9.3: (2021-12-17)

src/platform/3ds/3ds-vfs.c

@@ -137,6 +137,9 @@ ssize_t _vf3dWrite(struct VFile* vf, const void* buffer, size_t size) {
 static void* _vf3dMap(struct VFile* vf, size_t size, int flags) {
 	struct VFile3DS* vf3d = (struct VFile3DS*) vf;
 	UNUSED(flags);
+	if (!size) {
+		return NULL;
+	}
 	void* buffer = anonymousMemoryMap(size);
 	if (buffer) {
 		u32 sizeRead;

src/platform/psp2/sce-vfs.c

@@ -113,6 +113,9 @@ ssize_t _vfsceWrite(struct VFile* vf, const void* buffer, size_t size) {
 static void* _vfsceMap(struct VFile* vf, size_t size, int flags) {
 	struct VFileSce* vfsce = (struct VFileSce*) vf;
 	UNUSED(flags);
+	if (!size) {
+		return NULL;
+	}
 	void* buffer = anonymousMemoryMap(size);
 	if (buffer) {
 		SceOff cur = sceIoLseek(vfsce->fd, 0, SEEK_CUR);

src/util/vfs/vfs-fd.c

@@ -134,6 +134,9 @@ ssize_t _vfdWrite(struct VFile* vf, const void* buffer, size_t size) {
 #ifdef _POSIX_MAPPED_FILES
 static void* _vfdMap(struct VFile* vf, size_t size, int flags) {
 	struct VFileFD* vfd = (struct VFileFD*) vf;
+	if (!size) {
+		return NULL;
+	}
 	int mmapFlags = MAP_PRIVATE;
 	if (flags & MAP_WRITE) {
 		mmapFlags = MAP_SHARED;
@@ -153,6 +156,9 @@ static void _vfdUnmap(struct VFile* vf, void* memory, size_t size) {
 #elif defined(_WIN32)
 static void* _vfdMap(struct VFile* vf, size_t size, int flags) {
 	struct VFileFD* vfd = (struct VFileFD*) vf;
+	if (!size) {
+		return NULL;
+	}
 	int createFlags = PAGE_WRITECOPY;
 	int mapFiles = FILE_MAP_COPY;
 	if (flags & MAP_WRITE) {
@@ -192,12 +198,16 @@ static void _vfdUnmap(struct VFile* vf, void* memory, size_t size) {
 #else
 static void* _vfdMap(struct VFile* vf, size_t size, int flags) {
 	struct VFileFD* vfd = (struct VFileFD*) vf;
+	if (!size) {
+		vfd->writable = false;
+		return NULL;
+	}
 	if (flags & MAP_WRITE) {
 		vfd->writable = true;
 	}
 	void* mem = anonymousMemoryMap(size);
 	if (!mem) {
-		return 0;
+		return NULL;
 	}
 
 	off_t pos = lseek(vfd->fd, 0, SEEK_CUR);