panic: sandbox escaping file name "../file0"

goroutine 17 [running]:
github.com/google/syzkaller/prog.(*randGen).filename(0xc000591c00, 0xc00093b480, 0xbf4c40, 0x0, 0xc00093b400)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/rand.go:161 +0x2ac
github.com/google/syzkaller/prog.(*BufferType).mutate(0xbf4c40, 0xc000591c00, 0xc00093b480, 0x8fa9e0, 0xc000763b90, 0xc00093b208, 0xc000763b60, 0x0, 0xc000763c00, 0x3c0756ed5daa7823, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:264 +0x533
github.com/google/syzkaller/prog.(*Target).mutateArg(0xc0000711e0, 0xc000591c00, 0xc00093b480, 0x8fa9e0, 0xc000763b90, 0xc00093b208, 0xc000763b60, 0x0, 0x4f0f00, 0xc000877e0f, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:172 +0xe3
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc000877ed0, 0xa)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:153 +0x231
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc00093b1c0, 0x8f8660, 0xc000762630, 0x1e, 0xc000728840, 0xc000756000, 0x98d, 0xc00)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:34 +0x2db
main.(*Proc).loop(0xc0007288c0)
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:99 +0x446created by main.main
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/fuzzer.go:236 +0xfe2