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

goroutine 16 [running]:
github.com/google/syzkaller/prog.(*randGen).filename(0xc0006d2a40, 0xc0006cd340, 0xbf69a0, 0x0, 0x0)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/rand.go:161 +0x2ac
github.com/google/syzkaller/prog.(*BufferType).mutate(0xbf69a0, 0xc0006d2a40, 0xc0006cd340, 0x8fa9e0, 0xc0007bb7a0, 0xc0006cd108, 0xc0007bb770, 0x0, 0xc0007bb800, 0x7159f0a16cf8e074, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:264 +0x533
github.com/google/syzkaller/prog.(*Target).mutateArg(0xc0000711e0, 0xc0006d2a40, 0xc0006cd340, 0x8fa9e0, 0xc0007bb7a0, 0xc0006cd108, 0xc0007bb770, 0x0, 0x4f0f00, 0xc000883e0f, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:172 +0xe3
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc000883ed0, 0xa)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:153 +0x231
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc0006cd000, 0x8f8660, 0xc0005576e0, 0x1e, 0xc000684740, 0xc000626000, 0x8f7, 0x900)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:34 +0x2db
main.(*Proc).loop(0xc000684780)
	/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