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

goroutine 17 [running]:
github.com/google/syzkaller/prog.(*randGen).filename(0xc0004bc6e0, 0xc000a52a00, 0xbf78a0, 0x0, 0xc000a52a00)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/rand.go:161 +0x2ac
github.com/google/syzkaller/prog.(*BufferType).mutate(0xbf78a0, 0xc0004bc6e0, 0xc000a52a00, 0x8fa9e0, 0xc00097d6e0, 0xc000a52388, 0xc00097d6b0, 0x0, 0xc00097d800, 0x950766d7dd06f91, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:264 +0x533
github.com/google/syzkaller/prog.(*Target).mutateArg(0xc0000711e0, 0xc0004bc6e0, 0xc000a52a00, 0x8fa9e0, 0xc00097d6e0, 0xc000a52388, 0xc00097d6b0, 0x0, 0x4f0f00, 0xc0007c5e0f, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:172 +0xe3
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc0007c5ed0, 0xa)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:153 +0x231
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc000a52300, 0x8f8660, 0xc00047a3f0, 0x1e, 0xc0007a2140, 0xc00075c000, 0xad5, 0xc00)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:34 +0x2db
main.(*Proc).loop(0xc0007a2400)
	/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