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

goroutine 17 [running]:
github.com/google/syzkaller/prog.(*randGen).filename(0xc000615f20, 0xc000652b80, 0xbf4c40, 0x0, 0xc000652b00)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/rand.go:161 +0x2ac
github.com/google/syzkaller/prog.(*BufferType).mutate(0xbf4c40, 0xc000615f20, 0xc000652b80, 0x8fa9e0, 0xc0006a2990, 0xc000652a48, 0xc0006a2960, 0x0, 0xc0006a2a00, 0x88343a555458d15, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:264 +0x533
github.com/google/syzkaller/prog.(*Target).mutateArg(0xc0000711e0, 0xc000615f20, 0xc000652b80, 0x8fa9e0, 0xc0006a2990, 0xc000652a48, 0xc0006a2960, 0x0, 0x4f0f00, 0xc00084fe0f, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:172 +0xe3
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc00084fed0, 0xa)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:153 +0x231
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc000652a00, 0x8f8660, 0xc00064aff0, 0x1e, 0xc000727c80, 0xc0006fc000, 0x975, 0xc00)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:34 +0x2db
main.(*Proc).loop(0xc000727d00)
	/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