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

goroutine 16 [running]:
github.com/google/syzkaller/prog.(*randGen).filename(0xc0008f1f00, 0xc000786400, 0xbf69a0, 0x0, 0xc000786400)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/rand.go:161 +0x2ac
github.com/google/syzkaller/prog.(*BufferType).mutate(0xbf69a0, 0xc0008f1f00, 0xc000786400, 0x8fa9e0, 0xc00075be30, 0xc0007862c8, 0xc00075be00, 0x0, 0xc0006c0000, 0x262e9ea1a4912fbc, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:264 +0x533
github.com/google/syzkaller/prog.(*Target).mutateArg(0xc0000731e0, 0xc0008f1f00, 0xc000786400, 0x8fa9e0, 0xc00075be30, 0xc0007862c8, 0xc00075be00, 0x0, 0x4f0f00, 0xc000775e0f, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:172 +0xe3
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc000775ed0, 0xa)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:153 +0x231
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc0007861c0, 0x8f8660, 0xc0005a42a0, 0x1e, 0xc000712380, 0xc000750000, 0x94b, 0xc00)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:34 +0x2db
main.(*Proc).loop(0xc0007123c0)
	/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
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