In shiftfs, a non-upstream patch to the Linux kernel included in the Ubuntu 5.0 and 5.3 kernel series, shiftfsbtrfsioctlfdreplace() calls fdget(oldfd), then without further checks passes the resulting file* into shiftfsrealfdget(), which casts file->privatedata, a void* that points to a filesystem-dependent type, to a "struct shiftfsfileinfo *". As the privatedata is not required to be a pointer, an attacker can use this to cause a denial of service or possibly execute arbitrary code.