copy_from_user is supposed to transfer as much data as is
valid and then to return the number of bytes not tranferred.
That's how it works on x86.  On ppc it can be as much as 15
bytes short.

I initially saw the problem with the mount system call.

Note that the fifth argument to mount is an address 8 bytes from the end
of user data space.  There is a null byte at that address, since no mount
options are being passed.

In the kernel, sys_mount() allocates a page for the options and
does copy_from_user(new_page, 0x1005eff8, PAGE_SIZE).  copy_from_user
should copy 8 bytes and return (PAGE_SIZE-8).  Instead, on ppc it reads
8 bytes, faults, writes no bytes, and returns PAGE_SIZE, which causes the
EFAULT to be erroneously reported.