Errata

The code for silly_copy has a minor typo, the missing comma in "unsigned long len" (should be "unsigned long, len"). More serious is the ability to totally take down the Linux kernel from userland with this code since the buffer used is a limited size (the size of an unsigned long) and you copy data into it based on the user parameter. This is a security hole large enough to drive a space shuttle through :-)

The following code, as posted at http://stackoverflow.com/questions/6515227/source-code-example-from-linux-kernel-programming/6515426#6515426 has fixed both these problems, reproduced below to make this note complete.

SYSCALL_DEFINE3(silly_copy,
unsigned long *, src,
unsigned long *, dst,
unsigned long, len)
{
unsigned long buf[64]; /* Buffer for chunks */
unsigned long lenleft = len; /* Remaining size */
unsigned long chunklen = sizeof(buf); /* Initial chunk length */

/* Loop handling chunk sizes */

while (lenleft > 0) {
/* Change chunk length on last chunk */

if (lenleft < chunklen) chunklen = lenleft;

/* copy src(user) to buf(kernel) then dst(user) */

if (copy_from_user(&buf, src, chunklen)) return -EFAULT;
if (copy_to_user(dst, &buf, chunklen)) return -EFAULT;

/* Adjust pointers and remaining size */

src += chunklen; dst += chunklen; lenleft -= chunklen;
}

/* return amount of data copied */
return len;
}