pkeys: Add details of system call use to Documentation/

permissions are enforced on data access only and have no effect on

instruction fetches.

=========================== Syscalls ===========================

There are 2 system calls which directly interact with pkeys:

	int pkey_alloc(unsigned long flags, unsigned long init_access_rights)

	int pkey_free(int pkey);

	int pkey_mprotect(unsigned long start, size_t len,

			  unsigned long prot, int pkey);

Before a pkey can be used, it must first be allocated with

pkey_alloc().  An application calls the WRPKRU instruction

directly in order to change access permissions to memory covered

with a key.  In this example WRPKRU is wrapped by a C function

called pkey_set().

	int real_prot = PROT_READ|PROT_WRITE;

	pkey = pkey_alloc(0, PKEY_DENY_WRITE);

	ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);

	ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey);

	... application runs here

Now, if the application needs to update the data at 'ptr', it can

gain access, do the update, then remove its write access:

	pkey_set(pkey, 0); // clear PKEY_DENY_WRITE

	*ptr = foo; // assign something

	pkey_set(pkey, PKEY_DENY_WRITE); // set PKEY_DENY_WRITE again

Now when it frees the memory, it will also free the pkey since it

is no longer in use:

	munmap(ptr, PAGE_SIZE);

	pkey_free(pkey);

=========================== Behavior ===========================

The kernel attempts to make protection keys consistent with the

behavior of a plain mprotect().  For instance if you do this:

	mprotect(ptr, size, PROT_NONE);

	something(ptr);

you can expect the same effects with protection keys when doing this:

	pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ);

	pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey);

	something(ptr);

That should be true whether something() is a direct access to 'ptr'

like:

	*ptr = foo;

or when the kernel does the access on the application's behalf like

with a read():

	read(fd, ptr, 1);

The kernel will send a SIGSEGV in both cases, but si_code will be set

to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when

the plain mprotect() permissions are violated.

=========================== Config Option ===========================

This config option adds approximately 1.5kb of text. and 50 bytes of