[tip:mm/pkeys] pkeys: Add details of system call use to Documentation/

[tip-bot for Dave Hansen]

Commit-ID:  c74fe3940848c6afea83bfbda64a9baf9da547c8
Gitweb:     http://git.kernel.org/tip/c74fe3940848c6afea83bfbda64a9baf9da547c8
Author:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
AuthorDate: Fri, 29 Jul 2016 09:30:20 -0700
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Fri, 9 Sep 2016 13:02:28 +0200

pkeys: Add details of system call use to Documentation/

This spells out all of the pkey-related system calls that we have
and provides some example code fragments to demonstrate how we
expect them to be used.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
Cc: Dave Hansen <dave@xxxxxxxx>
Cc: mgorman@xxxxxxxxxxxxxxxxxxx
Cc: arnd@xxxxxxxx
Cc: linux-api@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: luto@xxxxxxxxxx
Cc: akpm@xxxxxxxxxxxxxxxxxxxx
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/20160729163020.59350E33@xxxxxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 Documentation/x86/protection-keys.txt | 62 +++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt
index c281ded..6da7689 100644
--- a/Documentation/x86/protection-keys.txt
+++ b/Documentation/x86/protection-keys.txt
@@ -18,6 +18,68 @@ even though there is theoretically space in the PAE PTEs.  These
 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