Documentation: x86: convert protection-keys.txt to reST

This converts the plain text documentation to reStructuredText format and
add it to Sphinx TOC tree. No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index f7012e4afacde103b9f9629eeb7586c24be10db6..e2c0db9fcd4ed6a8e19ee70f22a7ce7b6cc65e40 100644 (file)
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -18,3 +18,4 @@ x86-specific Documentation
    tlb
    mtrr
    pat
+   protection-keys

diff --git a/Documentation/x86/protection-keys.rst b/Documentation/x86/protection-keys.rst
new file mode 100644 (file)
index 0000000..49d9833
--- /dev/null
+++ b/Documentation/x86/protection-keys.rst
@@ -0,0 +1,99 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================
+Memory Protection Keys
+======================
+
+Memory Protection Keys for Userspace (PKU aka PKEYs) is a feature
+which is found on Intel's Skylake "Scalable Processor" Server CPUs.
+It will be avalable in future non-server parts.
+
+For anyone wishing to test or use this feature, it is available in
+Amazon's EC2 C5 instances and is known to work there using an Ubuntu
+17.04 image.
+
+Memory Protection Keys provides a mechanism for enforcing page-based
+protections, but without requiring modification of the page tables
+when an application changes protection domains.  It works by
+dedicating 4 previously ignored bits in each page table entry to a
+"protection key", giving 16 possible keys.
+
+There is also a new user-accessible register (PKRU) with two separate
+bits (Access Disable and Write Disable) for each key.  Being a CPU
+register, PKRU is inherently thread-local, potentially giving each
+thread a different set of protections from every other thread.
+
+There are two new instructions (RDPKRU/WRPKRU) for reading and writing
+to the new register.  The feature is only available in 64-bit mode,
+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 3 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_DISABLE_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_DISABLE_WRITE
+       *ptr = foo; // assign something
+       pkey_set(pkey, PKEY_DISABLE_WRITE); // set PKEY_DISABLE_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);
+
+.. note:: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
+          An example implementation can be found in
+          tools/testing/selftests/x86/protection_keys.c.
+
+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.

diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt
deleted file mode 100644 (file)
index ecb0d2d..0000000
--- a/Documentation/x86/protection-keys.txt
+++ /dev/null
@@ -1,90 +0,0 @@
-Memory Protection Keys for Userspace (PKU aka PKEYs) is a feature
-which is found on Intel's Skylake "Scalable Processor" Server CPUs.
-It will be avalable in future non-server parts.
-
-For anyone wishing to test or use this feature, it is available in
-Amazon's EC2 C5 instances and is known to work there using an Ubuntu
-17.04 image.
-
-Memory Protection Keys provides a mechanism for enforcing page-based
-protections, but without requiring modification of the page tables
-when an application changes protection domains.  It works by
-dedicating 4 previously ignored bits in each page table entry to a
-"protection key", giving 16 possible keys.
-
-There is also a new user-accessible register (PKRU) with two separate
-bits (Access Disable and Write Disable) for each key.  Being a CPU
-register, PKRU is inherently thread-local, potentially giving each
-thread a different set of protections from every other thread.
-
-There are two new instructions (RDPKRU/WRPKRU) for reading and writing
-to the new register.  The feature is only available in 64-bit mode,
-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 3 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_DISABLE_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_DISABLE_WRITE
-       *ptr = foo; // assign something
-       pkey_set(pkey, PKEY_DISABLE_WRITE); // set PKEY_DISABLE_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);
-
-(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
- An example implementation can be found in
- tools/testing/selftests/x86/protection_keys.c)
-
-=========================== 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.