[PATCH v4 49/63] Documentation: x86: convert intel_mpx.txt to reST

Changbin Du
2019-04-23T16:40:32Z

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>
---
Documentation/x86/index.rst | 1 +
.../x86/{intel_mpx.txt => intel_mpx.rst} | 120 ++++++++++--------
2 files changed, 65 insertions(+), 56 deletions(-)
rename Documentation/x86/{intel_mpx.txt => intel_mpx.rst} (75%)

diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index 576628b121cc..20091d3e5d97 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -19,3 +19,4 @@ Linux x86 Support
mtrr
pat
protection-keys
+ intel_mpx
diff --git a/Documentation/x86/intel_mpx.txt b/Documentation/x86/intel_mpx.rst
similarity index 75%
rename from Documentation/x86/intel_mpx.txt
rename to Documentation/x86/intel_mpx.rst
index 85d0549ad846..387a640941a6 100644
--- a/Documentation/x86/intel_mpx.txt
+++ b/Documentation/x86/intel_mpx.rst
@@ -1,5 +1,11 @@
-1. Intel(R) MPX Overview
-========================
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================================
+Intel(R) Memory Protection Extensions (MPX)
+===========================================
+
+Intel(R) MPX Overview
+=====================

Intel(R) Memory Protection Extensions (Intel(R) MPX) is a new capability
introduced into Intel Architecture. Intel MPX provides hardware features
@@ -7,7 +13,7 @@ that can be used in conjunction with compiler changes to check memory
references, for those references whose compile-time normal intentions are
usurped at runtime due to buffer overflow or underflow.

-You can tell if your CPU supports MPX by looking in /proc/cpuinfo:
+You can tell if your CPU supports MPX by looking in /proc/cpuinfo::

cat /proc/cpuinfo | grep ' mpx '

@@ -21,8 +27,8 @@ can be downloaded from
http://software.intel.com/en-us/articles/intel-software-development-emulator

-2. How to get the advantage of MPX
-==================================
+How to get the advantage of MPX
+===============================

For MPX to work, changes are required in the kernel, binutils and compiler.
No source changes are required for applications, just a recompile.
@@ -84,14 +90,15 @@ Kernel MPX Code:
is unmapped.

-3. How does MPX kernel code work
-================================
+How does MPX kernel code work
+=============================

Handling #BR faults caused by MPX
---------------------------------

When MPX is enabled, there are 2 new situations that can generate
#BR faults.
+
* new bounds tables (BT) need to be allocated to save bounds.
* bounds violation caused by MPX instructions.

@@ -124,37 +131,37 @@ the kernel. It can theoretically be done completely from userspace. Here
are a few ways this could be done. We don't think any of them are practical
in the real-world, but here they are.

-Q: Can virtual space simply be reserved for the bounds tables so that we
- never have to allocate them?
-A: MPX-enabled application will possibly create a lot of bounds tables in
- process address space to save bounds information. These tables can take
- up huge swaths of memory (as much as 80% of the memory on the system)
- even if we clean them up aggressively. In the worst-case scenario, the
- tables can be 4x the size of the data structure being tracked. IOW, a
- 1-page structure can require 4 bounds-table pages. An X-GB virtual
- area needs 4*X GB of virtual space, plus 2GB for the bounds directory.
- If we were to preallocate them for the 128TB of user virtual address
- space, we would need to reserve 512TB+2GB, which is larger than the
- entire virtual address space today. This means they can not be reserved
- ahead of time. Also, a single process's pre-populated bounds directory
- consumes 2GB of virtual *AND* physical memory. IOW, it's completely
- infeasible to prepopulate bounds directories.
-
-Q: Can we preallocate bounds table space at the same time memory is
- allocated which might contain pointers that might eventually need
- bounds tables?
-A: This would work if we could hook the site of each and every memory
- allocation syscall. This can be done for small, constrained applications.
- But, it isn't practical at a larger scale since a given app has no
- way of controlling how all the parts of the app might allocate memory
- (think libraries). The kernel is really the only place to intercept
- these calls.
-
-Q: Could a bounds fault be handed to userspace and the tables allocated
- there in a signal handler instead of in the kernel?
-A: mmap() is not on the list of safe async handler functions and even
- if mmap() would work it still requires locking or nasty tricks to
- keep track of the allocation state there.
+:Q: Can virtual space simply be reserved for the bounds tables so that we
+ never have to allocate them?
+:A: MPX-enabled application will possibly create a lot of bounds tables in
+ process address space to save bounds information. These tables can take
+ up huge swaths of memory (as much as 80% of the memory on the system)
+ even if we clean them up aggressively. In the worst-case scenario, the
+ tables can be 4x the size of the data structure being tracked. IOW, a
+ 1-page structure can require 4 bounds-table pages. An X-GB virtual
+ area needs 4*X GB of virtual space, plus 2GB for the bounds directory.
+ If we were to preallocate them for the 128TB of user virtual address
+ space, we would need to reserve 512TB+2GB, which is larger than the
+ entire virtual address space today. This means they can not be reserved
+ ahead of time. Also, a single process's pre-populated bounds directory
+ consumes 2GB of virtual *AND* physical memory. IOW, it's completely
+ infeasible to prepopulate bounds directories.
+
+:Q: Can we preallocate bounds table space at the same time memory is
+ allocated which might contain pointers that might eventually need
+ bounds tables?
+:A: This would work if we could hook the site of each and every memory
+ allocation syscall. This can be done for small, constrained applications.
+ But, it isn't practical at a larger scale since a given app has no
+ way of controlling how all the parts of the app might allocate memory
+ (think libraries). The kernel is really the only place to intercept
+ these calls.
+
+:Q: Could a bounds fault be handed to userspace and the tables allocated
+ there in a signal handler instead of in the kernel?
+:A: mmap() is not on the list of safe async handler functions and even
+ if mmap() would work it still requires locking or nasty tricks to
+ keep track of the allocation state there.

Having ruled out all of the userspace-only approaches for managing
bounds tables that we could think of, we create them on demand in
@@ -167,20 +174,20 @@ If a #BR is generated due to a bounds violation caused by MPX.
We need to decode MPX instructions to get violation address and
set this address into extended struct siginfo.

-The _sigfault field of struct siginfo is extended as follow:
-
-87 /* SIGILL, SIGFPE, SIGSEGV, SIGBUS */
-88 struct {
-89 void __user *_addr; /* faulting insn/memory ref. */
-90 #ifdef __ARCH_SI_TRAPNO
-91 int _trapno; /* TRAP # which caused the signal */
-92 #endif
-93 short _addr_lsb; /* LSB of the reported address */
-94 struct {
-95 void __user *_lower;
-96 void __user *_upper;
-97 } _addr_bnd;
-98 } _sigfault;
+The _sigfault field of struct siginfo is extended as follow::
+
+ 87 /* SIGILL, SIGFPE, SIGSEGV, SIGBUS */
+ 88 struct {
+ 89 void __user *_addr; /* faulting insn/memory ref. */
+ 90 #ifdef __ARCH_SI_TRAPNO
+ 91 int _trapno; /* TRAP # which caused the signal */
+ 92 #endif
+ 93 short _addr_lsb; /* LSB of the reported address */
+ 94 struct {
+ 95 void __user *_lower;
+ 96 void __user *_upper;
+ 97 } _addr_bnd;
+ 98 } _sigfault;

The '_addr' field refers to violation address, and new '_addr_and'
field refers to the upper/lower bounds when a #BR is caused.
@@ -209,9 +216,10 @@ Adding new prctl commands

Two new prctl commands are added to enable and disable MPX bounds tables
management in kernel.
+::

-155 #define PR_MPX_ENABLE_MANAGEMENT 43
-156 #define PR_MPX_DISABLE_MANAGEMENT 44
+ 155 #define PR_MPX_ENABLE_MANAGEMENT 43
+ 156 #define PR_MPX_DISABLE_MANAGEMENT 44

Runtime library in userspace is responsible for allocation of bounds
directory. So kernel have to use XSAVE instruction to get the base
@@ -223,8 +231,8 @@ into struct mm_struct to be used in future during PR_MPX_ENABLE_MANAGEMENT
command execution.

-4. Special rules
-================
+Special rules
+=============

1) If userspace is requesting help from the kernel to do the management
of bounds tables, it may not create or modify entries in the bounds directory.
--
2.20.1