Date:   Sat, 27 Apr 2019 14:54:47 -0300
From:   Mauro Carvalho Chehab <mchehab+samsung@...nel.org>
To:     Changbin Du <changbin.du@...il.com>
Cc:     Jonathan Corbet <corbet@....net>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/27] Documentation: x86: convert intel_mpx.txt to reST

Em Fri, 26 Apr 2019 23:31:36 +0800
Changbin Du <changbin.du@...il.com> escreveu:

> 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@...il.com>

Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@...nel.org>

> ---
>  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.



Thanks,
Mauro

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