Tools to process ARM's Machine Readable Architecture Specification
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README.md

Tools to extract ARM's Machine Readable Architecture Specification.

These tools unpack the ASL spec from inside the XML so that the spec is easy to process.

See blog post for an explanation of the structure of ARM's releases and a description of the innards of these tools and see blog post for some ideas on what can be done with the specification once it has been unpacked.

Usage

The following commands will download ARM's specification and unpack it.

mkdir -p v8.4
cd v8.4

wget https://developer.arm.com/-/media/developer/products/architecture/armv8-a-architecture/ARMv84A-SysReg-00bet7.tar.gz
wget https://developer.arm.com/-/media/developer/products/architecture/armv8-a-architecture/A64_v84A_ISA_xml_00bet7.tar.gz
wget https://developer.arm.com/-/media/developer/products/architecture/armv8-a-architecture/AArch32_v84A_ISA_xml_00bet7.tar.gz

tar zxf A64_v84A_ISA_xml_00bet7.tar.gz
tar zxf AArch32_v84A_ISA_xml_00bet7.tar.gz
tar zxf ARMv84A-SysReg-00bet7.tar.gz

tar zxf ISA_v84A_A64_xml_00bet7.tar.gz
tar zxf ISA_v84A_AArch32_xml_00bet7.tar.gz

cd ..

make all

Generates:

  • arch.asl: all the ASL support code (This file uses an alternative syntax for bitslices that is easier to parse. Remove the --altslicesyntax flag from the Makefile to get the original ASL.)
  • arch.tag: all the instruction encodings and decode/execute ASL
  • regs.asl: type of each system register

You can also extract various subsets of the full architecture specification. For example, if you want a subset of the usermode AArch64 instructions, you can use the following command.

make FILTER=--filter=usermode.json all

The subset selected may not contain all the instructions you would want --- see Subsetting for more details.

Help

$ bin/instrs2asl.py  -h
usage: instrs2asl.py [-h] [--verbose] [--tag FILE] [--asl FILE]
		 [--arch {AArch32,AArch64}]
		 <dir> [<dir> ...]

Unpack ARM instruction XML files extracting the encoding information and ASL
code within it.

positional arguments:
  <dir>                 input directories

optional arguments:
  -h, --help            show this help message and exit
  --verbose, -v         Use verbose output
  --tag FILE            Output tag file for instructions
  --asl FILE            Output asl file for support code
  --arch {AArch32,AArch64}
		    Optional list of architecture states to extract

Subsetting

Various subsets of the architecture can be generated using these additional flags

--arch=AArch32
--arch=AArch64
--arch=AArch32 --arch=AArch64

For finer control, you can specify a specific filter that selects exactly which instructions and subset of the call graph to include

make FILTER=--filter=usermode.json all

The filter is controlled by a json file that has this format:

{
    "instructions": [
        // regexp list goes here
    ],
    "roots": [
        // root definitions go here
    ],
    "cuts": [
        // cut functions go here
    ],
    "canaries": [
        // canary definitions go here
    ]
}

The four parts of this are:

  • 'instructions' and 'roots' define what you want to include

    • 'instructions' is a list of regexps that match instruction names For example "aarch64/branch/conditional/.*". You can find the list of instruction names by looking in the file arch.tag.

      grep TAG arch.tag | grep decode
      
    • 'roots' is a list of functions that you wish to keep even though they are not referred to by instructions. For example, after executing an instruction in Thumb mode, you should call "AArch32.ITAdvance()" (which has 0 arguments) so add "AArch32.ITAdvance.0" to the list of roots. The ".0" suffix indicates that the function has 0 arguments.

  • 'cuts' defines what you want to exclude.

    This should be a list of functions that you wish to provide your own implementations for. For example, if all you are interested in is usermode execution, you might want to omit all the code to implement page table lookups and replace the functions to read or write memory by adding the following to the cut list

      "AArch64.MemSingle.read.4",
      "AArch64.MemSingle.write.4",
    

    This will cause the definitions of these functions to be replaced by function prototypes.

    Choosing the right set of cuts will depend on what functionality from the part you extract and on what you want to implement in your analysis/simulation framework.

  • 'canaries' are optional but are useful when trying to understand why your 'cuts' are not behaving as intended.

    Any uncut path from the instructions or roots to a canary is reported.

    For example, if you are trying to eliminate as much of the AArch32 support as possible, you might want to omit the function "ELUsingAArch32.1". But there are many possible code paths to that function and it is hard to find which functions to cut. So add "ELUsingAArch32.1" to the list of canaries and you will get a report that looks a bit like this:

      Canary ELUsingAArch32.1 ELIsInHost.1 IsInHost.0 AArch64.TakeException.4 AArch64.UndefinedFault.0
      Canary ELUsingAArch32.1 ELIsInHost.1 S1TranslationRegime.0 ESR[ AArch64.ReportException.2 AArch64.TakeException.4 AArch64.UndefinedFault.0
      Canary ELUsingAArch32.1 ELIsInHost.1 S1TranslationRegime.0 AArch64.ReportException.2 AArch64.TakeException.4 AArch64.UndefinedFault.0
      ...
      Canary ELUsingAArch32.1 ELIsInHost.1 S1TranslationRegime.0 AArch64.TakeException.4 AArch64.UndefinedFault.0
      Canary ELUsingAArch32.1 ELIsInHost.1 AArch64.TakeException.4 AArch64.UndefinedFault.0
      Canary ELUsingAArch32.1 S1TranslationRegime.0 ESR[ AArch64.ReportException.2 AArch64.TakeException.4 AArch64.UndefinedFault.0
      Canary ELUsingAArch32.1 S1TranslationRegime.0 AArch64.ReportException.2 AArch64.TakeException.4 AArch64.UndefinedFault.0
      ...
      Canary ELUsingAArch32.1 S1TranslationRegime.0 VBAR.read.0 AArch64.TakeException.4 AArch64.UndefinedFault.0
      Canary ELUsingAArch32.1 S1TranslationRegime.0 AArch64.TakeException.4 AArch64.UndefinedFault.0
      Canary ELUsingAArch32.1 AArch64.TakeException.4 AArch64.UndefinedFault.0
    

    This shows that the final calls to ELUsingAArch32.1 are from ELIsInHost.1, S1TranslationRegime.0 and AArch64.TakeException. So we could choose to cut all those functions.

    It also shows that the root call to ELUsingAArch32.1 is AArch64.UndefinedFault.0 so the easiest fix is to cut just that function.

Currently implemented

  • Unpack all the ASL code in the 'shared_pseudocode' file to giant ASL file
  • Unpack instructions to 'tagfile' format
  • Quick and dirty unpack of system register spec to ASL file

All generated files include ARM's license notice.

Shared pseudocode

The shared pseudocode is sorted so that definitions come before uses.

Tagfile format for functions

A tagfile consists of sections that start with a line of the form "TAG:$label:$kind". There are five different kinds:

  • diagram: Instruction encoding consisting of:

    • An initial line that specifies the encoding: A64, A32, T32 or T16
    • Field specifiers of the form "hi:lo name constants" where the name "_" is used for anonymous fields and the each constant is of the form:
      • 0 or 1
      • x meaning don't care
      • (0) or (1) meaning 'should be 0/1' (UNPREDICTABLE if not)

    For example:

      T32
      31:25 _ 1110101
      24:21 op1 1000
      20:20 S x
      19:16 Rn 1101
      15:15 _ (0)
      14:12 imm3 xxx
      11:8 Rd xxxx
      7:6 imm2 xx
      5:4 type xx
      3:0 Rm xxxx
    
  • decode: ASL code to decode an instruction encoding

  • postdecode: Additional code to continue decoding an instruction encoding

  • execute: ASL code to execute after postdecode

  • index: Identifies the different parts of an instruction encoding and consists of:

    • Decode: [tag of decode section]@[tag of diagram section]
    • Postdecode: optional [tag of postdecode section]
    • Execute: [tag of execute section]

    There can be multiple Decode lines all sharing the same postdecode and execute parts.

  • asl: ASL definitions (e.g., function definitions)

Register spec

At the moment, we unpack all the information about fields and declare a variable with the right name and with named fields. This uses an unofficial ASL extension to declare a number the location of each field.

__register 32 {
    31:31 N, 30:30 Z, 29:29 C, 28:28 V, 27:27 Q, 24:24 J, 22:22 PAN, 19:16 GE,
    9:9 E, 8:8 A, 7:7 I, 6:6 F, 5:5 T, 7:2, 1:0 IT, 3:0 M
} CPSR;

The system register specification also contains a lot of information about how to refer to a system register, permission checking, constant value fields, etc. but none of that is being extracted at the moment.

Experimental parser, etc.

There is an experimental parser for the language written in ocaml. This requires some tools to be installed. The following instructions are for a Mac.

brew install ocaml opam
opam install menhir core

Test it using the following

make test

At the moment, all it does is parse the ASL code extracted from the XML files. It does not have a parser or typechecker.