100  Implementing a new disassembly plugin

Rizin has modular architecture, thus adding support for a new architecture is very easy, if you are fluent in C. For various reasons it might be easier to implement it out of the tree. For this we will need to create single C file, called asm_mycpu.c and a meson file for it.

The key thing of RzAsm plugin is a structure

RzAsmPlugin rz_asm_plugin_mycpu = {
    .name = "mycpu",
    .license = "LGPL3",
    .desc = "MYCPU disassembly plugin",
    .arch = "mycpu",
    .bits = 32,
    .endian = RZ_SYS_ENDIAN_LITTLE,
    .disassemble = &disassemble
};

where .disassemble is a pointer to disassembly function, which accepts the bytes buffer and length:

static int disassemble(RzAsm *a, RzAsmOp *op, const ut8 *buf, int len)

Meson

project('asm_mycpu', 'c')

rz_asm_dep = dependency('rz_asm')
plugins_dir = get_option('prefix') / rz_asm_dep.get_variable(pkgconfig: 'plugindir', cmake: 'rz_asm_PLUGINDIR')
message('Plugins install directory: ' + plugins_dir)

library('asm_mycpu',
  ['asm_mycpu.c'],
  dependencies: [rz_asm_dep],
  install: true,
  install_dir: plugins_dir,
)

asm_mycpu.c

/* rizin - LGPL - Copyright 2018 - user */

#include <stdio.h>
#include <string.h>
#include <rz_types.h>
#include <rz_lib.h>
#include <rz_asm.h>

static int disassemble(RzAsm *a, RzAsmOp *op, const ut8 *buf, int len) {
    struct op_cmd cmd = {
        .instr = "",
        .operands = ""
    };
    if (len < 2) return -1;
    int ret = decode_opcode (buf, len, &cmd);
    if (ret > 0) {
        snprintf (op->buf_asm, rz_asm_BUFSIZE, "%s %s",
              cmd.instr, cmd.operands);
    }
    return op->size = ret;
}

RzAsmPlugin rz_asm_plugin_mycpu = {
    .name = "mycpu",
    .license = "LGPL3",
    .desc = "MYCPU disassembly plugin",
    .arch = "mycpu",
    .bits = 32,
    .endian = RZ_SYS_ENDIAN_LITTLE,
    .disassemble = &disassemble
};

#ifndef RZ_PLUGIN_INCORE
RZ_API RzLibStruct rizin_plugin = {
    .type = RZ_LIB_TYPE_ASM,
    .data = &rz_asm_plugin_mycpu,
    .version = RZ_VERSION
};
#endif

After compiling rizin will list this plugin in the rz-asm output:

$ rz-asm -L |grep myc
_d__  _8_32      mycpu        LGPL3   MYCPU disassembly plugin

100.0.1 Moving plugin into the tree

Pushing a new architecture into the main branch of rizin requires to modify several files in order to make it fit into the way the rest of plugins are built.

List of affected files:

• librz/asm/p/asm_mycpu.c That’s where most of our code will be, the key part is to declare a RzAsmPlugin containing a valid disassemble field, a function pointer to the actual disassembler function.

• librz/asm/meson.build The build is handled by meson, we have to add our plugin to the list of things to be compiled :

@@ -49,6 +49,7 @@ asm_plugins_list = [
   'x86_nz',
   'xap',
   'xcore_cs',
+  'mycpu',
 ]

@@ -129,6 +130,7 @@ rz_asm_sources = [
   #'p/asm_x86_vm.c',
   'p/asm_xap.c',
   'p/asm_xcore_cs.c',
+  'p/asm_mycpu.c',
   #'arch/6502/6502dis.c',
   'arch/amd29k/amd29k.c',
   #'arch/8051/8051_disas.c',
+  'arch/mycpu/mycpu_disas.c',
   'arch/arm/armass.c',

@@ -129,6 +130,7 @@ rz_asm_inc = [
+   'arch/mycpu',
]

• librz/include/rz_asm.h Make Rizin aware of our plugin by defining our struct :

@@ -265,6 +265,7 @@ extern RzAsmPlugin rz_asm_plugin_xcore_cs;
 extern RzAsmPlugin rz_asm_plugin_xtensa;
 extern RzAsmPlugin rz_asm_plugin_z80;
 extern RzAsmPlugin rz_asm_plugin_pyc;
+extern RzAsmPlugin rz_asm_plugin_mycpu;

 #endif

Check out how the NIOS II CPU disassembly plugin was implemented by reading those commits:

The RzAsm plugin

The RzAnalysis plugin