libvmcu - Virtual MCU Library

libvmcu is a small engine for static and dynamic analysis of AVR Microcontroller binaries.

It takes care of the preparation of raw data, which can then be further processed by other programs. The goal here is to make it possible to interact programmatically with AVR source code.

libvmcu provides you with

• a simple way to analyze AVR assembly without further dependencies.
• directed control flow graphs extracted from your binary
• disassembly and a decomposed representation of instructions
• abstract implicit/explicit read/write memory access information
• analyzer modules for interrupt vectors, strings, ...
• cross references from/to disassembled instructions
• and more...

Note: This library is still in development.

Table of Contents

I Examples

II Showcase

III Setup VMCU

IV Supported MCUs

V Dynamic Analysis

VI Static Analysis

VII Instruction-Set

VIII Bindings

IX Contributing

X Credits

XI Documentation

Examples

Printing controlflow of a binary

/* A possible implementation of print_instruction can be found below */

int main(const int argc, const char **argv) {
    
    /* ignoring checks for this example */
    vmcu_model_t  *m328p  = vmcu_model_ctor(VMCU_DEVICE_M328P);
    vmcu_report_t *report = vmcu_analyze_file("file.hex", m328p);

    for(uint32_t i = 0; i < report->cfg->used; i++) {
        
        vmcu_cfg_node_t *node = &report->cfg->node[i];
        print_instruction(node->xto.i);
        
        if(node->t != NULL) {
            
            printf("true  -> ");
            print_instruction(node->t->xto.i);
        }
        
        if(node->f != NULL) {
            
            printf("false -> ");
            print_instruction(node->f->xto.i);
        }
        
        printf("\n");
    } 
    
    vmcu_report_dtor(report);
    vmcu_model_dtor(m328p);
    
    return EXIT_SUCCESS;
}

         0x0000  .... f1f3  breq -2         ; (ZF == 1): PC <- PC + -2 + 1
true  -> 0x3fff  .... 839a  sbi 0x10, 3     ; IO[0x10, 3] <- 1
false -> 0x0001  .... 0fef  ldi r16, 0xff   ; r16 <- 0xff
--------------------------------------------------------------------------------
         0x0001  .... 0fef  ldi r16, 0xff   ; r16 <- 0xff
true  -> 0x0002  .... 5817  cp r21, r24     ; r21 - r24
--------------------------------------------------------------------------------
         0x0002  .... 5817  cp r21, r24     ; r21 - r24
true  -> 0x0003  .... 19f4  brne 3          ; (ZF == 0): PC <- PC + 3 + 1
--------------------------------------------------------------------------------
         0x0003  .... 19f4  brne 3          ; (ZF == 0): PC <- PC + 3 + 1
true  -> 0x0007  .... 0127  eor r16, r17    ; r16 <- r16 ^ r17
false -> 0x0004  .... a895  wdr             ; watchdog reset
--------------------------------------------------------------------------------
         0x0004  .... a895  wdr             ; watchdog reset
true  -> 0x0005  .... 8895  sleep           ; circuit sleep
--------------------------------------------------------------------------------
         0x0005  .... 8895  sleep           ; circuit sleep
true  -> 0x0006  .... 0000  nop             ; no operation
--------------------------------------------------------------------------------
         0x0006  .... 0000  nop             ; no operation
true  -> 0x0007  .... 0127  eor r16, r17    ; r16 <- r16 ^ r17
--------------------------------------------------------------------------------
         0x0007  .... 0127  eor r16, r17    ; r16 <- r16 ^ r17
--------------------------------------------------------------------------------
         0x3fff  .... 839a  sbi 0x10, 3     ; IO[0x10, 3] <- 1
true  -> 0x0000  .... f1f3  breq -2         ; (ZF == 1): PC <- PC + -2 + 1
--------------------------------------------------------------------------------

Printing disassembly of an intel hex file

/* A possible implementation of print_instruction can be found below */

int main(const int argc, const char **argv) {
    
    /* ignoring checks for this example */
    vmcu_model_t  *m328p  = vmcu_model_ctor(VMCU_DEVICE_M328P); 
    vmcu_report_t *report = vmcu_analyze_file("file.srec", m328p);
    
    for(uint32_t i = 0; i < report->progsize; i++) {

        printf("0x%04x ", report->disassembly[i].addr);
        print_instruction(&report->disassembly[i]);
    }
        
    vmcu_report_dtor(report);
    vmcu_model_dtor(m328p);
    
    return EXIT_SUCCESS;
}

0x004e ldi r27, 0x06 ; r27 <- 0x06
0x004f rjmp 1        ; PC <- PC + 1 + 1
0x0050 st X+, r1     ; DS[X+] <- r1
0x0051 cpi r26, 0x20 ; r26 - 0x20
0x0052 cpc r27, r18  ; r27 - r18 - CF
0x0053 brne -4       ; (ZF == 0): PC <- PC + -4 + 1
0x0054 call 0x60b    ; PC <- 0x60b

Filtering read/write access on status flags

/* A possible implementation of print_instruction can be found below */

int main(const int argc, const char **argv) {

    /* ignoring checks for this example */
    vmcu_model_t  *m328p  = vmcu_model_ctor(VMCU_DEVICE_M328P); 
    vmcu_report_t *report = vmcu_analyze_file("file.hex", m328p);

    for(uint32_t i = 0; i < report->progsize; i++) {

        vmcu_instr_t *instr = &report->disassembly[i];
        
        if(instr->writes.c_flag == true)
            print_instruction(instr);
        
        if(instr->reads.c_flag == true)
            print_instruction(instr);
    }
    
    vmcu_report_dtor(report);
    vmcu_model_dtor(m328p);
    
    return EXIT_SUCCESS;
}

subi r18, 0x00     ; r18 <- r18 - 0x00
adiw r29:r28, 0x1a ; r29:r28 <- r29:r28 + 0x1a
sbci r23, 0xff     ; r23 <- r23 - 0xff - CF
cpc r19, r17       ; r19 - r17 - CF

Printing interrupt vectors and their xref-to

int main(const int argc, const char **argv) {

    /* ignoring checks for this example */
    vmcu_model_t  *m328p  = vmcu_model_ctor(VMCU_DEVICE_M328P); 
    vmcu_report_t *report = vmcu_analyze_file("file.hex", m328p);

    for(uint32_t i = 0; i < report->n_vector; i++) {

        vmcu_vector_t *vect = &report->vector[i];
        vmcu_instr_t  *isr  = vect->xto->i;
        
        printf("Vector ID %d @ 0x%04x\n", vect->id, vect->addr);
        printf(" interrupt service routine at 0x%04x", isr->addr);
        printf("\n\n");
    }
    
    vmcu_report_dtor(report);
    vmcu_model_dtor(m328p);
    
    return EXIT_SUCCESS;
}

Vector ID 16 @ 0x0020
 interrupt service routine at 0x03f5

Vector ID 17 @ 0x0022
 interrupt service routine at 0x008a

Vector ID 18 @ 0x0024
 interrupt service routine at 0x03c3

Vector ID 19 @ 0x0026
 interrupt service routine at 0x039d

Printing xrefs of potential labels

/* A possible implementation of print_instruction can be found below */

int main(const int argc, const char **argv) {
    
    /* ignoring checks for this example */
    vmcu_model_t  *m328p  = vmcu_model_ctor(VMCU_DEVICE_M328P); 
    vmcu_report_t *report = vmcu_analyze_file("file.hex", m328p);
    
    for(uint32_t i = 0; i < report->n_label; i++) {

        vmcu_label_t *lx = &report->label[i];
        printf("0x%04x\tL%d\n\n", lx->addr, lx->id);

        for(uint32_t j = 0; j < lx->n_xfrom; j++) {

            vmcu_xref_t *x = &lx->xfrom[j];
            
            printf(" xref from 0x%04x ", x->i->addr);
            print_instruction(x->i);
        }

        printf("\n");
    }
    
    vmcu_report_dtor(report);
    vmcu_model_dtor(m328p);
    
    return EXIT_SUCCESS;
}

0x04c6  L75

 xref from 0x04a1 call +1222 ; PC <- 0x4c6
 xref from 0x0a84 call +1222 ; PC <- 0x4c6
 xref from 0x0b5c call +1222 ; PC <- 0x4c6

0x04e2  L76

 xref from 0x05d4 rjmp -243  ; PC <- PC - 0xf3 + 1

0x05d0  L77

 xref from 0x04e1 rjmp +238  ; PC <- PC + 0xee + 1

Printing xrefs of special function registers

/* A possible implementation of print_instruction can be found below */

int main(const int argc, const char **argv) {

    /* ignoring checks for this example */
    vmcu_model_t  *m328p  = vmcu_model_ctor(VMCU_DEVICE_M328P); 
    vmcu_report_t *report = vmcu_analyze_file("file.hex", m328p);

    for(uint32_t i = 0; i < report->n_sfr; i++) {

        vmcu_sfr_t *sfr = &report->sfr[i];
        printf("SFR ID: %d\n\n", sfr->id);

        for(uint32_t j = 0; j < sfr->n_xfrom; j++) {

            vmcu_xref_t *x = &sfr->xfrom[j];

            printf(" xref from 0x%04x ", x->i->addr);
            print_instruction(x->i);
        }

        printf("\n");
    }

    vmcu_report_dtor(report);
    vmcu_model_dtor(m328p);
    
    return EXIT_SUCCESS;
}

SFR ID: 17
       
 xref from 0x00f4 sbi 0x1f, 2     ; IO[1f, 2] <- 0x01
 xref from 0x00f5 sbi 0x1f, 1     ; IO[1f, 1] <- 0x01
 
SFR ID: 50

 xref from 0x004c sts 0x006e, r1  ; DATA[0x6e] <- R1
 xref from 0x0051 lds r24, 0x006e ; R24 <- DATA[0x6e]
 xref from 0x0054 sts 0x006e, r24 ; DATA[0x6e] <- R24

Extracting details from opcode

/* 0x6a97 (little endian) <=> sbiw r29:r28, 0x1a */

int main(const int argc, const char **argv) {
    
    /* initialize a device model */
    vmcu_model_t *m328p = vmcu_model_ctor(VMCU_DEVICE_M328P);
    
    vmcu_instr_t instr;
    vmcu_disassemble_bytes(0x6a97, &instr, m328p);
    
    const VMCU_IKEY key    = instr.key;           // VMCU_IKEY_SBIW
    const VMCU_GROUP grp   = instr.group;         // VMCU_GROUP_MATH_LOGIC
    
    const uint32_t opcode  = instr.opcode;        // 0x976a (big endian)
    const uint32_t addr    = instr.addr;          // 0x0000 (undefined)

    const bool dword       = instr.dword;         // false
    const bool exec        = instr.exec;          // true
    
    vmcu_operand_t *src    = &instr.src;          // source operand
    vmcu_operand_t *dest   = &instr.dest;         // destination operand

    VMCU_OPTYPE src_type   = src->type;           // VMCU_OPTYPE_K6
    VMCU_OPTYPE dest_type  = dest->type;          // VMCU_OPTYPE_RP
    
    const uint8_t src_val  = src->k;              // 0x1a
    
    VMCU_REGISTER dest_rh  = dest->rp.high;       // VMCU_REGISTER_R29
    VMCU_REGISTER dest_rl  = dest->rp.low;        // VMCU_REGISTER_R28
    
    const bool writes_hf   = instr.writes.h_flag; // false
    const bool writes_cf   = instr.writes.c_flag; // true
    
    const bool reads_io    = instr.reads.io;      // false
    const bool reads_nf    = instr.reads.n_flag;  // false
    
    vmcu_mnemonic_t *mnem  = &instr.mnem;         // instruction mnemonic
    
    const char *base_str   = mnem->base;          // "sbiw"
    const char *dest_str   = mnem->dest;          // "r29:r28"
    const char *src_str    = mnem->src;           // "0x1a"
    const char *com_str    = mnem->comment;       // "r29:r28 <- r29:r28 - 0x1a"
    
    vmcu_model_dtor(m328p);
    
    return EXIT_SUCCESS;
}

Example of an instruction-printer function

/* this snippet can be used to assemble and print an instruction */

void print_instruction(const vmcu_instr_t *instr) {

    printf("%s",  instr->mnem.base);

    if(instr->dest.type != VMCU_OPTYPE_NONE)
        printf(" %s,", instr->mnem.dest);

    if(instr->src.type != VMCU_OPTYPE_NONE)
        printf(" %s", instr->mnem.src);

    printf(" %s\n", instr->mnem.comment);
}

Showcase

^{A small debugger written with libvmcu}

Setup

Currently this library comes with two headers, both can be found in engine/include/libvmcu:

• libvmcu_analyzer.h (static analysis)
• libvmcu_system.h (dynamic analysis, simulation)

Let's say, we have a file called prog.c on top level of this repository and we want to link it with libvmcu:

Include libvmcu headers

/* prog.c */

#include "libvmcu_analyzer.h"
#include "libvmcu_system.h"

int main(void) {
    
    /* do something */
    return 0;
}

Build libvmcu

You@Terminal:~$ cd build-release/
You@Terminal:~$ make -f build.mk

Build driver object

You@Terminal:~$ gcc -Iengine/include/libvmcu/ -c prog.c -o prog.o

Link with libvmcu (do not forget -lm)

You@Terminal:~$ gcc -o prog prog.o -Lbuild-release/ -lvmcu -lm

That's it. If you face issues, take look at some examples in the driver/ directory.

Supported Microcontroller

libvmcu tries to support as many AVR types as possible for static analysis. The dynamic analysis is currently only planned for the ATmega328 family but may be extended in the future.

It should be pretty easy to add new microcontrollers to the static analysis. For more information take a look at engine/*/arch/

Supported MCUs for static analysis

• AVR Device Core

  • ATtiny15
  • ...

• AVRe Device Core

  • ATtiny1634
  • ...

• AVRe+ Device Core

  • ATmega328(P)
  • ...

• AVRxm Device Core

  • ATxmega128A1
  • ...

• AVRxt Device Core

  • ATtiny827
  • ...

• AVRrc Device Core

  • ATtiny10
  • ...

Supported MCUs for dynamic analysis

• ATmega328(P)
• ATmega168
• ATmega88
• ATmega48

Static Analysis

• disassembler

• cross references (xref-from, xref-to)

• analyzer flags

• decompose and classify instructions

  • instruction groups
  • operands and operand types

• analyzer for AVR binaries

  • label analysis
  • vector analysis
  • controlflow analysis
  • function analysis
  • ISR analysis
  • SFR analysis
  • cycle analysis
  • string analysis
    • ASCII
    • UTF16
  • ...

• format reader

  • intel hex
  • motorola hex
  • bin
  • elf

Dynamic Analysis

• interrupt support

• cycle accurate realtime simulation

• support for 133 AVR assembly instructions

• accurate simulation of internal peripherals

  • 8-bit timer (partial)
  • 16-bit timer
  • EEPROM
  • UART
  • SPI
  • WDT
  • ...

Instructions

Currently VMCU supports: ~ 133 Instructions. Some few instructions are implemented as 'nop' instructions, therefore, have no real functionality. These instructions will be implemented as soon as possible. Following instructions require further work:

• WDR
• ELPM
• DES
• SLEEP
• SPM
• BREAK

All other assembly instructions are working just fine.

Bindings

• Java
• Python

libvmcu has Java bindings for basic functionalities. For more information take a look at bindings/java/

Also note that, bindings might not always work with the latest version due to development of the engine.

Contributing

Engine	Drivers	Bindings	Testing
closed for PR	open for PR	open for PR	open for PR

Credits

1. Huge thanks to Alexander Hansen for the new logo and architecture diagram. :)

Documentation

By the time of writing this the documentation is still in development. The (incomplete) documentation can be found at https://github.com/Milo-D/libvmcu-Virtual-MCU-Library/wiki

If you are missing information and don't want to wait for the wiki, the libvmcu header files are pretty well documented, too.