5 Aug 2016

On the subject of debuggers and tracers

Every programmer knows how important and useful debuggers are. They let us examine program memory and execution flow which is very helpful in finding bugs. And if you ever wanted to analyse what some program is doing (maybe in CTF/wargame) you probably know about strace or ltrace. In this post I will explain briefly how they work and create simple tracer as an example.

PTRACE

First we need to know how operating system allows one process to manipulate other processes.

From man:
long ptrace(enum __ptrace_request request, pid_t pid, void *addr, void *data);

The ptrace() system call provides a means by which one process (the "tracer") may 
observe and control the execution of another process (the "tracee"), and examine 
and change the tracee's memory and registers. It is primarily used to implement 
breakpoint debugging and system call tracing.

Ptrace makes it possible to attach to other process, read/write its memory and get notifications when tracee raises signals or makes syscalls. You choose which one by setting proper request in first argument. Knowing this let's try creating our own tracer.

Simple tracer

Our tracer will get path to binary (ELF) as argument and will output functions called by traced program. It will only be able to trace binary with symbols (not stripped) built for x64 architecture to keep it simple.

First thing tracer has to fork, call ptrace with PTRACE_TRACEME request in child process and execute program which we want to trace:
child_pid = fork();
if(child_pid == 0) // child process
{
    ptrace(PTRACE_TRACEME, 0, NULL, NULL);
    execl(argv[1], argv[1], NULL);
    printf("Failed to execl!!\n");
    exit(-1);
}
Parent process will wait for child to call TRACEME and prepare breakpoints. After that it will start tracing.
wait(NULL);
prepare_breakpoints();
trace();
To see which functions were called by tracee we need to know adresses of those functions and insert breakpoint at the beginning of each function. To get this information tracer will obtain symbols table from the elf file.
void prepare_breakpoints()
{
    parse_elf_file();
    insert_brakepoints();
}
I won't make detailed explanation about elf files here as it would take whole another post (which I may write if there is interest). For more info you can check man elf(5) or this description. I included "elf.h" to use elf structures for easier parsing.
void parse_elf_file()
{
    Elf64_Ehdr elf_header;
    Elf64_Shdr section_header;
    fp = fopen(filename, "r");
    if(!fp)
    {
        printf("Failed to open ELF file!\n");
        exit(-1);
    }
    
    fread(&elf_header, 1, sizeof(elf_header), fp);  // read elf header

    fseek(fp, elf_header.e_shoff, SEEK_SET);    // skip to section headers

    for(int i = 0; i < elf_header.e_shnum; ++i) // iterate through headers
    {
        fread(&section_header, 1, sizeof(section_header), fp);  // read section header
        if(section_header.sh_type == SHT_SYMTAB)    // check if this section is symbol table
        {
            
            Elf64_Shdr strtab_header;   // we need to get strtab associated with this symtab to get functions names
            long strtab_hdr_offset = elf_header.e_shoff+section_header.sh_link*sizeof(section_header); // calculate offset to strtab header
            fseek(fp, strtab_hdr_offset, SEEK_SET);
            fread(&strtab_header, 1, sizeof(strtab_header), fp);    // read strtab header
            fseek(fp, section_header.sh_offset, SEEK_SET);

            int entries = section_header.sh_size / section_header.sh_entsize;
            breakpoints = malloc(entries*sizeof(Breakpoint));   // there are more entries than just functions

            for(i = 0; i < entries; ++i)
            {
                Elf64_Sym symbol;
                fread(&symbol, 1, sizeof(symbol), fp);          // read symbol
                if(ELF64_ST_TYPE(symbol.st_info) == STT_FUNC    // symbol is a function
                    && symbol.st_name != 0                      // symbol has name
                    && symbol.st_value != 0)                    // symbol has address within binary
                {
                    
                    long pos = ftell(fp);
                    fseek(fp, strtab_header.sh_offset+symbol.st_name, SEEK_SET);

                    breakpoints[bp_count].addr = symbol.st_value;   // get address to beginning of function
                    fread(breakpoints[bp_count].name, SYMBOL_NAME_LEN, sizeof(char), fp);   // get function name

                    fseek(fp, pos, SEEK_SET);
                    bp_count++;
                }
            }
        }
    }
}
Here is how breakpoint structure looks like:
typedef struct 
{
    long addr;
    long original_code;
    char name[SYMBOL_NAME_LEN+1];
} Breakpoint;
Now we need to change first instruction of each function to breakpoint:
void insert_brakepoints()
{
    for(int i = 0; i < bp_count; ++i)
    {
        breakpoints[i].original_code = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)breakpoints[i].addr, 0);
        ptrace(PTRACE_POKETEXT, child_pid, (void*)breakpoints[i].addr, (breakpoints[i].original_code & BKPT_MASK) | BKPT); // insert breakpoint
    }
}
We are ready to continue child process and wait for breakpoint:
void trace()
{
    int status;
    ptrace(PTRACE_CONT, child_pid, 0, 0);   // start child process
    printf("Tracing started\n");
    while(1)
    {
        waitpid(child_pid, &status, 0);     // wait for change of status

        if(WIFEXITED(status))
        {
            printf("Child finished\n");
            return;
        }

        if(WIFSTOPPED(status))  // child stopped
        {
            if(WSTOPSIG(status) == SIGTRAP) // child stopped on sigtrap
            {
               ...
Tracee signaled SIGTRAP which probably is our breakpoint. We need to get RIP register value. If RIP-1 is equal to address of one of our breakpoints then we know that tracee called function which we trace. Why RIP-1? RIP points to next instruction and breakpoint is 1 byte long in opcode.
                ...

                struct user_regs_struct regs;
                ptrace(PTRACE_GETREGS, child_pid, 0, &regs);
                int id = get_bp_id(regs.rip-1); // -1 because rip is now set to next inst and BP is 1 byte
                if(id == -1)
                {
                    printf("Unexpected SIGTRAP %llx\n", regs.rip);
                    return;
                }           
After identifying stepping on breakpoint we have to insert original code in that place, set RIP back to that address (decrement it), single-step over this instruction and set breakpoint back. Then we can continue.
                ...

                else
                {
                    printf("%s();\n", breakpoints[id].name);
                    regs.rip = breakpoints[id].addr;
                    ptrace(PTRACE_SETREGS, child_pid, 0, &regs);    // set rip back to good position
                    ptrace(PTRACE_POKETEXT, child_pid, (void*)breakpoints[id].addr, breakpoints[id].original_code); // return original instruction
                    ptrace(PTRACE_SINGLESTEP, child_pid, 0, 0);     // step instruction
                    wait(NULL); // wait for singlestep
                    ptrace(PTRACE_POKETEXT, child_pid, (void*)breakpoints[id].addr, (breakpoints[id].original_code & BKPT_MASK) | BKPT); // insert breakpoint again
                } 
            }
        ptrace(PTRACE_CONT, child_pid, 0, 0);
    } // end of while loop
} // end of trace()      
You can get code HERE

Testing tracer

Here is small program which I will trace:
                
#include <stdio.h>

int func1()
{
    printf("A");
}

void func2()
{
    printf("B");
}

void func3()
{
}

int main()
{
    func1();
    func3();
    func2();
    func2();
    func3();
    return 0;
}          
Let's run it:
    
thun@avaritia:~/workspace/tracer$ ./tracer test 
PID 6960
Tracing started
===

_start();
__libc_csu_init();
_init();
frame_dummy();
register_tm_clones();
main();
func1();
func3();
func2();
func2();
func3();
__do_global_dtors_aux();
deregister_tm_clones();
_fini();
ABBChild finished
Looks correct! "ABB" before "Child finished" message is output from tracee.

Making next step

I wanted to keep this example simple to give you idea of how it works but you can make much more with it.
First of all, this tracer will show nested function execution the same way as if two functions were called one after another.
You can fix that by adding a bit of code in part when breakpoint is processed:
    - Get RSP register value.
    - Use PTRACE_PEEKTEXT to get function return address from stack (RSP).
    - Set breakpoint on return address.

Secondly, you can not only trace program which you launch but also attach to already running processes by using PTRACE_ATTACH.

You can also get function arguments by reading registers and stack memory when tracee steps on breakpoint at function beggining and read return value at breakpoint on return address.

If you manage to do these steps and set breakpoints on functions in libraries then you have your own ltrace.

Debuggers are based on same idea. Just give user way to interact with tracing process (set breakpoints, read registers etc.).

One more thing. Since Lollipop (5.0), Android uses ART. All apps are compiled but bytecode is also in use (giving much information). By parsing oat files you can get information about function names, parameters and addresses. So if you ever wanted to analyse execution of Pokemon Go (or other android app) now you know how to create trace tool for that.

NOTE: If you want to set breakpoints in attached mode you will need to adjust addresses to function by getting base address of binary or library from process maps and adding it to symbol address. Also remember that mapping changes due to ASLR so you cannot hardcode it.

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