#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <linux/user.h>

const int long_size = sizeof(long);

void getdata(pid_t child, long addr, char *str, int len)
{   char *laddr;
    int i, j;
    union u {
            long val;
            char chars[long_size];
    }data;
    
    i = 0;
    j = len / long_size;
    laddr = str;
    while(i < j) {
        data.val = ptrace(PTRACE_PEEKDATA, child, addr + i * 4, NULL);
        memcpy(laddr, data.chars, long_size);
        ++i;
        laddr += long_size;
    }
    
    j = len % long_size;
    if(j != 0) {
        data.val = ptrace(PTRACE_PEEKDATA, child, addr + i * 4, NULL);
        memcpy(laddr, data.chars, j);
    }
    str[len] = '\0';       
}

void putdata(pid_t child, long addr, char *str, int len)
{   char *laddr;
    int i, j;
    union u {
            long val;
            char chars[long_size];
    }data;
    
    i = 0;
    j = len / long_size;
    laddr = str;
    while(i < j) {
        memcpy(data.chars, laddr, long_size);
        ptrace(PTRACE_POKEDATA, child, addr + i * 4, data.val);
        ++i;
        laddr += long_size;
    }
    
    j = len % long_size;
    if(j != 0) {
        memcpy(data.chars, laddr, j);
        ptrace(PTRACE_POKEDATA, child, addr + i * 4, data.val);
    }
}

int main(int argc, char *argv[])
{   pid_t traced_process;
    struct user_regs_struct regs, newregs;
    long ins;
    char code[] = {0xcd,0x80,0xcc,0};
    char backup[4];

    if(argc != 2) {
        printf("Usage: %s <pid to be traced>\n", argv[0], argv[1]);
        exit(1);
    }
    traced_process = atoi(argv[1]);

    ptrace(PTRACE_ATTACH, traced_process, NULL, NULL);
    wait(NULL);
   
    ptrace(PTRACE_GETREGS, traced_process, NULL, &regs);

    /* Get the instructions into a backup variable */
    getdata(traced_process, regs.eip, backup, 3);

    /* Put the break point */
    putdata(traced_process, regs.eip, code, 3);
  
    /* Let the process continue and execute the int 3 instruction */
    ptrace(PTRACE_CONT, traced_process, NULL, NULL);

    wait(NULL);
    printf("The process stopped, putting back the original instructions\n");
    printf("Press <enter> to continue\n");
    getchar();
    putdata(traced_process, regs.eip, backup, 3);

    /* Setting the eip back to the original instruction to let */
    /* the process continue */
    ptrace(PTRACE_SETREGS, traced_process, NULL, &regs);
   
    ptrace(PTRACE_DETACH, traced_process, NULL, NULL);
    return 0;
}