6——Linux二进制分析——学习——ptrace调试器V2

缺乏、安全感 2022-03-17 09:44 210阅读 0赞

这是3.5章节的内容，这个例子对上一章节的ptrace工具进行了完善，主要是添加了attach功能，也就是获取正在运行的程序的信息，gdb就有这个功能，用的应该也是这个方法。本来作者写此书的时候就已经很简略了，省去了很多详细的介绍，测试代码也没有给出。手头的中文版中的代码更是经常有错误，如果照搬书中的例子注定无法得到预期的结果。我进行了修改和调试，能够得到书中作者提到的正确的结果。

这个例子只是对上一个例子的补充，上一篇文章全文都可以参考。代码中有XBter字段的注释是我添加的。***增加的是PTRACE\_ATTACH和getopt()接口，可以参考代码和注释。***下面贴出代码，模块清晰很简单，编译参考上一篇文章即可。

### test2.c ###

#include <stdio.h>
    #include <unistd.h>
    
    void print_string(char *);
    
    int main(void){
        int i = 0;
        for(; i < 20; i++){
            print_string("Hello 1");
            sleep(2);
        }
        return 0;
    }
    
    void print_string(char *str){
        printf("%s\n",str);
    }

### tracer2.c ###

#include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <fcntl.h>
    #include <errno.h>
    #include <signal.h>
    #include <elf.h>
    #include <sys/types.h>
    #include <sys/user.h>
    #include <sys/stat.h>
    #include <sys/ptrace.h>
    #include <sys/mman.h>
    #include <sys/wait.h>//The sample of the book don't include this lib, it's wrong.
    
    typedef struct handle{
        char *exec;
        char *symname;
        Elf64_Addr symaddr;//symname's address
        Elf64_Ehdr *ehdr;
        Elf64_Phdr *phdr;
        Elf64_Shdr *shdr;
        uint8_t *mem;
        struct user_regs_struct pt_reg;
    }handle_t;
    
    int global_pid;
    Elf64_Addr lookup_symbol(handle_t *, const char *);
    char * get_exe_name(int);
    void sighandler(int);
    #define EXE_MODE 0
    #define PID_MODE 1
    
    int main(int argc, char **argv, char **envp)
    {
        int fd, c, mode = 0;
        handle_t h;
        struct stat st;
        long trap, orig;
        int status, pid;
        char * args[2];
    
        printf("Usage: %s [-ep <exe>/<pid>] [f <fname>]\n", argv[0]);
        memset(&h, 0, sizeof(handle_t));
    
        /* This is a important and amazing function--getopt(),
         * getopt() must be the function almost all linux tools
         * used.  XBter*/
        while((c = getopt(argc, argv, "p:e:f:")) != -1){
            switch(c){
                case 'p':
                    pid = atoi(optarg);
                    h.exec = get_exe_name(pid);
                    if(h.exec == NULL){
                        printf("Unable to retrieve executable path for pid: %d\n", pid);
                        exit(-1);
                    }
                    mode = PID_MODE;
                    break;
                case 'e':
                    if((h.exec = strdup(optarg)) == NULL){
                        perror("strdup");
                        exit(-1);
                    }
                    mode = EXE_MODE;
                    break;
                case 'f':
                    if((h.symname = strdup(optarg)) == NULL){
                        perror("strdup");
                        exit(-1);
                    }
                    break;
                default:
                    printf("Unknown option\n");
                    break;
            }
        }
    
        /* I think this is needless, program can't run in it.  XBter */
        if(h.symname == NULL){
            printf("Specifying a function name with -f option is required\n");
            exit(-1);
        }
    
        if(mode == EXE_MODE){
            args[0] = h.exec;
            args[1] = NULL;
        }
    
        signal(SIGINT, sighandler);
        if((fd = open(h.exec, O_RDONLY)) < 0){
            perror("open");
            exit(-1);
        }
    
        if(fstat(fd, &st) < 0){
            perror("fstat");
            exit(-1);
        }
    
        h.mem = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
        if(h.mem == MAP_FAILED){
            perror("mmap");
            exit(-1);
        }
    
        h.ehdr = (Elf64_Ehdr*)h.mem;
        h.phdr = (Elf64_Phdr*)(h.mem + h.ehdr->e_phoff);
        h.shdr = (Elf64_Shdr*)(h.mem + h.ehdr->e_shoff);
    
        if(h.mem[0] != 0x7f || strncmp((char*)&h.mem[1], "ELF", 3)){
            printf("%s is not an ELF file\n", h.exec);
            exit(-1);
        }
    
        /* I use gcc, so I should add the -no-pie flag. XBter */
        if(h.ehdr->e_type != ET_EXEC){
            printf("%s is not an ELF executable. The type: %d\n", h.exec, h.ehdr->e_type);
            exit(-1);
        }
    
        if(h.ehdr->e_shstrndx == 0 || h.ehdr->e_shoff == 0 || h.ehdr->e_shnum == 0){
            printf("Section header table not found\n");
            exit(-1);
        }
    
        if((h.symaddr = lookup_symbol(&h, h.symname)) == 0){
            printf("Unable to find symbol: %s not found in executable\n", h.symname);
            exit(-1);
        }
    
        close(fd);
    
        if(mode == EXE_MODE){
            if((pid = fork()) < 0){
                perror("fork");
                exit(-1);
            }
    
            if(pid == 0){
                if(ptrace(PTRACE_TRACEME, pid, NULL, NULL) < 0){
                    perror("PTRACE_TRACEME");
                    exit(-1);
                }
                execve(h.exec, args, envp);//run the child pid.  XBter
                exit(0);
            }
        }else{ //PID_MODE
            if(ptrace(PTRACE_ATTACH, pid, NULL, NULL) < 0){
                perror("PTRACE_ATTACH");
                exit(-1);
            }
        }
    
        wait(&status);//wait tracee to stop
        global_pid = pid;
        printf("\nXBter\n\nBeginning analysis of pid: %d at %lx\n", pid, h.symaddr);
    
        // Read the 8 byte at h.symaddr
        if((orig = ptrace(PTRACE_PEEKTEXT, pid, h.symaddr, NULL)) < 0){
            perror("PTRCE_PEEKTEXT");
            exit(-1);
        }
    
        /* Set breakpoint, 0xcc. XBter*/
        trap = (orig & ~0xff) | 0xcc;
        if(ptrace(PTRACE_POKETEXT, pid, h.symaddr, trap) < 0){
            perror("PTRACE_PEEKTEXT");
            exit(-1);
        }
    
        // Begin tracing execution
    trace:
        if(ptrace(PTRACE_CONT, pid, NULL, NULL) < 0){
            perror("PTRACE_CONT");
            exit(-1);
        }
    
        wait(&status);
        /**
         * WIFSTOPPED(status): 如果进程在被ptrace调用监控的时候被信号暂停/停止，返回True
         * WSTOPSIG(status): WIFSTOPPED为true时，返回导致子进程停止的信号类型
         * WIFEXITED(status):子进程正常退出情况下为true
         */
         /**
          * If we receive a SIgTRAP then we presumably hit a break
          * Point instrunction. In which case we will print out the
          * current register state.
          */
        if(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP)
        {
            if(ptrace(PTRACE_GETREGS, pid, NULL, &h.pt_reg) < 0)
            {
                perror("PTRACE_GETREGS");
                exit(-1);
            }
    
            printf("\nExecutable %s (pid: %d) has hit breakpoint 0x%lx\n\n", h.exec, pid, h.symaddr);
    
            printf("%%rcx: %llx\n%%rdx: %llx\n%%rbx: %llx\n"
                   "%%rax: %llx\n%%rdi: %llx\n%%rsi: %llx\n"
                   "%%r8: %llx\n%%r9: %llx\n%%r10: %llx\n"
                   "%%r11: %llx\n%%r12: %llx\n%%r13: %llx\n"
                   "%%r14: %llx\n%%r15: %llx\n%%rsp: %llx,\n"
                   "%%rip: %llx\n",
                   h.pt_reg.rcx, h.pt_reg.rdx, h.pt_reg.rbx,
                   h.pt_reg.rax, h.pt_reg.rdi, h.pt_reg.rsi,
                   h.pt_reg.r8, h.pt_reg.r9, h.pt_reg.r10,
                   h.pt_reg.r11, h.pt_reg.r12, h.pt_reg.r13,
                   h.pt_reg.r14, h.pt_reg.r15, h.pt_reg.rsp,
                   h.pt_reg.rip);
            printf("\nPlease hit any key to contunue: ");
    
            getchar();
    
            /* restore the program, delete the breakpoint. XBter*/
            if(ptrace(PTRACE_POKETEXT, pid, h.symaddr, orig) < 0)
            {
                perror("PTRACE_POKETEXT");
                exit(-1);
            }
    
            /* rip: next cmd address.  XBter */
            h.pt_reg.rip = h.pt_reg.rip - 1;
            printf("%d rip=%llx\n",__LINE__, h.pt_reg.rip);
            if(ptrace(PTRACE_SETREGS, pid, NULL, &h.pt_reg) < 0)
            {
                perror("PTRACE_SETREGS");
                exit(-1);
            }
            if(ptrace(PTRACE_SINGLESTEP, pid, NULL, NULL) < 0)
            {
                perror("PTRACE_SINGLESTEP");
                exit(-1);
            }
            wait(NULL);
            if(ptrace(PTRACE_POKETEXT, pid, h.symaddr, trap) < 0)
            {
                perror("PTRACE_POKETEXT");
                exit(-1);
            }
            goto trace;
        }
        if(WIFEXITED(status))
            printf("Completed tracing pid: %d\n", pid);
        exit(0);
    }
    
    /**
     * This function will lookup a symbol by name, specifically from
     * The .symtab section, and return the symbol value.
     */
    Elf64_Addr lookup_symbol(handle_t *h, const char *symname)
    {
        int i, j;
        char *strtab;      //.strtab section
        Elf64_Sym *symtab; //.symtab section
        for(i = 0; i <h->ehdr->e_shnum; i++){
            if(h->shdr[i].sh_type == SHT_SYMTAB){
                strtab = (char*)&h->mem[h->shdr[h->shdr[i].sh_link].sh_offset];
                symtab = (Elf64_Sym*)&h->mem[h->shdr[i].sh_offset];
                for(j = 0; j < h->shdr[i].sh_size/sizeof(Elf64_Sym); j++){
                    if(strcmp(&strtab[symtab->st_name], symname) == 0)
                        return (symtab->st_value);
                    symtab++;
                }
            }
        }
        return 0;
    }
    
    /**
     * This function will parse the cmdline proc entry to retrieve
     * the executable name of the process.
     */
    char * get_exe_name(int pid){
        char cmdline[255], path[512], *p;
        int fd;
        snprintf(cmdline, 255, "/proc/%d/cmdline", pid);
        if((fd = open(cmdline, O_RDONLY)) < 0){
            perror("open");
            exit(-1);
        }
    
        if(read(fd, path, 512) < 0){
            perror("open");
            exit(-1);
        }
    
        if((p = strdup(path)) == NULL){
            perror("strdup");
            exit(-1);
        }
    
        return p;
    }
    
    void sighandler(int sig){
         printf("Caught SIGINT: Detaching from %d\n", global_pid);
         if(ptrace(PTRACE_DETACH, global_pid, NULL, NULL) < 0 && errno){
             perror("");
             exit(-1);
         }
         exit(0);
     }

gcc调试一切正常，支持-p和-e两种模式。执行指令：

***./tracer2 -p \`pidof ./test2\` -f print\_string***

***./tracer2 -e ./test2 -f print\_string***

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhbmdxdW54aWFuZmVuZw_size_16_color_FFFFFF_t_70][]

文章中有这么一段话，***......附加到一个已经处于运行状态的进程上......***。所以在运行的时候需要先把要附加的程序运行起来，我这里也就是test2程序。在一个终端上./test2，然后在另外一个终端上执行***./tracer2 -p \`pidof ./test2\` -f print\_string。***这样就能在运行的程序上加断点了，gdb应该用的就是这种办法。

谢谢读者指出问题，谢谢。

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhbmdxdW54aWFuZmVuZw_size_16_color_FFFFFF_t_70]: /images/20220317/c6cffd5d68ee4ba28f0f067ab86ae9f2.png