3——Linux二进制分析——学习——简易ELF解析器

Myth丶恋晨 2022-03-19 10:28 294阅读 0赞

在第二章的结尾，作者贴上了一段ELF解析器的代码，我进行了反复的尝试，加深了对ELF格式的理解。这个其实就是一个简单的readelf，打印所有的section header名称和在可执行程序中的虚拟地址，program header的地址。

为了便于理解，在某些地方我加入了自己的注释。例如***mmap()***处的注释，line92之前的3行对***e\_shstrndx***的解释，还有开头对***3个结构体***的解释。

之前在看《深入理解Linux内核》的时候看到过mmap()，这次再次遇到才真正查了一下mmap()的具体功能，也知识有了肤浅的认识。我没有想到的是***共享内存***居然使用的就是mmap()技术实现的，大家对同一块内存进行映射（map），大家就能操作相同一块内存，可读也可以写，这样也就实现了进程间通信。除了共享内存外还有一个更广的应用场景，就是划拨一块硬盘作为内存使用，也是这个思想。

下面我把代码贴出来，就是一个简单的main()函数，一点也不复杂。按照第一行的注释进行编译即可。代码里用到的结构体都在***elf.h***系统头文件中定义。任何一个完整的linux系统中都存在，在linux内核代码中也存在。如果使用window编辑的话，建议拷贝一份elf.h到自己的工程中。

/* elfparse.c - gcc elfparse.c -o elfparse */
    
    #include <stdio.h>
    #include <string.h>
    #include <errno.h>
    #include <elf.h>
    #include <unistd.h>
    #include <stdlib.h>
    #include <sys/mman.h>
    #include <stdint.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    
    int main(int argc, char **argv)
    {
        printf("XBter!\n");
    
        int fd, i;
        uint8_t *mem;
        struct stat st;
        char *StringTable, *interp;
    
        Elf32_Ehdr *ehdr;//ELF file header
        Elf32_Phdr *phdr;//program header
        Elf32_Shdr *shdr;//section header
    
        if(argc < 2){
            printf("Usage: %s <executable>\n", argv[0]);
            exit(0);
        }
    
        if((fd = open(argv[1], O_RDONLY)) < 0){
            perror("open");
            exit(-1);
        }
    
        if(fstat(fd, &st) < 0){
            perror("fstat");
            exit(-1);
        }
    
        /* Map the executable into memory */
        mem = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
        if(mem == MAP_FAILED){/*mmap may have many fuctions. I read*/
            perror("mmap");/*a blog, share memory, Changing disk to*/
            exit(-1);/*memory when memory is not enough may all use*/
        }/*mmap(). This is real mapping. This is the blog address: */
        /*https://blog.csdn.net/yanerhao/article/details/77829191  */
    
        /**
         * The initial ELF Header starts at offset 0
         * of our mapped memory
         */
        ehdr = (Elf32_Ehdr*)mem;
    
        /**
         * The shdr table and phdr table offsets are
         * given by e_shoff and e_phoff members of the
         * ELF32_Ehdr
         */
        phdr = (Elf32_Phdr*)&mem[ehdr->e_phoff];
        shdr = (Elf32_Shdr*)&mem[ehdr->e_shoff];
    
        /**
         * Check to see if the ELF magic (The first 4 bytes)
         * match up as 0x7f E L F (0x7f 45 4c 46)
         */
        if(mem[0] != 0x7f && strcmp(&mem[1], "ELF")){
           fprintf(stderr, "%s is not an ELF file\n", argv[1]);
           exit(-1);
        }
    
        /**
         * We are only parsing executables with this code
         * so ET_EXEC marks an executables.
         */
        if(ehdr->e_type != ET_EXEC){
            fprintf(stderr, "%s is not an executable\n", argv[1]);
            exit(-1);
        }
    
        printf("Program Entry point: 0x%x\n", ehdr->e_entry);
    
        /**
         * We find the string table for the section header
         * names with e_shstrndx which gives the index of
         * which section holds the string table.
         * e_shstrndx is the index of section .shstrtab
         * .shstrtab holds all sections's name, just a strings
         * ending with /0.
         */
         StringTable = &mem[shdr[ehdr->e_shstrndx].sh_offset];
         /**
          * Print each section header name and address.
          * Notice we get the index into the string table
          * that contains each section header name with
          * the shdr.sh_name member.
          */
         printf("Section header total:%d list:\n\n", ehdr->e_shnum);
         for(i = 0; i < ehdr->e_shnum; i++)
             printf("[%d]%s: 0x%x\n", i, &StringTable[shdr[i].sh_name], shdr[i].sh_addr);
    
         /**
          * Print out each segment name, and address.
          * Except for PT_INTERP we print the path to
          * the dynamic linker (Interpreter).
          */
         printf("\nProgram header total:%d list:\n\n", ehdr->e_phnum);
         for(i = 0; i < ehdr->e_phnum; i++){
    //         printf("phdr[%d].p_type=%d\n", i, phdr[i].p_type);
             switch(phdr[i].p_type){
                 case PT_LOAD:
                     /**
                      * We know that text segment starts
                      * at offset 0. And only one other
                      * possible loadable segment exists
                      * which is the data segment.
                      */
                     if(phdr[i].p_offset == 0)
                         printf("Text segment: 0x%x\n", phdr[i].p_vaddr);
                     else
                         printf("Data segment: 0x%x\n",phdr[i].p_vaddr);
                 break;
                 case PT_INTERP:
                     interp = strdup((char*)&mem[phdr[i].p_offset]);
                     printf("Interpreter: %s\n", interp);
                 break;
                 case PT_NOTE:
                     printf("Note segment: 0x%x\n", phdr[i].p_vaddr);
                 break;
                 case PT_DYNAMIC:
                     printf("Dynamic segment: 0x%x\n", phdr[i].p_vaddr);
                 break;
                 case PT_PHDR:
                     printf("Phdr segment: 0x%x\n", phdr[i].p_vaddr);
                 break;
             }
         }
    
        exit(0);
    }