通过dlopen、dlsym获取共享库函数地址、全局变量是一种经常使用到的编程技巧，尤其是在Hook框架中。然而无论是dlsym还是一些常用框架（如Nougat_dlfunctions
），都只能搜索.dynsym段，而无法搜索.symtab段。因此实现.symtab段搜索是一个亟待解决的问题。
本文将介绍在Nougat_dlfunctions框架基础上，如何实现搜索.symtab段的功能。（如果对FastHook不了解，请查阅FastHook——一种高效稳定、简洁易用的Android Hook框架）
项目地址：Enhanced_dlfunctions

ELF文件实际是个表结构，以段为单位，每个段存储不同的信息，段与段之间以索引来关联形成一个表。所以只要有一个头，就可以通过这些关系访问所有的段，这个头便是ELF文件头。下面我们来看看需要获取那些信息：

1. 获取so加载的地址，存储在load_addr

2. 重新映射so，获取ELF文件头

3. 获取.shstrtab段。

4.获取.dynsym段、.dynstr段、.symtab段、.strtab段

1. 先查询.dynsym段。遍历.dymsym所有符号，查找符号名与给定字符串一致的符号。用load_addr、bias结合符号偏移来获取实际地址。

2. 如果.dynsym段找不到目标符号且.symtab段存在时，遍历.symtab所有符号，查找符号名与给定字符串一致的符号。用load_addr、bias结合符号偏移来获取实际地址。

Nougat_dlfunctions只支持arm平台，而我在实际工作中也不涉及到x86平台，所以Enhanced dlfunctions暂时只支持arm32和arm64，如果要兼容x86等其他平台也不难，不需要改实现，主要就是elf.h一些结构体的不同。

void *enhanced_dlopen(const char *libpath, int flags) {
    FILE *maps;
    char buff[256];
    struct ctx *ctx = 0;
    off_t load_addr, size;
    int k, fd = -1, found = 0;
    void *shoff;
    Elf_Ehdr *elf = (Elf_Ehdr *) MAP_FAILED;

#define fatal(fmt, args...) do { log_err(fmt,##args); goto err_exit; } while(0)

    maps = fopen("/proc/self/maps", "r");
    if (!maps) fatal("failed to open maps");

    while (!found && fgets(buff, sizeof(buff), maps))
        if (strstr(buff, "r-xp") && strstr(buff, libpath)) found = 1;

    fclose(maps);

    if (!found) fatal("%s not found in my userspace", libpath);

    if (sscanf(buff, "%lx", &load_addr) != 1)
        fatal("failed to read load address for %s", libpath);

    log_info("%s loaded in Android at 0x%08lx", libpath, load_addr);

    /* Now, mmap the same library once again */

    fd = open(libpath, O_RDONLY);
    if (fd < 0) fatal("failed to open %s", libpath);

    size = lseek(fd, 0, SEEK_END);
    if (size <= 0) fatal("lseek() failed for %s", libpath);

    elf = (Elf_Ehdr *) mmap(0, size, PROT_READ, MAP_SHARED, fd, 0);
    close(fd);
    fd = -1;

    if (elf == MAP_FAILED) fatal("mmap() failed for %s", libpath);

    ctx = (struct ctx *) calloc(1, sizeof(struct ctx));
    if (!ctx) fatal("no memory for %s", libpath);

    ctx->load_addr = (void *) load_addr;
    shoff = ((void *) elf) + elf->e_shoff;

    Elf_Shdr *shstrtab = (Elf_Shdr *)(shoff + elf->e_shstrndx * elf->e_shentsize);
    char * shstr = malloc(shstrtab->sh_size);
    memcpy(shstr, ((void *) elf) + shstrtab->sh_offset, shstrtab->sh_size);

    for (k = 0; k < elf->e_shnum; k++, shoff += elf->e_shentsize) {
        Elf_Shdr *sh = (Elf_Shdr *) shoff;
        log_dbg("%s: k=%d shdr=%p type=%d", __func__, k, sh, sh->sh_type);
        switch (sh->sh_type) {
            case SHT_DYNSYM:
                if (ctx->dynsym) fatal("%s: duplicate DYNSYM sections", libpath); /* .dynsym */
                ctx->dynsym = malloc(sh->sh_size);
                if (!ctx->dynsym) fatal("%s: no memory for .dynsym", libpath);
                memcpy(ctx->dynsym, ((void *) elf) + sh->sh_offset, sh->sh_size);
                ctx->dynsym_num = (sh->sh_size / sizeof(Elf_Sym));
                break;
            case SHT_SYMTAB:
                if (ctx->symtab) fatal("%s: duplicate SYMTAB sections", libpath); /* .symtab */
                ctx->symtab = malloc(sh->sh_size);
                if (!ctx->symtab) fatal("%s: no memory for .symtab", libpath);
                memcpy(ctx->symtab, ((void *) elf) + sh->sh_offset, sh->sh_size);
                ctx->symtab_num = (sh->sh_size / sizeof(Elf_Sym));
                break;
            case SHT_STRTAB:
                if(!strcmp(shstr+sh->sh_name,".dynstr")) {
                    if (ctx->dynstr) break;    /* .dynstr is guaranteed to be the first STRTAB */
                    ctx->dynstr = malloc(sh->sh_size);
                    if (!ctx->dynstr) fatal("%s: no memory for .dynstr", libpath);
                    memcpy(ctx->dynstr, ((void *) elf) + sh->sh_offset, sh->sh_size);
                }else if(!strcmp(shstr+sh->sh_name,".strtab")) {
                    if (ctx->strtab) break;
                    ctx->strtab = malloc(sh->sh_size);
                    if (!ctx->strtab) fatal("%s: no memory for .strtab", libpath);
                    memcpy(ctx->strtab, ((void *) elf) + sh->sh_offset, sh->sh_size);
                }
                break;
            case SHT_PROGBITS:
                if (!ctx->dynstr || !ctx->dynsym || ctx->bias) break;
                /* won't even bother checking against the section name */
                ctx->bias = (off_t) sh->sh_addr - (off_t) sh->sh_offset;
                break;
        }
    }

    munmap(elf, size);
    elf = 0;
    if (!ctx->dynstr || !ctx->dynsym) fatal("dynamic sections not found in %s", libpath);
#undef fatal
    log_dbg("%s: ok, dynsym = %p, dynstr = %p symtab = %p strtab = %p", libpath, ctx->dynsym, ctx->dynstr, ctx->symtab, ctx->strtab);
    return ctx;
   }

    maps = fopen("/proc/self/maps", "r");

    while (!found && fgets(buff, sizeof(buff), maps))
        if (strstr(buff, "r-xp") && strstr(buff, libpath)) found = 1;

    fclose(maps);

    if (sscanf(buff, "%lx", &load_addr) != 1)
        fatal("failed to read load address for %s", libpath);

    fd = open(libpath, O_RDONLY);

    size = lseek(fd, 0, SEEK_END);

    elf = (Elf_Ehdr *) mmap(0, size, PROT_READ, MAP_SHARED, fd, 0);

    Elf_Shdr *shstrtab = (Elf_Shdr *)(shoff + elf->e_shstrndx * elf->e_shentsize);

    char * shstr = malloc(shstrtab->sh_size);

    memcpy(shstr, ((void *) elf) + shstrtab->sh_offset, shstrtab->sh_size);

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