FastHook——实现.dynsym段和.symtab段符号查询

1、概述

经过dlopen、dlsym获取共享库函数地址、全局变量是一种常常使用到的编程技巧,尤为是在Hook框架中。然而不管是dlsym仍是一些经常使用框架(如Nougat_dlfunctions),都只能搜索**.dynsym段,而没法搜索.symtab**段。所以实现.symtab段搜索是一个亟待解决的问题。git

本文将介绍在Nougat_dlfunctions框架基础上,如何实现搜索.symtab段的功能。(若是对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_dlfunctionsgithub

2、Enhanced dlfunctions实现

ELF文件实际是个表结构,以段为单位,每一个段存储不一样的信息,段与段之间以索引来关联造成一个表。因此只要有一个头,就能够经过这些关系访问全部的段,这个头即是ELF文件头。下面咱们来看看须要获取那些信息:编程

  1. ELF文件头:存储全部段头部的信息,段头部是描述段信息的元数据,能够经过段头部来获取段信息。
  2. .shstrtab段。存储全部的段的段名。经过ELF文件头,能够实现段的遍历,而没法识别具体的段(不一样段类型能够相同),所以须要用段名来肯定段。
  3. .dynsym段:动态符号表,存储与动态连接相关的导入导出符号,不包括模块内部的符号。Nougat_dlfunctions只查询这个段,所以会漏掉不少符号。
  4. .dynstr段:存储.dynsym段符号对应的符号名。
  5. .symtab段:符号表。存储在程序中被定义和引用的函数和全局变量的信息。
  6. .strtab段:存储.symtab段符号对应的符号名。
  7. .comment段:程序相关信息,用与定位符号地址。

2.1 enhanced_dlopen

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; } 复制代码

1. 获取so加载的地址,存储在load_addrbash

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);
复制代码

2. 从新映射so,获取ELF文件头微信

fd = open(libpath, O_RDONLY);

    size = lseek(fd, 0, SEEK_END);

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

3. 获取.shstrtab段。框架

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);
复制代码

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

for (k = 0; k < elf->e_shnum; k++, shoff += elf->e_shentsize) {
        Elf_Shdr *sh = (Elf_Shdr *) shoff;
        switch (sh->sh_type) {
            case SHT_DYNSYM:
                ctx->dynsym = malloc(sh->sh_size);
                memcpy(ctx->dynsym, ((void *) elf) + sh->sh_offset, sh->sh_size);
                ctx->dynsym_num = (sh->sh_size / sizeof(Elf_Sym));
                break;
            case SHT_SYMTAB:
                ctx->symtab = malloc(sh->sh_size);
                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")) {
                    ctx->dynstr = malloc(sh->sh_size);
                    memcpy(ctx->dynstr, ((void *) elf) + sh->sh_offset, sh->sh_size);
                }else if(!strcmp(shstr+sh->sh_name,".strtab")) {
                    ctx->strtab = malloc(sh->sh_size);
                    memcpy(ctx->strtab, ((void *) elf) + sh->sh_offset, sh->sh_size);
                }
                break;
            case SHT_PROGBITS:
                if (!ctx->dynstr || !ctx->dynsym || ctx->bias) break;
                ctx->bias = (off_t) sh->sh_addr - (off_t) sh->sh_offset;
                break;
        }
    }
复制代码

2.2 enhanced_dlsym

void *enhanced_dlsym(void *handle, const char *name) {
    int k;
    struct ctx *ctx = (struct ctx *) handle;
    Elf_Sym *dynsym = (Elf_Sym *) ctx->dynsym;
    Elf_Sym *symtab = (Elf_Sym *) ctx->symtab;
    char *dynstr = (char *) ctx->dynstr;
    char *strtab = (char *) ctx->strtab;

    for (k = 0; k < ctx->dynsym_num; k++, dynsym++) {
        if (strcmp(dynstr + dynsym->st_name, name) == 0) {
            void *ret = ctx->load_addr + dynsym->st_value - ctx->bias;
            log_info("%s found at %p", name, ret);
            return ret;
        }
    }

    if(symtab) {
        for (k = 0; k < ctx->symtab_num; k++, symtab++) {
            sym_tab->st_name,strings + sym_tab->st_name,k);
            if (strcmp(strtab + symtab->st_name, name) == 0) {
                void *ret = ctx->load_addr + symtab->st_value - ctx->bias;
                log_info("%s found at %p", name, ret);
                return ret;
            }
        }
    }
    return 0;
}
复制代码

1. 先查询.dynsym段。遍历.dymsym全部符号,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。post

for (k = 0; k < ctx->dynsym_num; k++, dynsym++) {
        if (strcmp(dynstr + dynsym->st_name, name) == 0) {
            void *ret = ctx->load_addr + dynsym->st_value - ctx->bias;
            log_info("%s found at %p", name, ret);
            return ret;
        }
    }
复制代码

2. 若是.dynsym段找不到目标符号且.symtab段存在时,遍历.symtab全部符号,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。ui

for (k = 0; k < ctx->symtab_num; k++, symtab++) {
            sym_tab->st_name,strings + sym_tab->st_name,k);
            if (strcmp(strtab + symtab->st_name, name) == 0) {
                void *ret = ctx->load_addr + symtab->st_value - ctx->bias;
                log_info("%s found at %p", name, ret);
                return ret;
            }
        }
复制代码

3、结语

Nougat_dlfunctions只支持arm平台,而我在实际工做中也不涉及到x86平台,因此Enhanced dlfunctions暂时只支持arm32和arm64,若是要兼容x86等其余平台也不难,不须要改实现,主要就是elf.h一些结构体的不一样。spa

4、参考

  1. FastHook——一种高效稳定、简洁易用的Android Hook框架
  2. FastHook——巧妙利用动态代理实现非侵入式AOP
  3. FastHook——远超YAHFA的优异稳定性
  4. 如何使用FastHook免root hook微信
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