最近，在学习BCTF和0CTF的writeup时，注意到了一种通过DT_DEBUG来获得库的基址的方式：BCTF里的pattern用这一方法来获得ld-linux.so的地址，0CTF里的sandbox用这一方法来获得sandbox.so的基址。之前面对ASLR，我只知道可以通过GOT来获取libc.so的地址，而其他库的地址还不清楚应该怎样取得。于是，我稍微研究了下，在此记录。
首先，通过readelf -d，可以得到.dynamic的信息。而有些二进制文件里的.dynamic里包含DT_DEBUG：
Dynamic section at offset 0x7c8 contains 20 entries: Tag Type Name/Value ... 0x0000000000000015 (DEBUG) 0x0 ...
这里DT_DEBUG的值是0。在实际运行时，DT_DEBUG的值是指向struct r_debug的指针。其定义如下：
/* Rendezvous structure used by the run-time dynamic linker to communicate details of shared object loading to the debugger. If the executable's dynamic section has a DT_DEBUG element, the run-time linker sets that element's value to the address where this structure can be found. */structr_debug{intr_version;/* Version number for this protocol. */structlink_map*r_map;/* Head of the chain of loaded objects. *//* This is the address of a function internal to the run-time linker, that will always be called when the linker begins to map in a library or unmap it, and again when the mapping change is complete. The debugger can set a breakpoint at this address if it wants to notice shared object mapping changes. */ElfW(Addr)r_brk;enum{/* This state value describes the mapping change taking place when the `r_brk' address is called. */RT_CONSISTENT,/* Mapping change is complete. */RT_ADD,/* Beginning to add a new object. */RT_DELETE/* Beginning to remove an object mapping. */}r_state;ElfW(Addr)r_ldbase;/* Base address the linker is loaded at. */};可以看到，其第二个元素是指向struct link_map的指针。其定义如下：
/* Structure describing a loaded shared object. The `l_next' and `l_prev' members form a chain of all the shared objects loaded at startup. These data structures exist in space used by the run-time dynamic linker; modifying them may have disastrous results. */structlink_map{/* These first few members are part of the protocol with the debugger. This is the same format used in SVR4. */ElfW(Addr)l_addr;/* Difference between the address in the ELF file and the addresses in memory. */char*l_name;/* Absolute file name object was found in. */ElfW(Dyn)*l_ld;/* Dynamic section of the shared object. */structlink_map*l_next,*l_prev;/* Chain of loaded objects. */};于是，遍历link_map，对比l_name，找到目标之后，就可以通过l_addr获得那个库的基址。
实例如下，比如说我们要找ld-linux.so的基址。首先，检查.dynamic的内容：
gdb-peda$ x/20gx 0x6007c8
0x6007c8: 0x0000000000000001 0x0000000000000010
0x6007d8: 0x000000000000000c 0x0000000000400400
0x6007e8: 0x000000000000000d 0x00000000004006d8
0x6007f8: 0x000000006ffffef5 0x0000000000400260
0x600808: 0x0000000000000005 0x0000000000400310
0x600818: 0x0000000000000006 0x0000000000400280
0x600828: 0x000000000000000a 0x000000000000004b
0x600838: 0x000000000000000b 0x0000000000000018
0x600848: 0x0000000000000015 0x00007f38cd4bd140
0x600858: 0x0000000000000003 0x0000000000600960DT_DEBUG是0x15，所以0x600848那里就是DT_DEBUG的条目，其值是0x00007f38cd4bd140，即struct r_debug的地址。
gdb-peda$ x/6x 0x00007f38cd4bd140
0x7f38cd4bd140 <_r_debug>: 0x0000000000000001 0x00007f38cd4bd168
0x7f38cd4bd150 <_r_debug+16>: 0x00007f38cd2aba90 0x0000000000000000
0x7f38cd4bd160 <_r_debug+32>: 0x00007f38cd29c000 0x0000000000000000对照定义，我们知道第二个元素，0x00007f38cd4bd168是link_map链表的第一个元素的地址。
gdb-peda$ x/6x 0x00007f38cd4bd168
0x7f38cd4bd168: 0x0000000000000000 0x00007f38cd2b69fb
0x7f38cd4bd178: 0x00000000006007c8 0x00007f38cd4bd700
0x7f38cd4bd188: 0x0000000000000000 0x00007f38cd4bd168
gdb-peda$ x/s 0x00007f38cd2b69fb
0x7f38cd2b69fb: ""而这里第二个元素l_name是空，不是我们要找的库。于是通过第四个元素l_next，即0x00007f38cd4bd700，来看下一个
gdb-peda$ x/6x 0x00007f38cd4bd700
0x7f38cd4bd700: 0x00007fff09fe9000 0x00007f38cd2b69fb
0x7f38cd4bd710: 0x00007fff09fe9318 0x00007f38cd4ba658
0x7f38cd4bd720: 0x00007f38cd4bd168 0x00007f38cd4bd700
gdb-peda$ x/s 0x00007f38cd2b69fb
0x7f38cd2b69fb: ""同理，这里也不是。继续看下一个0x00007f38cd4ba658
gdb-peda$ x/6x 0x00007f38cd4ba658
0x7f38cd4ba658: 0x00007f38ccede000 0x00007f38cd4ba640
0x7f38cd4ba668: 0x00007f38cd294b40 0x00007f38cd4bc998
0x7f38cd4ba678: 0x00007f38cd4bd700 0x00007f38cd4ba658
gdb-peda$ x/s 0x00007f38cd4ba640
0x7f38cd4ba640: "/lib64/libc.so.6"这里是libc.so，那么我们继续看下一个0x00007f38cd4bc998
gdb-peda$ x/6x 0x00007f38cd4bc998
0x7f38cd4bc998 <_rtld_local+2456>: 0x00007f38cd29c000 0x0000000000400200
0x7f38cd4bc9a8 <_rtld_local+2472>: 0x00007f38cd4bbe10 0x0000000000000000
0x7f38cd4bc9b8 <_rtld_local+2488>: 0x00007f38cd4ba658 0x00007f38cd4bc998
gdb-peda$ x/s 0x0000000000400200
0x400200: "/lib64/ld-linux-x86-64.so.2"OK，这里就是ld-linux.so了。l_addr的值是0x00007f38cd29c000，我这里开了ASLR，而ld-linux.so的基址就正好是0x00007f38cd29c000
gdb-peda$ vmmap
Start End Perm Name
0x00400000 0x00401000 r-xp /root/heap.out
0x00600000 0x00601000 rw-p /root/heap.out
0x00007f38ccede000 0x00007f38cd092000 r-xp /usr/lib64/libc-2.18.so
0x00007f38cd092000 0x00007f38cd291000 ---p /usr/lib64/libc-2.18.so
0x00007f38cd291000 0x00007f38cd295000 r--p /usr/lib64/libc-2.18.so
0x00007f38cd295000 0x00007f38cd297000 rw-p /usr/lib64/libc-2.18.so
0x00007f38cd297000 0x00007f38cd29c000 rw-p mapped
0x00007f38cd29c000 0x00007f38cd2bc000 r-xp /usr/lib64/ld-2.18.so
0x00007f38cd4ae000 0x00007f38cd4b1000 rw-p mapped
0x00007f38cd4ba000 0x00007f38cd4bb000 rw-p mapped
0x00007f38cd4bb000 0x00007f38cd4bc000 r--p /usr/lib64/ld-2.18.so
0x00007f38cd4bc000 0x00007f38cd4bd000 rw-p /usr/lib64/ld-2.18.so
0x00007f38cd4bd000 0x00007f38cd4be000 rw-p mapped
0x00007fff09f9f000 0x00007fff09fc0000 rw-p [stack]
0x00007fff09fe7000 0x00007fff09fe9000 r--p [vvar]
0x00007fff09fe9000 0x00007fff09feb000 r-xp [vdso]
0xffffffffff600000 0xffffffffff601000 r-xp [vsyscall]由此，我们得到了ld-linux.so的地址。同样的道理，我们通过遍历link_map，就可以得到所有库的地址。当然，前提是二进制文件需要有DT_DEBUG。
Edit:最近研究才意识到，不一定需要从DT_DEBUG去获得link_map的地址。事实上，.got.plt的前3项，分别是.dynamic的地址，link_map的地址和_dl_runtime_resolve的地址。在解析函数地址时，link_map会被作为参数推到栈上传递给_dl_runtime_resolve。关于这一过程及利用方式，详情可见http://rk700.github.io/article/2015/08/09/return-to-dl-resolve