备用glibc动态链接器的库路径顺序(ld.so)

我需要使用备用glibc版本,比我系统上安装的版本更新(2.18vs 2.15).这里和这里都涉及几个相关的问题 .我在这里问的具体问题如下:

我成立了新的动态连接的库路径(ld-2.18.so),以使新的libc(libc-2.18.so)被发现提前老libc(libc-2.15.so).但是,当我尝试使用新程序运行程序时ld,旧版本libc被选中,生成一个SEGV.为什么会这样？

注意:我知道这可以通过--rpath在编译时或LD_LIBRARY_PATH运行时使用来修复.但是,我仍然想了解为什么还需要其中一个.

详情如下:

我下载glibc-2.18并构建了它/opt/glibc-2.18.默认情况下,文件/opt/glibc-2.18/etc/ld.so.conf丢失.我创建了它,并更新了新的库缓存,glibc如下所示.我强调:新的libc是在旧的之前找到的libc:

$ cat /opt/glibc-2.18/etc/ld.so.conf
/opt/glibc-2.18/lib
/usr/local/lib
/lib/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu/mesa
/lib
/usr/lib
$ /opt/glibc-2.18/sbin/ldconfig -v |& grep -E '^[^'$'\t'']|libc\.'
/opt/glibc-2.18/sbin/ldconfig: Path `/opt/glibc-2.18/lib' given more than once
/opt/glibc-2.18/sbin/ldconfig: Can't stat /opt/glibc-2.18/lib64: No such file or directory
/opt/glibc-2.18/sbin/ldconfig: Can't stat /opt/glibc-2.18/libx32: No such file or directory
/opt/glibc-2.18/lib:
        libc.so.6 -> libc-2.18.so
/usr/local/lib:
/lib/x86_64-linux-gnu:
        libc.so.6 -> libc-2.15.so
/usr/lib/x86_64-linux-gnu:
/usr/lib/x86_64-linux-gnu/mesa:
/lib:
/usr/lib:

然后,我创建了一个简单的C程序:

$ cat <a.c
> #include 
> int main()
> {
>     fprintf(stdout, "ok\n");
>     return 0;
> }
> EOF
$ g++ a.c
$ file a.out
a.out: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=0x43b8484e3910072375d68418cb6327478266c0e9, not stripped
$ ldd a.out
    linux-vdso.so.1 =>  (0x00007fffd7ffe000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa7c47bd000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fa7c4b9b000)
$ readelf -a a.out | grep lib
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
000000601000  000100000007 R_X86_64_JUMP_SLO 0000000000000000 __libc_start_main + 0
     1: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.2.5 (2)
    46: 00000000004005f0     2 FUNC    GLOBAL DEFAULT   13 __libc_csu_fini
    52: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@@GLIBC_
    57: 0000000000400560   137 FUNC    GLOBAL DEFAULT   13 __libc_csu_init
  000000: Version: 1  File: libc.so.6  Cnt: 1
$ objdump -x a.out | grep -A3 Version
Version References:
  required from libc.so.6:
    0x09691a75 0x00 02 GLIBC_2.2.5

如上所示,该程序具有旧的ld硬编码内部.我可以用new强制运行它ld,我希望ld使用new的路径(你可以看到新的ld.so.cache被打开).但是,由于某些原因,我试图理解,旧的libc是在新的之前找到的libc,产生了一个SEGV:

$ /opt/glibc-2.18/lib/ld-2.18.so ./a.out
Segmentation fault (core dumped)
$ strace /opt/glibc-2.18/lib/ld-2.18.so ./a.out |& grep open
open("./a.out", O_RDONLY|O_CLOEXEC)     = 3
open("/opt/glibc-2.18/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3

我也可以使用新库编译ld并按如下方式烘焙新内容:

$ g++ -L/opt/glibc-2.18/lib -Wl,--dynamic-linker=/opt/glibc-2.18/lib/ld-2.18.so a.c -o a.2.18.out
$ file a.2.18.out
a.2.18.out: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=0x25ab43f3d29b49fa21385a15e43325e9fb904e81, not stripped
$ ldd a.2.18.out
    linux-vdso.so.1 =>  (0x00007fffa68da000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f9df5cbe000)
    /opt/glibc-2.18/lib/ld-2.18.so => /lib64/ld-linux-x86-64.so.2 (0x00007f9df609c000)
$ readelf -a a.2.18.out | grep lib
      [Requesting program interpreter: /opt/glibc-2.18/lib/ld-2.18.so]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
000000601000  000100000007 R_X86_64_JUMP_SLO 0000000000000000 __libc_start_main + 0
     1: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.2.5 (2)
    54: 0000000000400600     2 FUNC    GLOBAL DEFAULT   13 __libc_csu_fini
    60: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@@GLIBC_
    65: 0000000000400570   137 FUNC    GLOBAL DEFAULT   13 __libc_csu_init
  000000: Version: 1  File: libc.so.6  Cnt: 1
$ objdump -x a.2.18.out | grep -A3 Version
Version References:
  required from libc.so.6:
    0x09691a75 0x00 02 GLIBC_2.2.5

尽管如此,如果我尝试运行新程序,同样的事情发生了,旧的libc是用来代替新的libc:

$ ./a.2.18.out
Segmentation fault (core dumped)
$ strace ./a.2.18.out |& grep open
open("/opt/glibc-2.18/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3

使用任一可执行文件,指定LD_LIBRARY_PATH=/opt/glibc-2.18/lib使其工作.但是,我的问题是为什么仍然需要这个,因为新的路径ld在开始时被配置libc为在旧的之前拾取新的libc.