LD_PRELOAD 导致动态库加载器中的分段错误

Question

我写了一个库，打算通过LD_PRELOAD 加载。在某些 Linux 系统上，这会导致动态库加载器在初始化期间出现段错误。

我有一个显示这种行为的简单测试用例，但前提是我与-lm 链接。例如：

# Works fine
gcc -o vecadd.normal -std=c99 vecadd.c -lOpenCL
LD_PRELOAD=/path/to/my/library.so ./vecadd.normal

# Causes segmentation fault
gcc -o vecadd.broken -std=c99 vecadd.c -lOpenCL -lm
LD_PRELOAD=/path/to/my/library.so ./vecadd.broken

奇怪的是libm.so 似乎包含在两个版本中：ldd 显示完全相同的库集，只是顺序不同：

vecadd.normal:
  linux-vdso.so.1 =>  (0x00007fffed9ff000)
  libOpenCL.so => /usr/lib64/libOpenCL.so (0x00007f135c9b1000)
  libc.so.6 => /lib64/libc.so.6 (0x00007f135c61c000)
  libdl.so.2 => /lib64/libdl.so.2 (0x00007f135c418000)
  libnuma.so.1 => /usr/lib64/libnuma.so.1 (0x00007f135c20f000)
  libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007f135bf08000)
  libm.so.6 => /lib64/libm.so.6 (0x00007f135bc84000)
  libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f135ba6e000)
  /lib64/ld-linux-x86-64.so.2 (0x00007f135cbd4000)
vecadd.broken:
  linux-vdso.so.1 =>  (0x00007fff25c74000)
  libOpenCL.so => /usr/lib64/libOpenCL.so (0x00007fb8c071e000)
  libm.so.6 => /lib64/libm.so.6 (0x00007fb8c0499000)
  libc.so.6 => /lib64/libc.so.6 (0x00007fb8c0105000)
  libdl.so.2 => /lib64/libdl.so.2 (0x00007fb8bff01000)
  libnuma.so.1 => /usr/lib64/libnuma.so.1 (0x00007fb8bfcf7000)
  libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007fb8bf9f1000)
  libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fb8bf7db000)
  /lib64/ld-linux-x86-64.so.2 (0x00007fb8c0941000)

Google 将我定向到 LD_DEBUG，这也指向 libm.so 作为问题的一部分：

 14143: symbol=fma;  lookup in file=./vecadd.broken [0]
 14143: symbol=fma;  lookup in file=/path/to/my/library.so [0]
 14143: symbol=fma;  lookup in file=/usr/lib64/libOpenCL.so [0]
 14143: symbol=fma;  lookup in file=/lib64/libm.so.6 [0]
 14143: binding file /path/to/my/library.so [0] to /lib64/libm.so.6 [0]: normal symbol `fma' [GLIBC_2.2.5]
Segmentation fault (core dumped)

不幸的是，我可以重现此问题的唯一机器似乎没有可用的动态库加载器的调试符号（而且我没有任何管理权限），因此 GDB 不会产生任何有用的东西：

gdb ./vecadd.broken
(gdb) set environment LD_PRELOAD /path/to/my/library.so
(gdb) run
Starting program: vecadd.broken 

Program received signal SIGSEGV, Segmentation fault.
0x0000000000003dce in ?? ()
Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.132.el6_5.1.x86_64
(gdb) bt
#0  0x0000000000003dce in ?? ()
#1  0x00007fff7f755369 in ?? ()
#2  0x00007fffffffd840 in ?? ()
#3  0x00007fff7fde9e91 in _dl_relocate_object ()
   from /lib64/ld-linux-x86-64.so.2
#4  0x00007fff7fde18a3 in dl_main () from /lib64/ld-linux-x86-64.so.2
#5  0x00007fff7fdf3a0e in _dl_sysdep_start () from /lib64/ld-linux-x86-64.so.2
#6  0x00007fff7fddf4a4 in _dl_start () from /lib64/ld-linux-x86-64.so.2
#7  0x00007fff7fddeb08 in _start () from /lib64/ld-linux-x86-64.so.2
#8  0x0000000000000001 in ?? ()
#9  0x00007fffffffde01 in ?? ()
#10 0x0000000000000000 in ?? ()

找出问题根源的最佳方法是什么？

Answer 1

您可以上传自己的使用符号构建的动态加载器并调用它来链接您的应用，例如：

∞ /lib/ld-linux-x86-64.so.2 /bin/echo linked linked