使用patchelf修改rpath

自动解决共享库链接问题

当从软件包获取一个程序时，经常需要手动设置 LD_LIBRARY_PATH 环境变量，以确保程序能够正确加载共享库并正常运行。为了避免每次都进行繁琐的配置，可以通过为程序设置 RPATH 来实现自动查找共享库，从而简化操作。

问题背景

例如，当你从 SourceForge 下载并解压 srecord RPM 包后，你会发现其目录结构如下：

[wanlinwang@computing-server-01 usr]$ ls *
bin:
srec_cat  srec_cmp  srec_info

include:
srecord

lib:
ld-linux-x86-64.so.2  libgcc_s.so.1    libgcrypt.so.20.3.4  libgpg-error.so.0.32.1  libm.so.6       libstdc++.so.6.0.30
libc.so.6             libgcrypt.so.20  libgpg-error.so.0    liblib_srecord.a        libstdc++.so.6

share:
doc  man

直接运行 srec_cat 等命令时，可能会提示共享库未找到。

解决方案

为了让程序能够自动找到其依赖的共享库，而无需用户每次手动设置环境变量，可以通过设置 ELF 文件的 RPATH，使得动态链接器在指定目录（如 lib 目录）中查找共享库。

1. 设置 RPATH

使用 patchelf 工具可以轻松修改 ELF 文件的 RPATH。RPATH 告诉动态链接器去哪里查找共享库，$ORIGIN 代表程序所在目录，因此设置 RPATH 为 $ORIGIN/../lib，表示程序将从其上级目录的 lib 文件夹中查找共享库。

[wanlinwang@computing-server-01 bin]$ patchelf --set-rpath '$ORIGIN/../lib' --force-rpath *

2. 验证 RPATH 设置

修改 RPATH 后，可以通过 readelf 查看每个二进制文件的 RPATH 设置，确保其正确：

[wanlinwang@computing-server-01 bin]$ readelf -d * | grep -i rpath
 0x000000000000000f (RPATH)              Library rpath: [$ORIGIN/../lib]
 0x000000000000000f (RPATH)              Library rpath: [$ORIGIN/../lib]
 0x000000000000000f (RPATH)              Library rpath: [$ORIGIN/../lib]

3. 验证共享库加载情况

通过 ldd 命令检查共享库是否正确加载，确保程序能够从预期目录加载共享库：

[wanlinwang@computing-server-01 bin]$ ldd *
srec_cat:
	linux-vdso.so.1 =>  (0x00007fff02d8d000)
	libgcrypt.so.20 => /nfs/home/wanlinwang/usr/bin/./../lib/libgcrypt.so.20 (0x00007f9b740a0000)
	libstdc++.so.6 => /nfs/home/wanlinwang/usr/bin/./../lib/libstdc++.so.6 (0x00007f9b73e76000)
	libgcc_s.so.1 => /nfs/home/wanlinwang/usr/bin/./../lib/libgcc_s.so.1 (0x00007f9b742d3000)
	libc.so.6 => /nfs/home/wanlinwang/usr/bin/./../lib/libc.so.6 (0x00007f9b73c4e000)
	libgpg-error.so.0 => /nfs/home/wanlinwang/usr/bin/./../lib/libgpg-error.so.0 (0x00007f9b742ac000)
	libm.so.6 => /nfs/home/wanlinwang/usr/bin/./../lib/libm.so.6 (0x00007f9b73b67000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f9b741de000)
srec_cmp:
	linux-vdso.so.1 =>  (0x00007fffa639e000)
	libgcrypt.so.20 => /nfs/home/wanlinwang/usr/bin/./../lib/libgcrypt.so.20 (0x00007f5fd42ca000)
	libstdc++.so.6 => /nfs/home/wanlinwang/usr/bin/./../lib/libstdc++.so.6 (0x00007f5fd4067000)
	libgcc_s.so.1 => /nfs/home/wanlinwang/usr/bin/./../lib/libgcc_s.so.1 (0x00007f5fd4047000)
	libc.so.6 => /nfs/home/wanlinwang/usr/bin/./../lib/libc.so.6 (0x00007f5fd3e1f000)
	libgpg-error.so.0 => /nfs/home/wanlinwang/usr/bin/./../lib/libgpg-error.so.0 (0x00007f5fd3df9000)
	libm.so.6 => /nfs/home/wanlinwang/usr/bin/./../lib/libm.so.6 (0x00007f5fd3d12000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f5fd4291000)
srec_info:
	linux-vdso.so.1 =>  (0x00007ffe78da2000)
	libgcrypt.so.20 => /nfs/home/wanlinwang/usr/bin/./../lib/libgcrypt.so.20 (0x00007f97be3fc000)
	libstdc++.so.6 => /nfs/home/wanlinwang/usr/bin/./../lib/libstdc++.so.6 (0x00007f97be19c000)
	libgcc_s.so.1 => /nfs/home/wanlinwang/usr/bin/./../lib/libgcc_s.so.1 (0x00007f97be17c000)
	libc.so.6 => /nfs/home/wanlinwang/usr/bin/./../lib/libc.so.6 (0x00007f97bdf54000)
	libgpg-error.so.0 => /nfs/home/wanlinwang/usr/bin/./../lib/libgpg-error.so.0 (0x00007f97bdf2e000)
	libm.so.6 => /nfs/home/wanlinwang/usr/bin/./../lib/libm.so.6 (0x00007f97bde47000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f97be3c6000)

此时，所有的共享库都已成功加载，且程序会自动从解压目录下的 lib 文件夹中查找所需的共享库。

共享库查找顺序

当动态链接器查找共享库时，会按照以下顺序进行：

1. RPATH：如果可执行文件的 RPATH 存在且不为空，链接器将首先检查这些路径。
2. LD_LIBRARY_PATH：如果设置了环境变量 LD_LIBRARY_PATH，链接器将依次检查其中列出的目录。
3. RUNPATH：如果可执行文件包含 RUNPATH，链接器将使用这些路径。
4. 缓存：检查 /etc/ld.so.cache 是否包含所需的库。
5. 系统默认路径：如果以上都没有找到，链接器将检查 /lib 和 /usr/lib 目录。

总结

通过为程序设置 RPATH，可以避免手动设置 LD_LIBRARY_PATH，并确保共享库能够自动找到。这样做的好处是用户无需额外配置，软件运行时自动加载必要的库，提高了用户体验和软件的可移植性。

参考资料

共享库查找顺序，来自The Linux Programming

When resolving library dependencies, the dynamic linker first inspects each dependency string to see if it contains a slash (/), which can occur if we specified an explicit library pathname when linking the executable. If a slash is found, then the dependency string is interpreted as a pathname (either absolute or relative), and the library is loaded using that pathname. Otherwise, the dynamic linker searches for the shared library using the following rules:

1. If the executable has any directories listed in its DT_RPATH run-time library path list (rpath) and the executable does not contain a DT_RUNPATH list, then these directories are searched (in the order that they were supplied when linking the program).

2. If the LD_LIBRARY_PATH environment variable is defined, then each of the colon-separated directories listed in its value is searched in turn. If the executable is a set-user-ID or set-group-ID program, then LD_LIBRARY_PATH is ignored. This is a security measure to prevent users from tricking the dynamic linker into loading a private version of a library with the same name as a library required by the executable.

3. If the executable has any directories listed in its DT_RUNPATH run-time library path list, then these directories are searched (in the order that they were supplied when linking the program).

4. The file /etc/ld.so.cache is checked to see if it contains an entry for the library.

5. The directories /lib and /usr/lib are searched (in that order).