Usefulness of Linux dynamic linker/loader

The programs ld.so and ld-linux.so* is Linux dynamic linker/loader.

It can find and load the shared libraries needed by a program, prepare the program to run, and then run it

Let us use the loader for loading a program which does not have executable permission

Default permission of chmod command provided below

[root@localhost ~]# ls -l /bin/chmod
-rwxr-xr-x 1 root root 58656 /bin/chmod

chmod has executable permission by default, hence chmod can be executed to change permission of other files in Linux system

Assume a scenario where chmod command lost its executable permission, like the below

[root@localhost ~]# chmod -x /bin/chmod

[root@localhost ~]# ls -l /bin/chmod
-rw-r--r-- 1 root root 58656 Oct 30  2018 /bin/chmod
[root@localhost ~]#

The command available for changing file permission in linux system is chmod, now it has lost its executable permission

Since chmod command become non executable, then think about the way to provide or restore executable permission for chmod command.

[root@localhost ~]# chmod +x /bin/chmod
-bash: /usr/bin/chmod: Permission denied 

[root@localhost ~]# touch /tmp/testfile
[root@localhost ~]# chmod +x /tmp/testfile
-bash: /usr/bin/chmod: Permission denied

Solution:

Let us use Linux loader for loading and executing chmod command

[root@localhost ~]# /lib64/ld-linux-x86-64.so.2 /bin/chmod 755 /bin/chmod 

[root@localhost ~]# ls -l /bin/chmod

-rwxr-xr-x 1 root root 58656 /bin/chmod

Now the chmod command has got executable permission with the help linux loader for execution.

As chmod command became executable, now it can be used for changing file permissions

[root@localhost ~]# chmod +x /tmp/testfile

[root@localhost ~]# ls -l /tmp/testfile

-rwxr-xr-x 1 root root 0 Jun  3 10:32 /tmp/testfile

Linux loader dynamically finds and loads shared libraries needed by a program and runs the program even when the program do not have executable permission