System unable to boot to login screen

[Janya]

I am using centos 7 (3.10.0-514.6.2) and have mistakenly given wrong permission to /usr/bin by typing chmod -R 666 /usr/bin.
After that no command was working and rebooting the system got stuck at started update UTMP about system run level change
Neither rescue or emergency mode is working.

Using installation CD I was able to again set permission 755 to /usr/bin, but still system gets stuck at UTMP run level change every time.

So can someone suggest how to solve this issue.

[TrevorH]

Boot the installation DVD in rescue mode and tell it to mount your system partitions when it asks. Once booted up, chroot into the system using

chroot /mnt/sysimage
then run

for p in $(rpm -qa); do rpm --setugids $p; rpm --setperms $p; done

Since you are so far backlevel, that will hit a bug in rpm that removes the capabilities flags from some executables so you then need to run rpm -Va and look for messages about missing capabilities (a P in the flags at the start of each output line). For those files, you need to run rpm -qf $thatfile and work out what package it belongs to from that and then yum reinstall that package from scratch.

And 7.3 is nearly 3 years out of date. You need to be running yum update much more regularly than every 3 years. High severity patches come out all the time and to keep your system secure you should be updating it. 7.3 has numerous high severity vulnerabilities present that are fixed in later releases. Update.

[Janya]

chroot /mnt/sysimage command gives permission denied..

But every command i fire works as root
I couldn't understand this behaviour also.