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i cant login as super user using su command ,previously it was successful and i am sure that password is correct,previosly there was an issue by changing file permission of www folder and i follow this link to correct it after that i cant login as su

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If I am right, you can not login directly as root with su in Ubuntu. You need to run the command with the prefix sudo, or in your case obtain a shell with sudo su.

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no,it is not the case,it was previously able to use su, the out put of sudo su is "sudo: must be setuid root" – sajith May 8 '13 at 6:45
The /usr/bin/sudo must be setuid normally, and to do that you can run chmod 4755 /usr/bin/sudo. But since you need root access to execute this command, you are trapped. One way I see to solve this is to boot from a live cd, mount the root partition, execute this command to setuid sudo and reboot the system. – StreakyCobra May 8 '13 at 7:07
i have one terminal with root access opened before this operation,so i think i can do that – sajith May 8 '13 at 7:09

You should boot into live mode, and then mount the drives and then change the permission for the file /usr/bin/sudo

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