how passwd bypasses permisssion for /etc/shadow in linux

[sknowonweb]

Hello every one,
My instructor throw me a question in linux.
As a normal user one cannot append or edit something in /etc/passwd or /etc/shadow files as they have 400 permision code.But for a normal user the usage of passwd(1) command changes his password which means that this edits the /etc/shadow files password field entry.How and what is passwd(1) doing when it cannot have even read permission for /etc/shadow.

Please bare me im a newbie.and help me solve this.

[__Virus__]

:according to my thought:

When a user changes his password from say think-digit from digit-think, the OS does recognize that the user has permission to edit part of the file and without user intervention the file will be edited. Remember the user must the permission to change his password.

[praka123]

I think it has to do something with PAM(pluggable--) i saw a file /etc/pam.d/passwd..perhaps i need to study about pam .... also should examine SUID and SGID...

[ujjwal]

$ ls -l /usr/bin/passwd
-rws--x--x  1 root bin 37880 2004-06-22 00:50 /usr/bin/passwd*

As you can see, the program passwd runs with suid, and therefore runs with root priviledges. Check this for more info -

http://www.linuxgeek.net/index.pl/suid

[__Virus__]

Originally Posted by ujjwal

Check this for more info -

http://www.linuxgeek.net/index.pl/suid

Thanks ujjwal that was indeed good site which by now cleared it.