how is shadow file modified
 

how is the shadow file modified when its permissions are -r--------

i know that when passwd is executed the user is set to root
priveleges by setuid(), but how does passwd modify the shadow file

thank you

root doesn't have to obey posix filesystem rights, it's happy to override them if that's what the code says. What it can't / won't, to my knowledge, get around is if the file is set to be immutable (chattr +i /etc/shadow). I guess it's set as not writable by default as it's never meant to be manually edited, but can be if you really need to.

how is shadow file written to
 

so if i login as root can i change the shadow file using vi or you meant kernel instead of rootwhich can modify the file

thank you

yeah, you'll need a :w! to force a write, but it's easy enough in vi.