linux permission, when things go wrong, mishaps/ gotcha's for newbies

Question

Are there any 'gotcha' permission issues that newbie sys admins might not take into consideration.

Basically what I am looking for, is experienced linux admins to share some permission 'mishaps' that newbie admins might overlook.

(I am a linux newbie, just read over file ownership and permissions, and what to get some real life stores of when things go wrong')

Answer 1

Don't use chown -R unless you're really, really sure you know what you're doing.

I was once at a place where the sysadmin (not me, I was a programmer) decided to do a "chown -R" on our source directory tree (this was back in the days of SCCS, so everybody looked at the same directory) and managed to chown the SCCS directories, making it so nobody could check anything out or in.

Answer 2

It's important to know that if you want people to be able access a directory, the execute bit must be set on all of the parent directories for that particular user. For instance:

user1@host$ mkdir -p one/two
user1@host$ echo "hi" > one/two/readme
user1@host$ chmod 700 one
user1@host$ su user2
user2@host$ cd one/two
bash: cd: one/two: Permission denied
user2@host$ ls -al one/two/readme
ls: cannot access one/two/readme: Permission denied
user2@host$  cat one/two/readme
cat: one/two/readme: Permission denied
user2@host$ exit
user1@host$ chmod +x one
user1@host$ su user2
user2@host$ cat one/two/readme
hi

This mostly comes into play in things like web servers, where a user will want to present files to the populace in general (typically in ~/public_html for instance) but wants to keep the parent directory secure. There is no need or reason to set the read bit, unless you want your user to be able to execute a directory listing.

Answer 3

As a general rule, don't make files or directories world-writable. (temporary directories count as an exception, but be sure to set the "sticky" bit.)

Removing the executable permissions from the dynamic linker (/lib/ld*.so) is a "fun" way to shoot yourself in the foot.