Sensible umask for users

Question

Why is a umask of 022 used for non-root users? Why not use 007 for non-root users, since: 1. the user-private group (UPG) scheme is used (thus there would be no security issue); 2. it would allow other users to write to files created in a "shared" directory (i.e., dir with setgid set and belonging to a group other than a UPG; e.g., project1 containing several users) which otherwise would not be possible with 022; 3. it would prevent others (human users in the group, human users not in the group, system users used by daemons) from reading/writing files not created in a "shared" directory, and for "shared" directories, it would prevent others (human users not in the group, system users used by daemons) from reading/writing files.
Am I missing something? I do not see a purpose of changing the root's umask, but I do see a purpose of changing the umask of non-root users.

Answer 1

Leonid.I wrote:Why not 027, for a group-based solution? 007 is dangerous since /tmp might contain user-specific data which you'll make rw for others in the same group (e.g. users).
But with the UPG scheme, the primary group (obviously the same group that will be the owner of user-specific data in /tmp) will be the user's private group. I do not see how this is a security concern.