A possible security bug fix

[bob at ucla-locus]

From:            Bob English <bob at ucla-locus>

My comment was in reply to a criticism of a suggestion that
exec require the presence of an open stdin, stdout, and stderr.
The reason this was suggested was to prevent opens after the
exec from falling accidentally into fd's 0, 1 or 2, resulting
in interference between program output and diagnostic output.

The criticism was indeed that a user might want to close 0, 1, or
2.  I replied that re-directing the output to stdio might be
wiser.  Perhaps exec should open /dev/null itself if 0, 1, or 2
are open (but that sounds like a real bad idea).

I'm more worried about what the shell does, anyway.  If some
hacker wants to close a conventional file descriptor and take
his chances on subsequent exec's, I see no reason to stop him. A
naive user, however, should be protected from such foolishness.

I should point out that I have yet to hear any reasons (good or
bad) for allowing 0, 1, or 2 to be closed.  The only one I can
think of involves the number of keystrokes typed at the command
level in currently existing shells...

--bob--