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From: mouse@lightning.mcrcim.mcgill.EDU (der Mouse)
Newsgroups: comp.windows.x
Subject: Re: Xterm cannot write log files
Message-ID: <9105021525.AA19153@lightning.McRCIM.McGill.EDU>
Date: 2 May 91 15:25:56 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 45

>> So I'd conclude that, unless I've missed something suble,
>> setuid-root isn't necessary for xterm, and setgid-system suffices,
>> if all the appropriate files are in the system group and are group
>> writable.

I think you're mostly right, though tty ownerships still won't get set
properly.

> Doesn't the -C option, which redirects console output to the xterm
> window, use an ioctl that requires root access?

(I am assuming the systems in question are Suns.)

TIOCCONS, it is.  No, anyone can use it.  This is somewhat dangerous,
as it allows anyone to steal the console from whoever (if anyone) is
using it - even when there's no bitmap screen.  I understand there is a
fix that disallows TIOCCONS if the caller can't already read from the
console, which seems reasonable.  I would also like to have a variable
somewhere in the kernel, patchable with adb for example, that would
disallow TIOCCONS entirely.

> Also, does xterm need to chown() the tty device special file to the
> user?

Yes, and for this xterm needs setuid-root, though a proper rework of
the pty stuff would cure it.

> Or does the tty driver do this automatically when the process opens
> it?

It pretty much can't; ownership is an attribute of the special device
file, not of the device itself.  The driver has no access to the
pathname with which the process opened the file.  In the case of
pseudo-ttys, it might be workable to assume

	sprintf(...,"/dev/tty%c%x",'p'+(ptyno/16),ptyno%16)

but that seems like a dreadful kludge to me.  I would much rather build
a proper pty mechanism.  (Just what that would be, I'm not sure.  I
haven't thought about it much.)

					der Mouse

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