Re: [Qemu-devel] pty/tty functions for BSD too

[Todd T. Fries]

>From the OpenBSD man page...

DESCRIPTION
     The openpty(), login_tty(), and forkpty() functions perform manipulations
     on ttys and pseudo-ttys.

     The openpty() function finds an available pseudo-tty and returns file de-
     scriptors for the master and slave in amaster and aslave.  If name is
     non-null, the filename of the slave is returned in name (a string of at
     least 16 characters).  If termp is non-null, the terminal parameters of
     the slave will be set to the values in termp.  If winp is non-null, the
     window size of the slave will be set to the values in winp.

     The openpty() function works in the following way: first it attempts to
     allocate the pseudo-tty through the /dev/ptm device (see pty(4) for de-
     tails) and if that fails it searches for a free pseudo-tty by iterating
     through all existing pseudo-tty devices in /dev.  When a free pseudo-tty
     is found, its ownership is changed to the UID of the caller, permissions
     are set to correct values, and all earlier uses of that device are re-
     voked (see revoke(2) for details).  The first method can work for any us-
     er, the second method requires super-user privileges in most cases.

In OpenBSD, it's a pretty safe bet that 16chars is overkill.  The tty and/or
pty name will never be longer than /dev/?ty??.

Thanks,
-- 
Todd Fries .. address@hidden

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                        http://todd.fries.net/pgp.txt

Penned by Daniel P. Berrange on 20080818 17:26.18, we have:
| On Mon, Aug 18, 2008 at 07:16:36PM +0300, Blue Swirl wrote:
| > On 8/18/08, Samuel Thibault <address@hidden> wrote:
| > > Anthony Liguori, le Mon 18 Aug 2008 09:06:41 -0500, a ?crit :
| > >
| > > > Samuel Thibault wrote:
| > >  > >In Xen, pty/tty functions are enabled for BSD too, shouldn't we enable
| > >  > >them in upstream qemu too, as patched below?
| > >  > >
| > >  >
| > >  > And you're sure that these functions compile/work on NetBSD/OpenBSD?
| > >
| > >
| > > The defines are explicit in Xen, so I guess somebody tested it.  I
| > >  haven't myself.  I wonder why there is no FreeBSD however.
| > 
| > The patch does not work on OpenBSD, because while openpty() is
| > available, ptsname() isn't.
| > 
| > I tested the attached version on OpenBSD and Linux, pty name is
| > printed correctly.
| 
| Passing a non-NULL value to openpty()'s name parameter is not safe
| 
| [quote openpty(1)]
| BUGS
|        Nobody knows how much space should be reserved for name.  So, call-
|        ing openpty() or forkpty() with non-NULL name may not be secure.
| [/quote]
| 
| If BSD has no other way to determine the PTY name, then at least it
| should be conditionalized so that systems with ptsname() use it, only
| falling back to using the 'name' arg to openpty() for OS lacking ptsname
| 
| Regards,
| Daniel
| -- 
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