Date: Tue, 4 Dec 2007 11:23:28 +0100 From: Ed Schouten <ed@fxq.nl> To: Robert Watson <rwatson@FreeBSD.org> Cc: stable@FreeBSD.org, current@FreeBSD.org Subject: Re: Attention 7.x and 8.x ptmx/pts users (read if you set kern.pts.enable=1) Message-ID: <20071204102328.GK72574@hoeg.nl> In-Reply-To: <20071203225800.S30376@fledge.watson.org> References: <20071203225800.S30376@fledge.watson.org>
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[-- Attachment #1 --] * Robert Watson <rwatson@FreeBSD.org> wrote: > Unfortunately, the current implementation is subject to a potential > resource leak: the pty is created when the lookup occurs, but if the open > never takes place, then the pty is leaked. In principle, we have > facilities to GC unused device nodes "eventually", although not a race-free > way to determine that no race occurs, assuming that we implemented that. > This leakage turns out to interact particularly poorly with our resource > limits on pty/pts pairs -- both the administrative limit imposed by sysctl > and also the functional limit on the number of entries in /etc/ttys. It's > possible to imagine various sometimes messy techniques of performing this > garbage collection. So this is the same issue I sent a message to arch@ about some time ago, that /dev/ptmx already returns a reference to the new pty, already when you stat(2) it (for example by running `ls -l /dev/ptmx')? > Instead, what I'd like to do is modify the ptmx code to have a race-free > protocol, in which eventual termination of processes referencing the node > results in freeing of the nodes. On some systems, ptmx performs a > "bait-and-switch", in which the file descriptor of the pty node is silently > substituted for the file descriptor of the ptmx code--similar to our model, > only no window between lookup and open, but also not easily supported in > our current VFS. Another possibility is to introduce a new system call and > bypass ptmx entirely -- similar to pipe(), socketpair(), etc. I actually think that this sounds pretty nice. You mean something like an in-kernel implementation for openpty()? Another thing that would make the TTY code a little bit cleaner in my opinion is removing the PRIV_TTY_PRISON check and making something generic inside devfs. If we have proper garbage collecting on TTY's, then we can just change make_dev_cred() to bind the new device node to a certain jail. That way you could even choose to hide nodes in /dev that don't belong to the jail in question. -- Ed Schouten <ed@fxq.nl> WWW: http://g-rave.nl/ [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (FreeBSD) iD8DBQFHVSqf52SDGA2eCwURAlh9AJoD/Iatz0HLAjeZAZPRhWYzvNCx+wCaAkqs KoagPlubGYniKnSg4ap+ZeE= =ZzPP -----END PGP SIGNATURE-----
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