linux-kernel - Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

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Date:	Mon, 21 Dec 2015 16:03:42 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Peter Hurley <peter@...leysoftware.com>, Greg KH <greg@...ah.com>,
	Jiri Slaby <jslaby@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Aurelien Jarno <aurelien@...el32.net>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security\@kernel.org" <security@...nel.org>,
	"security\@ubuntu.com \>\> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

ebiederm@...ssion.com (Eric W. Biederman) writes:

> "H. Peter Anvin" <hpa@...or.com> writes:
>
>> Does it matter if it mounts devpts twice?  It seems like a waste of a
>> minuscule amount of memory, and nothing else.

> It breaks system("mknod /tmp/ptmx c 5 2"); open("/tmp/ptmx");

Correction.

It does break the above but that isn't the real reason we need to
support that.  We only have evidence of pople doing:
"mkdir -p dev/pts; mknod c dev/ptmx 5 2; mount -t devpts dev/pts/"
Where the relatives paths would work.

What actually breaks is "echo NNN > /proc/sys/kernel/pty/reserve"
Which allows the primary instance of devpts to have access to more
ptys than any other instance.

Ultimately if we are going to be backwards compatible we need to
preserve as much of the current behavior as possible so we don't forget
something in the analysis and break something we don't intend to break
by accident.

Eric
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