Messages in this thread | | | Subject | Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance | From | "H. Peter Anvin" <> | Date | Fri, 11 Dec 2015 15:07:09 -0800 |
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On December 11, 2015 3:00:49 PM PST, Andy Lutomirski <luto@amacapital.net> wrote: >On Fri, Dec 11, 2015 at 2:58 PM, Jann Horn <jann@thejh.net> wrote: >> On Fri, Dec 11, 2015 at 02:52:01PM -0800, Andy Lutomirski wrote: >>> On Fri, Dec 11, 2015 at 2:35 PM, Eric W. Biederman >>> <ebiederm@xmission.com> wrote: >>> > Andy Lutomirski <luto@amacapital.net> writes: >>> > >>> >> On Fri, Dec 11, 2015 at 2:07 PM, H. Peter Anvin <hpa@zytor.com> >wrote: >>> >>> On 12/11/15 13:48, Andy Lutomirski wrote: >>> >>>> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman >>> >>>> <ebiederm@xmission.com> wrote: >>> >>>>> Al Viro <viro@ZenIV.linux.org.uk> writes: >>> >>>>> >>> >>>>>> On Fri, Dec 11, 2015 at 01:40:40PM -0600, Eric W. Biederman >wrote: >>> >>>>>> >>> >>>>>>> + inode = path.dentry->d_inode; >>> >>>>>>> + filp->f_path = path; >>> >>>>>>> + filp->f_inode = inode; >>> >>>>>>> + filp->f_mapping = inode->i_mapping; >>> >>>>>>> + path_put(&old); >>> >>>>>> >>> >>>>>> Don't. You are creating a fairly subtle constraint on what >the code in >>> >>>>>> fs/open.c and fs/namei.c can do, for no good reason. You can >bloody >>> >>>>>> well maintain the information you need without that. >>> >>>>> >>> >>>>> There is a good reason. We can not write a race free version >of ptsname >>> >>>>> without it. >>> >>>> >>> >>>> As long as this is for new userspace code, would it make sense >to just >>> >>>> add a new ioctl to ask "does this ptmx fd match this /dev/pts >fd?" >>> >>>> >>> >>> >>> >>> For the newinstance case st_dev should match between the master >and the >>> >>> slave. Unfortunately this is not the case for a legacy ptmx, as >a >>> >>> stat() on the master descriptor still returns the st_dev, >st_rdev, and >>> >>> st_ino for the ptmx device node. >>> >> >>> >> Sure, but I'm not talking about stat. I'm saying that we could >add a >>> >> new ioctl that works on any ptmx fd (/dev/ptmx or /dev/pts/ptmx) >that >>> >> answers the question "does this ptmx logically belong to the >given >>> >> devpts filesystem". >>> >> >>> >> Since it's not stat, we can make it do whatever we want, >including >>> >> following a link to the devpts instance that isn't f_path or >f_inode. >>> > >>> > The useful ioctl to add in my opinion would be one that actually >opens >>> > the slave, at which point ptsname could become ttyname, and that >closes >>> > races in grantpt. >>> >>> Unfortunately, ptsname is POSIX, so we can't get rid of it. It's a >>> bad idea, but it's in the standard. >> >> But then ptsname could become "open the slave, call ttyname() on it, >close >> the slave". Unless opening the slave would have side effects? > >Hmm, fair enough. So maybe that does make sense after all. > >Anyway, I still think there are two pieces here: > >1. Fix /dev/ptmx so that we can banish newinstance=0. > >2. Fix libc. If that needs kernel help, then so be it. > >ISTM we could still implement the "open the slave" operation for (2) >as an ioctl that does the appropriate magic the fd is /dev/ptmx as >opposed to /dev/pts/ptmx. > > >--Andy
I want to be clear:
If /dev/ptmx -> pts/ptmx and devpts is mounted with the proper options, I believe ask the remaining parts of userspace should be fine, and pt_chown can be removed even with glibc.
The magic ptmx we are talking about is all about dealing with a mismanaged /dev. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.
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