Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 11 Dec 2015 14:52:01 -0800 | Subject | Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance |
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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.
> > I even posted an implementation earlier in the discussion and no one was > interested. > > Honestly the more weird special cases we add to devpts the less likely > userspace will be to get things right. We have been trying since 1998 > and devpts is still a poor enough design we have not been able to get > rid of /usr/lib/pt_chown. Adding another case where we have to sand on > one foot and touch our nose does not seem to likely to achieve > widespread adoption. How many version of libc are there now?
Old libc can stay buggy, I think. Given that this mess is partially a libc mis-design, I don't see why it needs to be fixed entirely in the kernel.
For new systems, it would be really nice if we can make a /dev/ptmx symlink be 100% functional.
In any event, this is semi-moot. If we actually want to retire the newinstance=0 thing from the kernel, we apparently need a magic /dev/ptmx node. That doesn't mean that new userspace needs to *use* that magic node. So why not implement the magic node without fiddling with f_path?
--Andy
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