Re: Does anyone care about a race free ptsname?

[Serge E. Hallyn]

On Tue, Apr 19, 2016 at 04:39:07PM -0700, H. Peter Anvin wrote:
> On 04/19/2016 04:23 PM, Linus Torvalds wrote:
> > On Tue, Apr 19, 2016 at 11:44 AM, Eric W. Biederman
> > <ebiederm@xxxxxxxxxxxx> wrote:
> >>
> >> I will take a look in a minute.   Before I do that I want to mention
> >> why I care about /dev/pts/ptmx.
> >>
> >> There is a posix function that is widely used called ptsname.  It's
> >> function is to take a master file descriptor and returns the path to the
> >> slave.
> >>
> >> All we have in the kernel to support ptsname is an ioctl TIOCGPTN that
> >> returns the pty number in the appropriate instance of devpts.
> > 
> > Don't bother with that completely mis-designed interface.It's crap.
> > 
> > So we'll keep it working for legacy models, but the whole "return an
> > integer index" is just pure shit. It's not worth worrying about.
> > 
> > We can (and probably should) just introduce a new ioctl or even a
> > system call that just does the sane thing and returns the pathname
> > from the kernel.
> > 
> 
> What do you think of the idea of TIOCPTSOPEN (or whatever) to get a file
> descriptor for the slave device given the master device?  Then we can

That's what i was thinking, since as I said many users just want

        blah = ptsname(fd);
        slavefd = open(blah, ...);

so they really just want an open fd.

> use realpath() or a readlink on /proc/self/fd/# to get the pathname if
> needed.
> 
> (Incidentally, we added getcwd() as a system call.  Should we add
> [f]realpath() as a system call too?)
> 
> 	-hpa