Re: Does anyone care about a race free ptsname?

[H. Peter Anvin]

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
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