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

[Andy Lutomirski]

On Fri, Dec 11, 2015 at 2:07 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 12/11/15 13:48, Andy Lutomirski wrote:
>> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman
>> <ebiederm@xxxxxxxxxxxx> wrote:
>>> Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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.

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/