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

[H. Peter Anvin]

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.

	-hpa

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