Re: procfs/d_invalidate break userland

["Mickaël Salaün" <mic@digikod.net>]

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On 04/23/2015 01:51 AM, Al Viro wrote:
> On Thu, Apr 23, 2015 at 01:41:02AM +0200, Mickaël Salaün wrote:
>> Hi,
>>
>> For your information, the commit bbd5192412fdedbae00888316bfe350bf89d0458 (proc: Update proc_flush_task_mnt to use d_invalidate), in the v3.17, break some userland use of procfs.
>>
>> In a user namespace, I'm using /proc/<pid>/task/<tid>/fdinfo to mount a tmpfs, chdir into it, terminate the <tid> thread, and use the tmpfs as a private working directory for PID 1. This trick is useful to disconnect/d_drop a directory from the current file system and make it inaccessible to the other processes (kind of an O_TMPFILE for directory).
>>
>> With the v3.17 this is no more possible because it's no more possible to mount anything in this tmpfs nor use the previously mounted file systems in this tmpfs.
>>
>> The new d_invalidate is really good to lazily invalidate/umount an entire hierarchy but I'm not sure the intention was to remove this useful "feature" ;)
> 
> And what will happen if you mount stuff there and exit?  Dirty filesystem
> after reboot?
> 

I didn't notice any but only do tmpfs or bind mounts because of user namespace anyway.


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