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From: Dmitrii Tcvetkov <demfloro@demfloro.ru>
To: Ulli Horlacher <framstag@rus.uni-stuttgart.de>,
        Linux fs Btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: user snapshots
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> >Also in https://btrfs.wiki.kernel.org/index.php/Mount_options
> >"user_subvol_rm_allowed (...) Use with caution."
> >
> >Why? What is the problem?  
> 
> Because with the mount option any user can delete any subvolume,
> including root one (subvol_id=5)

Apologies, it works somewhat different:
filesystem doesn't allow to delete subvolume with id 5 and POSIX access
is checked before deleting subvolume with user_subvol_rm_allowed mount
option.

>>From btrfs-progs cmds-subvolume.c:

res = ioctl(fd, BTRFS_IOC_SNAP_DESTROY, &args);
if(res < 0 ){
         error("cannot delete '%s/%s': %s", dname, vname,
                 strerror(errno));