On Fri, Jul 30, 2021 at 05:01:33PM +0200, Max Reitz wrote:
When the inode_file_handles option is set, try to generate a file handle
for new inodes instead of opening an O_PATH FD.
Being able to open these again will require CAP_DAC_READ_SEARCH, so the
description text tells the user they will also need to specify
-o modcaps=+dac_read_search.
Generating a file handle returns the mount ID it is valid for. Opening
it will require an FD instead. We have mount_fds to map an ID to an FD.
get_file_handle() fills the hash map by opening the file we have
generated a handle for. To verify that the resulting FD indeed
represents the handle's mount ID, we use statx(). Therefore, using file
handles requires statx() support.
So opening the file and storing that fd in mount_fds table might be
a potential problem with inotify work Ioannis is doing.
So say a file foo.txt was opened O_RDONLY and fd stored in mount_fs. Now
say user unlinks foo.txt. If notifications are enabled, final notification
will not be generated till this mount_fds fd is closed.
Now question is when will this fd be closed? If it closed at some
later point and then notification is generated, that will break
notificaitons.
In fact even O_PATH fd is delaying notifications due to same reason.
But its not too bad as we close O_PATH fd pretty quickly after
unlinking. And we were hoping that file handle support will get rid
of this problem because we will not keep O_PATH fd open.
But, IIUC, mount_fds stuff will make it even worse. I did not see
the code which removes this fd from mount_fds. So I am not sure what's
the life time of this fd.