Limit size of various tmpfs mounts to 10% of RAM, except volatile root and /var
to 25%. Another exception is made for /dev (also /devs for PrivateDevices) and
/sys/fs/cgroup since no (or very few) regular files are expected to be used.
In addition, since directories, symbolic links, device specials and xattrs are
not counted towards the size= limit, number of inodes is also limited
correspondingly: 4MB size translates to 1k of inodes (assuming 4k each), 10% of
RAM (using 16GB of RAM as baseline) translates to 400k and 25% to 1M inodes.
Because nr_inodes option can't use ratios like size option, there's an
unfortunate side effect that with small memory systems the limit may be on the
too large side. Also, on an extremely small device with only 256MB of RAM, 10%
of RAM for /run may not be enough for re-exec of PID1 because 16MB of free
space is required.
When in a userns environment we cannot take away per-mount point flags
set on a mount point that was passed to us. Hence we need to be careful
to always check the actual mount flags in place and manipulate only
those flags of them that we actually want to change and not reset more
as side-effect.
We mostly got this right already in
bind_remount_recursive_with_mountinfo(), but didn't in the simpler
bind_remount_one_with_mountinfo(). Catch up.
(The old code assumed that the MountEntry.flags field contained the
right flag settings, but it actually doesn't for new mounts we just
established as for those mount() establishes the initial flags for us,
and we have to read them back to figure out which ones the kernel
picked.)
Fixes: #13622
To support ProtectHome=y in a user namespace (which mounts the inaccessible
nodes), the nodes need to be accessible by the user. Create these paths and
devices in the user runtime directory so they can be used later if needed.
The function is otherwise generic enough to toggle other bind mount
flags beyond MS_RDONLY (for example: MS_NOSUID or MS_NODEV), hence let's
beef it up slightly to support that too.
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