Fix failure with rw bind mount of a ro fuse

As reported in [1], in a case where read-only fuse (sshfs) mount
is used as a volume without specifying ro flag, the kernel fails
to remount it (when adding various flags such as nosuid and nodev),
returning EPERM.

Here's the relevant strace line:

> [pid 333966] mount("/tmp/bats-run-PRVfWc/runc.RbNv8g/bundle/mnt", "/proc/self/fd/7", 0xc0001e9164, MS_NOSUID|MS_NODEV|MS_REMOUNT|MS_BIND|MS_REC, NULL) = -1 EPERM (Operation not permitted)

I was not able to reproduce it with other read-only mounts as the source
(tried tmpfs, read-only bind mount, and an ext2 mount), so somehow this
might be specific to fuse.

The fix is to check whether the source has RDONLY flag, and retry the
remount with this flag added.

A test case (which was kind of hard to write) is added, and it fails
without the fix. Note that rootless user need to be able to ssh to
rootless@localhost in order to sshfs to work -- amend setup scripts
to make it work, and skip the test if the setup is not working.

[1] https://github.com/containers/podman/issues/12205

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/test.yml

@@ -46,13 +46,13 @@ jobs:
         curl -fSsl $REPO/Release.key | sudo apt-key add -
         echo "deb $REPO/ /" | sudo tee /etc/apt/sources.list.d/criu.list
         sudo apt update
-        sudo apt install libseccomp-dev criu
+        sudo apt install libseccomp-dev criu sshfs
 
     - name: install deps (criu ${{ matrix.criu }})
       if: matrix.criu != ''
       run: |
         sudo apt -q update
-        sudo apt -q install libseccomp-dev \
+        sudo apt -q install libseccomp-dev sshfs \
           libcap-dev libnet1-dev libnl-3-dev \
           libprotobuf-c-dev libprotobuf-dev protobuf-c-compiler protobuf-compiler
         git clone https://github.com/checkpoint-restore/criu.git ~/criu
@@ -81,9 +81,10 @@ jobs:
       if: matrix.rootless == 'rootless'
       run: |
         sudo useradd -u2000 -m -d/home/rootless -s/bin/bash rootless
-        # Allow root to execute `ssh rootless@localhost` in tests/rootless.sh
+        # Allow root and rootless itself to execute `ssh rootless@localhost` in tests/rootless.sh
         ssh-keygen -t ecdsa -N "" -f $HOME/rootless.key
         sudo mkdir -m 0700 -p /home/rootless/.ssh
+        sudo cp $HOME/rootless.key /home/rootless/.ssh/id_ecdsa
         sudo cp $HOME/rootless.key.pub /home/rootless/.ssh/authorized_keys
         sudo chown -R rootless.rootless /home/rootless