procd jail incorrect mount behavior when openwrt is run in a container (namespaces)?

从 strace 中显示的 /sbin/ujail 挂载序列开始，重点关注 /dev/log 的 bind 和只读 remount，并将其与 Linux 挂载命名空间的 flag 锁定行为进行比较。在容器内复现 dnsmasq jail，并跟踪失败的 remount；当 jail 能够处理容器的挂载 flag，不再出现报告的 EPERM 错误，同时保留其预期的限制时，即视为完成。

see the following strace for dnsmasq

strace: Process 74923 attached                                                                                                                
[pid 74923] execve("/sbin/ujail", ["/sbin/ujail", "-t", "5", "-n", "dnsmasq", "-u", "-l", "-r", "/bin/ubus", "-r", "/etc/TZ", "-r", "/etc/dnsmasq.conf", "-r", "/etc/ethers", "-r", "/etc/group", "-r", "/etc/hosts", "-r", "/etc/passwd", "-w", "/tmp/dhcp.leases", "-r", "/tmp/dnsmasq.cfg01411c.d", "-r", "/tmp/hosts", "-r", "/tmp/resolv.conf.d", "-r", "/usr/bin/jshn", "-r", ...], 0x7fff4ddbc2f8 /* 10 vars */) = 0               
[pid 74923] clone(child_stack=0x5636ee84a718, flags=CLONE_NEWNS|CLONE_NEWIPC|CLONE_NEWPID|SIGCHLDstrace: Process 74924 attached) = 4367
[pid 74924] mount("none", "/", 0x5636ee7446dd, MS_REC|MS_PRIVATE, NULL) = 0                                                                   
[pid 74924] mount("tmpfs", "/tmp/ujail-EJimnB", "tmpfs", MS_NOATIME, "mode=0755") = 0                                                         
[pid 74924] mount("/bin/sh", "/tmp/ujail-EJimnB/bin/sh", 0x5636ee745e09, MS_BIND, NULL) = 0                                                   
[pid 74924] mount("/bin/sh", "/tmp/ujail-EJimnB/bin/sh", 0x5636ee7446dd, MS_RDONLY|MS_REMOUNT|MS_BIND, NULL) = 0                              
[pid 74924] mount("/bin/ubus", "/tmp/ujail-EJimnB/bin/ubus", 0x5636ee745e09, MS_BIND, NULL) = 0                                               
[pid 74924] mount("/bin/ubus", "/tmp/ujail-EJimnB/bin/ubus", 0x5636ee7446dd, MS_RDONLY|MS_REMOUNT|MS_BIND, NULL) = 0                          
[pid 74924] mount(NULL, "/tmp/ujail-EJimnB/dev", "tmpfs", MS_NOSUID|MS_NOEXEC|MS_NOATIME, "size=1M") = 0                                      
[pid 74924] mount("/dev/log", "/tmp/ujail-EJimnB/dev/log", 0x5636ee745e09, MS_BIND, NULL) = 0                                                 
[pid 74924] mount("/dev/log", "/tmp/ujail-EJimnB/dev/log", 0x5636ee7446dd, MS_REMOUNT|MS_BIND, NULL) = -1 EPERM (Operation not permitted)     
[pid 74924] +++ exited with 1 +++                                                                                                             
[pid 74923] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=4367, si_uid=0, si_status=1, si_utime=0, si_stime=0} ---                
[pid 74923] +++ exited with 1 +++                                                                                                             

in mount_namespaces - overview of Linux mount namespaces [5]. it is stated that

        The mount flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the
        "atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME)
        settings become locked when propagated from a more privileged
        to a less privileged mount namespace, and may not be changed
        in the less privileged mount namespace.

        This point is illustrated in the following example where, in
        a more privileged mount namespace, we create a bind mount
        that is marked as read-only.  For security reasons, it should
        not be possible to make the mount writable in a less
        privileged mount namespace, and indeed the kernel prevents
        this:

            $ sudo mkdir /mnt/dir
            $ sudo mount --bind -o ro /some/path /mnt/dir
            $ sudo unshare --user --map-root-user --mount \
                           mount -o remount,rw /mnt/dir
            mount: /mnt/dir: permission denied.

There's a workaround in lxc https://github.com/lxc/lxc-ci/issues/586 by unjailing dnsmasq. But I guess the proper way is to OR the original flags which may include nosuid?

bash-5.2# cat /proc/mounts | grep "/dev "                                                                    
tmpfs /dev tmpfs rw,nosuid,size=4096k,nr_inodes=65536,mode=755,uid=921370624,gid=921370624,inode64 0 0       
sysfs /sys/dev sysfs ro,nosuid,nodev,noexec,relatime 0 0                                                     

Note: ftrace seems to suggest EPERM from can_change_locked_flags()