Yeah, the appended testcase shows that mount(2) with
MS_REMOUNT|MS_RDONLY and without MS_BIND does remount
the sb readonly, as we expected. So as Jäkel pointed out
privately, mount(1) is sometimes looking at /etc/mtab, seeing
a bind mount, and adding MS_BIND to be nice.
Meaning that we absolutely cannot use a bind mount to
prevent ro-remount at container shutdown.
Running the below, I get:
serge@tp:~/test$ sudo ./testmounts
creat bc - t1 was remounted ro?: Read-only file system
Here's the testcase:
#include <stdio.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/fcntl.h>
#include <stdlib.h>
int main()
{
int ret = 1;
if (mkdir("/tmp/t1", 0755)) {
perror("mkdir t1");
goto err;
}
if (mkdir("/tmp/t2", 0755)) {
perror("mkdir t2");
goto err;
}
if (mount("tmpfs", "/tmp/t1", "tmpfs", 0, NULL)) {
perror("mount t1");
goto err;
}
if (mount("/tmp/t1", "/tmp/t2", "none", MS_BIND, NULL)) {
perror("bind mount t2");
goto err;
}
if (mount("/tmp/t2", "/tmp/t2", "ro", MS_REMOUNT | MS_BIND | MS_RDONLY,
NULL)) {
perror("bind-remount ro");
goto err;
}
int fd = creat("/tmp/t1/ab", 0755);
if (fd < 0) {
perror("creat ab - t1 was remounted ro with bind-remount?");
goto err;
}
close(fd);
if (mount("/tmp/t2", "/tmp/t2", "ro", MS_REMOUNT | MS_RDONLY, NULL)) {
perror("remount ro");
goto err;
}
fd = creat("/tmp/t1/bc", 0755);
if (fd < 0) {
perror("creat bc - t1 was remounted ro?");
goto err;
}
close(fd);
ret = 0;
err:
umount2("/tmp/t2", MNT_DETACH);
umount2("/tmp/t1", MNT_DETACH);
rmdir("/tmp/t2");
rmdir("/tmp/t1");
exit(ret);
}
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