[PATCH 0/1] man2/mount.2: expand and clarify docs for MS_REMOUNT | MS_BIND

My edit is based on experiments and reading Linux code

You will find C code I used for experiments below

Askar Safin (1):
  man2/mount.2: expand and clarify docs for MS_REMOUNT | MS_BIND

 man/man2/mount.2 | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

-- 
2.47.2

// You need to be root in initial user namespace

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sched.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/sysmacros.h>
#include <linux/openat2.h>

#define MY_ASSERT(cond) do { \
    if (!(cond)) { \
        fprintf (stderr, "%d: %s: assertion failed\n", __LINE__, #cond); \
        exit (1); \
    } \
} while (0)

int
main (void)
{
    // Init
    {
        MY_ASSERT (chdir ("/") == 0);
        MY_ASSERT (unshare (CLONE_NEWNS) == 0);
        MY_ASSERT (mount (NULL, "/", NULL, MS_PRIVATE | MS_REC, NULL) == 0);
        MY_ASSERT (mount (NULL, "/tmp", "tmpfs", 0, NULL) == 0);
    }

    MY_ASSERT (mkdir ("/tmp/a", 0777) == 0);
    MY_ASSERT (mkdir ("/tmp/b", 0777) == 0);

    // MS_REMOUNT sets options for superblock
    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, NULL) == 0);
        MY_ASSERT (mount ("/tmp/a", "/tmp/b", NULL, MS_BIND, NULL) == 0);
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT | MS_RDONLY, NULL) == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (mkdir ("/tmp/b/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (umount ("/tmp/a") == 0);
        MY_ASSERT (umount ("/tmp/b") == 0);
    }

    // MS_REMOUNT | MS_BIND sets options for vfsmount
    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, NULL) == 0);
        MY_ASSERT (mount ("/tmp/a", "/tmp/b", NULL, MS_BIND, NULL) == 0);
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT | MS_BIND | MS_RDONLY, NULL) == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (mkdir ("/tmp/b/c", 0777) == 0);
        MY_ASSERT (rmdir ("/tmp/b/c") == 0);
        MY_ASSERT (umount ("/tmp/a") == 0);
        MY_ASSERT (umount ("/tmp/b") == 0);
    }

    // fspick sets options for superblock
    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, NULL) == 0);
        MY_ASSERT (mount ("/tmp/a", "/tmp/b", NULL, MS_BIND, NULL) == 0);
        {
            int fsfd = fspick (AT_FDCWD, "/tmp/a", 0);
            MY_ASSERT (fsfd >= 0);
            MY_ASSERT (fsconfig (fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0) == 0);
            MY_ASSERT (fsconfig (fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0) == 0);
            MY_ASSERT (close (fsfd) == 0);
        }
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (mkdir ("/tmp/b/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (umount ("/tmp/a") == 0);
        MY_ASSERT (umount ("/tmp/b") == 0);
    }

    // mount_setattr sets options for vfsmount
    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, NULL) == 0);
        MY_ASSERT (mount ("/tmp/a", "/tmp/b", NULL, MS_BIND, NULL) == 0);
        {
            struct mount_attr attr;
            memset (&attr, 0, sizeof attr);
            attr.attr_set = MOUNT_ATTR_RDONLY;
            MY_ASSERT (mount_setattr (AT_FDCWD, "/tmp/a", 0, &attr, sizeof attr) == 0);
        }
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (mkdir ("/tmp/b/c", 0777) == 0);
        MY_ASSERT (rmdir ("/tmp/b/c") == 0);
        MY_ASSERT (umount ("/tmp/a") == 0);
        MY_ASSERT (umount ("/tmp/b") == 0);
    }

    // "ro" as a string works for MS_REMOUNT
    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, NULL) == 0);
        MY_ASSERT (mount ("/tmp/a", "/tmp/b", NULL, MS_BIND, NULL) == 0);
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT, "ro") == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (mkdir ("/tmp/b/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (umount ("/tmp/a") == 0);
        MY_ASSERT (umount ("/tmp/b") == 0);
    }

    // "ro" as a string doesn't work for MS_REMOUNT | MS_BIND
    // Option string is ignored
    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, NULL) == 0);
        MY_ASSERT (mount ("/tmp/a", "/tmp/b", NULL, MS_BIND, NULL) == 0);
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT | MS_BIND, "ro") == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == 0);
        MY_ASSERT (rmdir ("/tmp/a/c") == 0);
        MY_ASSERT (mkdir ("/tmp/b/c", 0777) == 0);
        MY_ASSERT (rmdir ("/tmp/b/c") == 0);
        MY_ASSERT (umount ("/tmp/a") == 0);
        MY_ASSERT (umount ("/tmp/b") == 0);
    }

    // Removing MS_RDONLY makes mount writable again (in case of MS_REMOUNT | MS_BIND)
    // Same for other options (not tested, but I did read code)
    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, NULL) == 0);
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT | MS_BIND | MS_RDONLY, NULL) == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT | MS_BIND, NULL) == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == 0);
        MY_ASSERT (umount ("/tmp/a") == 0);
    }

    // Removing "ro" from option string makes mount writable again (in case of MS_REMOUNT)
    // I. e. mount(2) works exactly as documented
    // This works even if option string is NULL, i. e. NULL works as default option string
    {
        typedef const char *c_string;
        c_string opts[3] = {NULL, "", "rw"};
        for (int i = 0; i != 3; ++i)
            {
                for (int j = 0; j != 3; ++j)
                    {
                        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, opts[i]) == 0);
                        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == 0);
                        MY_ASSERT (rmdir ("/tmp/a/c") == 0);
                        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT, "ro") == 0);
                        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == -1);
                        MY_ASSERT (errno == EROFS);
                        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT, opts[j]) == 0);
                        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == 0);
                        MY_ASSERT (umount ("/tmp/a") == 0);
                    }
            }
    }

    // Removing MS_RDONLY makes mount writable again (in case of MS_REMOUNT)
    // I. e. mount(2) works exactly as documented
    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, NULL) == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == 0);
        MY_ASSERT (rmdir ("/tmp/a/c") == 0);
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT | MS_RDONLY, NULL) == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == -1);
        MY_ASSERT (errno == EROFS);
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT, NULL) == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == 0);
        MY_ASSERT (rmdir ("/tmp/a/c") == 0);
        MY_ASSERT (umount ("/tmp/a") == 0);
    }

    // Setting MS_RDONLY (without other flags) removes all other flags, such as MS_NODEV (in case of MS_REMOUNT | MS_BIND)
    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", 0, NULL) == 0);
        MY_ASSERT (mknod ("/tmp/a/mynull", S_IFCHR | 0666, makedev (1, 3)) == 0);

        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == 0);
        MY_ASSERT (rmdir ("/tmp/a/c") == 0);
        {
            int fd = open ("/tmp/a/mynull", O_WRONLY);
            MY_ASSERT (fd >= 0);
            MY_ASSERT (write (fd, "a", 1) == 1);
            MY_ASSERT (close (fd) == 0);
        }
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT | MS_BIND | MS_NODEV, NULL) == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == 0);
        MY_ASSERT (rmdir ("/tmp/a/c") == 0);
        MY_ASSERT (open ("/tmp/a/mynull", O_WRONLY) == -1);
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT | MS_BIND | MS_RDONLY, NULL) == 0);
        MY_ASSERT (mkdir ("/tmp/a/c", 0777) == -1);
        {
            int fd = open ("/tmp/a/mynull", O_WRONLY);
            MY_ASSERT (fd >= 0);
            MY_ASSERT (write (fd, "a", 1) == 1);
            MY_ASSERT (close (fd) == 0);
        }
        MY_ASSERT (umount ("/tmp/a") == 0);
    }
    printf ("All tests passed\n");
    exit (0);
}