Re: [PATCH v3 05/12] man/man2/fspick.2: document "new" mount API

[Askar Safin <safinaskar@xxxxxxxxxxxx>]

 ---- On Sat, 09 Aug 2025 00:39:49 +0400  Aleksa Sarai <cyphar@xxxxxxxxxx> wrote --- 
 > +The above procedure is functionally equivalent to
 > +the following mount operation using
 > +.BR mount (2):

This is not true.

fspick adds options to superblock. It doesn't remove existing ones.

mount(MS_REMOUNT) replaces options. I. e. mount(2) call provided in
example will unset all other options.

In the end of this message you will find C code, which proves this.

Also, recently I sent patch to mount(2) manpage,
which clarifies MS_REMOUNT | MS_BIND behavior.
This is somewhat related.
The patch comes with another reproducer.
Here is a link: https://lore.kernel.org/linux-man/20250822114315.1571537-1-safinaskar@xxxxxxxxxxxx/

--
Askar Safin
https://types.pl/@safinaskar

// You need to be root (non-initial user namespace will go)

#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/vfs.h>
#include <sys/sysmacros.h>
#include <sys/statvfs.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);

    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", MS_SYNCHRONOUS, NULL) == 0);
        {
            struct statfs buf;
            memset (&buf, 0, sizeof buf);
            MY_ASSERT (statfs ("/tmp/a", &buf) == 0);
            MY_ASSERT (buf.f_flags & ST_SYNCHRONOUS);
            MY_ASSERT (!(buf.f_flags & ST_RDONLY));
        }
        {
            int fsfd = fspick (AT_FDCWD, "/tmp/a", FSPICK_CLOEXEC);
            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);
        }
        {
            struct statfs buf;
            memset (&buf, 0, sizeof buf);
            MY_ASSERT (statfs ("/tmp/a", &buf) == 0);
            MY_ASSERT (buf.f_flags & ST_SYNCHRONOUS);
            MY_ASSERT (buf.f_flags & ST_RDONLY);
        }
        MY_ASSERT (umount ("/tmp/a") == 0);
    }

    {
        MY_ASSERT (mount (NULL, "/tmp/a", "tmpfs", MS_SYNCHRONOUS, NULL) == 0);
        {
            struct statfs buf;
            memset (&buf, 0, sizeof buf);
            MY_ASSERT (statfs ("/tmp/a", &buf) == 0);
            MY_ASSERT (buf.f_flags & ST_SYNCHRONOUS);
            MY_ASSERT (!(buf.f_flags & ST_RDONLY));
        }
        MY_ASSERT (mount (NULL, "/tmp/a", NULL, MS_REMOUNT | MS_RDONLY, NULL) == 0);
        {
            struct statfs buf;
            memset (&buf, 0, sizeof buf);
            MY_ASSERT (statfs ("/tmp/a", &buf) == 0);
            MY_ASSERT (!(buf.f_flags & ST_SYNCHRONOUS));
            MY_ASSERT (buf.f_flags & ST_RDONLY);
        }
        MY_ASSERT (umount ("/tmp/a") == 0);
    }

    printf ("All tests passed\n");
    exit (0);
}