Kernel+Userland: Support remounting filesystems :^)

This makes it possible to change flags of a mount after the fact, with the
caveats outlined in the man page.

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -91,6 +91,20 @@ KResult VFS::bind_mount(Custody& source, Custody& mount_point, int flags)
     return KSuccess;
 }
 
+KResult VFS::remount(Custody& mount_point, int new_flags)
+{
+    LOCKER(m_lock);
+
+    dbg() << "VFS: Remounting " << mount_point.absolute_path();
+
+    Mount* mount = find_mount_for_guest(mount_point.inode().identifier());
+    if (!mount)
+        return KResult(-ENODEV);
+
+    mount->set_flags(new_flags);
+    return KSuccess;
+}
+
 KResult VFS::unmount(InodeIdentifier guest_inode_id)
 {
     LOCKER(m_lock);

Kernel/FileSystem/VirtualFileSystem.h

@@ -66,6 +66,7 @@ public:
         String absolute_path() const;
 
         int flags() const { return m_flags; }
+        void set_flags(int flags) { m_flags = flags; }
 
     private:
         InodeIdentifier m_host;
@@ -83,6 +84,7 @@ public:
     bool mount_root(FS&);
     KResult mount(FS&, Custody& mount_point, int flags);
     KResult bind_mount(Custody& source, Custody& mount_point, int flags);
+    KResult remount(Custody& mount_point, int new_flags);
     KResult unmount(InodeIdentifier guest_inode_id);
 
     KResultOr<NonnullRefPtr<FileDescription>> open(StringView path, int options, mode_t mode, Custody& base, Optional<UidAndGid> = {});

Kernel/Process.cpp

@@ -4103,6 +4103,11 @@ int Process::sys$mount(const Syscall::SC_mount_params* user_params)
 
     auto& target_custody = custody_or_error.value();
 
+    if (params.flags & MS_REMOUNT) {
+        // We're not creating a new mount, we're updating an existing one!
+        return VFS::the().remount(target_custody, params.flags & ~MS_REMOUNT);
+    }
+
     if (params.flags & MS_BIND) {
         // We're doing a bind mount.
         if (description.is_null())

Kernel/UnixTypes.h

@@ -53,6 +53,7 @@
 #define MS_NOSUID (1 << 2)
 #define MS_BIND (1 << 3)
 #define MS_RDONLY (1 << 4)
+#define MS_REMOUNT (1 << 5)
 
 #define PERF_EVENT_MALLOC 1
 #define PERF_EVENT_FREE 2