convert do_remount_sb() to fs_context

Replace do_remount_sb() with a function, reconfigure_super(), that's
fs_context aware.  The fs_context is expected to be parameterised already
and have ->root pointing to the superblock to be reconfigured.

A legacy wrapper is provided that is intended to be called from the
fs_context ops when those appear, but for now is called directly from
reconfigure_super().  This wrapper invokes the ->remount_fs() superblock op
for the moment.  It is intended that the remount_fs() op will be phased
out.

The fs_context->purpose is set to FS_CONTEXT_FOR_RECONFIGURE to indicate
that the context is being used for reconfiguration.

do_umount_root() is provided to consolidate remount-to-R/O for umount and
emergency remount by creating a context and invoking reconfiguration.

do_remount(), do_umount() and do_emergency_remount_callback() are switched
to use the new process.

[AV -- fold UMOUNT and EMERGENCY_REMOUNT in; fixes the
umount / bug, gets rid of pointless complexity]
[AV -- set ->net_ns in all cases; nfs remount will need that]
[AV -- shift security_sb_remount() call into reconfigure_super(); the callers
that didn't do security_sb_remount() have NULL fc->security anyway, so it's
a no-op for them]

Signed-off-by: David Howells <dhowells@redhat.com>
Co-developed-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 33 insertions, 2 deletions

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 857cd46a687b..5e2c3aba1dd8 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -69,6 +69,13 @@ static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,
 	case FS_CONTEXT_FOR_MOUNT:
 		fc->user_ns = get_user_ns(fc->cred->user_ns);
 		break;
+	case FS_CONTEXT_FOR_RECONFIGURE:
+		/* We don't pin any namespaces as the superblock's
+		 * subscriptions cannot be changed at this point.
+		 */
+		atomic_inc(&reference->d_sb->s_active);
+		fc->root = dget(reference);
+		break;
 	}
 
 	ret = legacy_init_fs_context(fc);
@@ -90,6 +97,15 @@ struct fs_context *fs_context_for_mount(struct file_system_type *fs_type,
 }
 EXPORT_SYMBOL(fs_context_for_mount);
 
+struct fs_context *fs_context_for_reconfigure(struct dentry *dentry,
+					unsigned int sb_flags,
+					unsigned int sb_flags_mask)
+{
+	return alloc_fs_context(dentry->d_sb->s_type, dentry, sb_flags,
+				sb_flags_mask, FS_CONTEXT_FOR_RECONFIGURE);
+}
+EXPORT_SYMBOL(fs_context_for_reconfigure);
+
 void fc_drop_locked(struct fs_context *fc)
 {
 	struct super_block *sb = fc->root->d_sb;
@@ -99,6 +115,7 @@ void fc_drop_locked(struct fs_context *fc)
 }
 
 static void legacy_fs_context_free(struct fs_context *fc);
+
 /**
  * put_fs_context - Dispose of a superblock configuration context.
  * @fc: The context to dispose of.
@@ -118,8 +135,7 @@ void put_fs_context(struct fs_context *fc)
 		legacy_fs_context_free(fc);
 
 	security_free_mnt_opts(&fc->security);
-	if (fc->net_ns)
-		put_net(fc->net_ns);
+	put_net(fc->net_ns);
 	put_user_ns(fc->user_ns);
 	put_cred(fc->cred);
 	kfree(fc->subtype);
@@ -173,6 +189,21 @@ int legacy_get_tree(struct fs_context *fc)
 }
 
 /*
+ * Handle remount.
+ */
+int legacy_reconfigure(struct fs_context *fc)
+{
+	struct legacy_fs_context *ctx = fc->fs_private;
+	struct super_block *sb = fc->root->d_sb;
+
+	if (!sb->s_op->remount_fs)
+		return 0;
+
+	return sb->s_op->remount_fs(sb, &fc->sb_flags,
+				    ctx ? ctx->legacy_data : NULL);
+}
+
+/*
  * Initialise a legacy context for a filesystem that doesn't support
  * fs_context.
  */