linux-kernel - [PATCH 10/27] afs: Distinguish mountpoints from symlinks by file mode alone

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Message-ID: <148908581614.16794.11126261692420529431.stgit@warthog.procyon.org.uk>
Date:   Thu, 09 Mar 2017 18:56:56 +0000
From:   David Howells <dhowells@...hat.com>
To:     viro@...IV.linux.org.uk
Cc:     dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
        Jeffrey Altman <jaltman@...istor.com>,
        linux-kernel@...r.kernel.org, linux-afs@...ts.infradead.org
Subject: [PATCH 10/27] afs: Distinguish mountpoints from symlinks by file
 mode alone

In AFS, mountpoints appear as symlinks with mode 0644 and normal symlinks
have mode 0777, so use this to distinguish them rather than reading the
content and parsing it.  In the case of a mountpoint, the symlink body is a
formatted string indicating the location of the target volume.

Note that with this, kAFS no longer 'pre-fetches' the contents of symlinks,
so afs_readpage() may fail with an access-denial because when the VFS calls
d_automount(), it wraps the call in an credentials override that sets the
initial creds - thereby preventing access to the caller's keyrings and the
authentication keys held therein.

To this end, a patch reverting that change to the VFS is required also.

Reported-by: Jeffrey Altman <jaltman@...istor.com>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/afs/inode.c    |   29 +++++++++++++++--------------
 fs/afs/internal.h |    1 -
 fs/afs/mntpt.c    |   53 -----------------------------------------------------
 3 files changed, 15 insertions(+), 68 deletions(-)

diff --git a/fs/afs/inode.c b/fs/afs/inode.c
index 299dbaeb2e2a..ade6ec3873cf 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -54,8 +54,21 @@ static int afs_inode_map_status(struct afs_vnode *vnode, struct key *key)
 		inode->i_fop	= &afs_dir_file_operations;
 		break;
 	case AFS_FTYPE_SYMLINK:
-		inode->i_mode	= S_IFLNK | vnode->status.mode;
-		inode->i_op	= &page_symlink_inode_operations;
+		/* Symlinks with a mode of 0644 are actually mountpoints. */
+		if ((vnode->status.mode & 0777) == 0644) {
+			inode->i_flags |= S_AUTOMOUNT;
+
+			spin_lock(&vnode->lock);
+			set_bit(AFS_VNODE_MOUNTPOINT, &vnode->flags);
+			spin_unlock(&vnode->lock);
+
+			inode->i_mode	= S_IFDIR | 0555;
+			inode->i_op	= &afs_mntpt_inode_operations;
+			inode->i_fop	= &afs_mntpt_file_operations;
+		} else {
+			inode->i_mode	= S_IFLNK | vnode->status.mode;
+			inode->i_op	= &page_symlink_inode_operations;
+		}
 		inode_nohighmem(inode);
 		break;
 	default:
@@ -79,18 +92,6 @@ static int afs_inode_map_status(struct afs_vnode *vnode, struct key *key)
 	inode->i_generation	= vnode->fid.unique;
 	inode->i_version	= vnode->status.data_version;
 	inode->i_mapping->a_ops	= &afs_fs_aops;
-
-	/* check to see whether a symbolic link is really a mountpoint */
-	if (vnode->status.type == AFS_FTYPE_SYMLINK) {
-		afs_mntpt_check_symlink(vnode, key);
-
-		if (test_bit(AFS_VNODE_MOUNTPOINT, &vnode->flags)) {
-			inode->i_mode	= S_IFDIR | vnode->status.mode;
-			inode->i_op	= &afs_mntpt_inode_operations;
-			inode->i_fop	= &afs_mntpt_file_operations;
-		}
-	}
-
 	return 0;
 }
 
diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index af1d91ec7f2c..39de154fb42e 100644
--- a/fs/afs/internal.h
+++ b/fs/afs/internal.h
@@ -559,7 +559,6 @@ extern const struct inode_operations afs_autocell_inode_operations;
 extern const struct file_operations afs_mntpt_file_operations;
 
 extern struct vfsmount *afs_d_automount(struct path *);
-extern int afs_mntpt_check_symlink(struct afs_vnode *, struct key *);
 extern void afs_mntpt_kill_timer(void);
 
 /*
diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
index d4fb0afc0097..bd3b65cde282 100644
--- a/fs/afs/mntpt.c
+++ b/fs/afs/mntpt.c
@@ -47,59 +47,6 @@ static DECLARE_DELAYED_WORK(afs_mntpt_expiry_timer, afs_mntpt_expiry_timed_out);
 static unsigned long afs_mntpt_expiry_timeout = 10 * 60;
 
 /*
- * check a symbolic link to see whether it actually encodes a mountpoint
- * - sets the AFS_VNODE_MOUNTPOINT flag on the vnode appropriately
- */
-int afs_mntpt_check_symlink(struct afs_vnode *vnode, struct key *key)
-{
-	struct page *page;
-	size_t size;
-	char *buf;
-	int ret;
-
-	_enter("{%x:%u,%u}",
-	       vnode->fid.vid, vnode->fid.vnode, vnode->fid.unique);
-
-	/* read the contents of the symlink into the pagecache */
-	page = read_cache_page(AFS_VNODE_TO_I(vnode)->i_mapping, 0,
-			       afs_page_filler, key);
-	if (IS_ERR(page)) {
-		ret = PTR_ERR(page);
-		goto out;
-	}
-
-	ret = -EIO;
-	if (PageError(page))
-		goto out_free;
-
-	buf = kmap(page);
-
-	/* examine the symlink's contents */
-	size = vnode->status.size;
-	_debug("symlink to %*.*s", (int) size, (int) size, buf);
-
-	if (size > 2 &&
-	    (buf[0] == '%' || buf[0] == '#') &&
-	    buf[size - 1] == '.'
-	    ) {
-		_debug("symlink is a mountpoint");
-		spin_lock(&vnode->lock);
-		set_bit(AFS_VNODE_MOUNTPOINT, &vnode->flags);
-		vnode->vfs_inode.i_flags |= S_AUTOMOUNT;
-		spin_unlock(&vnode->lock);
-	}
-
-	ret = 0;
-
-	kunmap(page);
-out_free:
-	put_page(page);
-out:
-	_leave(" = %d", ret);
-	return ret;
-}
-
-/*
  * no valid lookup procedure on this sort of dir
  */
 static struct dentry *afs_mntpt_lookup(struct inode *dir,