switch non-create side of open() to use of do_last()

Instead of path_lookupat() doing trailing symlink resolution,
use the same scheme as on the O_CREAT side.  Walk with
LOOKUP_PARENT, then (in do_last()) look the final component
up, then either open it or return error or, if it's a symlink,
give the symlink back to path_openat() to be resolved there.

The really messy complication here is RCU.  We don't want to drop
out of RCU mode before the final lookup, since we don't want to
bounce parent directory ->d_count without a good reason.

Result is _not_ pretty; later in the series we'll clean it up.
For now we are roughly back where we'd been before the revert
done by Nick's series - top-level logics of path_openat() is
cleaned up, do_last() does actual opening, symlink resolution is
done uniformly.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 67 insertions, 33 deletions

diff --git a/fs/namei.c b/fs/namei.c
index a260a306daf5..9595b4a55c39 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2178,13 +2178,14 @@ exit:
 }
 
 /*
- * Handle O_CREAT case for do_filp_open
+ * Handle the last step of open()
  */
 static struct file *do_last(struct nameidata *nd, struct path *path,
                             const struct open_flags *op, const char *pathname)
 {
         struct dentry *dir = nd->path.dentry;
         struct file *filp;
+        struct inode *inode;
         int error;
 
         nd->flags &= ~LOOKUP_PARENT;
@@ -2192,17 +2193,27 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
 
         switch (nd->last_type) {
         case LAST_DOTDOT:
-                follow_dotdot(nd);
-                dir = nd->path.dentry;
         case LAST_DOT:
+                error = handle_dots(nd, nd->last_type);
+                if (error)
+                        return ERR_PTR(error);
                 /* fallthrough */
         case LAST_ROOT:
+                if (nd->flags & LOOKUP_RCU) {
+                        if (nameidata_drop_rcu_last(nd))
+                                return ERR_PTR(-ECHILD);
+                }
                 error = handle_reval_path(nd);
                 if (error)
                         goto exit;
-                error = -EISDIR;
-                goto exit;
+                audit_inode(pathname, nd->path.dentry);
+                if (op->open_flag & O_CREAT) {
+                        error = -EISDIR;
+                        goto exit;
+                }
+                goto ok;
         case LAST_BIND:
+                /* can't be RCU mode here */
                 error = handle_reval_path(nd);
                 if (error)
                         goto exit;
@@ -2210,6 +2221,51 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
                 goto ok;
         }
 
+        if (!(op->open_flag & O_CREAT)) {
+                if (nd->last.name[nd->last.len])
+                        nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
+                /* we _can_ be in RCU mode here */
+                error = do_lookup(nd, &nd->last, path, &inode);
+                if (error) {
+                        terminate_walk(nd);
+                        return ERR_PTR(error);
+                }
+                if (!inode) {
+                        path_to_nameidata(path, nd);
+                        terminate_walk(nd);
+                        return ERR_PTR(-ENOENT);
+                }
+                if (unlikely(inode->i_op->follow_link)) {
+                        /* We drop rcu-walk here */
+                        if (nameidata_dentry_drop_rcu_maybe(nd, path->dentry))
+                                return ERR_PTR(-ECHILD);
+                        return NULL;
+                }
+                path_to_nameidata(path, nd);
+                nd->inode = inode;
+                /* sayonara */
+                if (nd->flags & LOOKUP_RCU) {
+                        if (nameidata_drop_rcu_last(nd))
+                                return ERR_PTR(-ECHILD);
+                }
+
+                error = -ENOTDIR;
+                if (nd->flags & LOOKUP_DIRECTORY) {
+                        if (!inode->i_op->lookup)
+                                goto exit;
+                }
+                audit_inode(pathname, nd->path.dentry);
+                goto ok;
+        }
+
+        /* create side of things */
+
+        if (nd->flags & LOOKUP_RCU) {
+                if (nameidata_drop_rcu_last(nd))
+                        return ERR_PTR(-ECHILD);
+        }
+
+        audit_inode(pathname, dir);
         error = -EISDIR;
         /* trailing slashes? */
         if (nd->last.name[nd->last.len])
@@ -2303,6 +2359,7 @@ exit:
 static struct file *path_openat(int dfd, const char *pathname,
                 const struct open_flags *op, int flags)
 {
+        struct file *base = NULL;
         struct file *filp;
         struct nameidata nd;
         struct path path;
@@ -2318,39 +2375,15 @@ static struct file *path_openat(int dfd, const char *pathname,
         nd.intent.open.flags = open_to_namei_flags(op->open_flag);
         nd.intent.open.create_mode = op->mode;
 
-        if (op->open_flag & O_CREAT)
-                goto creat;
-
-        /* !O_CREAT, simple open */
-        error = path_lookupat(dfd, pathname, flags | op->intent, &nd);
+        error = path_init(dfd, pathname, flags | LOOKUP_PARENT, &nd, &base);
         if (unlikely(error))
                 goto out_filp;
-        error = -ELOOP;
-        if (!(nd.flags & LOOKUP_FOLLOW)) {
-                if (nd.inode->i_op->follow_link)
-                        goto out_path;
-        }
-        error = -ENOTDIR;
-        if (nd.flags & LOOKUP_DIRECTORY) {
-                if (!nd.inode->i_op->lookup)
-                        goto out_path;
-        }
-        audit_inode(pathname, nd.path.dentry);
-        filp = finish_open(&nd, op->open_flag, op->acc_mode);
-        release_open_intent(&nd);
-        return filp;
 
-creat:
-        /* OK, have to create the file. Find the parent. */
-        error = path_lookupat(dfd, pathname, LOOKUP_PARENT | flags, &nd);
+        current->total_link_count = 0;
+        error = link_path_walk(pathname, &nd);
         if (unlikely(error))
                 goto out_filp;
-        if (unlikely(!audit_dummy_context()))
-                audit_inode(pathname, nd.path.dentry);
 
-        /*
-         * We have the parent and last component.
-         */
         filp = do_last(&nd, &path, op, pathname);
         while (unlikely(!filp)) { /* trailing symlink */
                 struct path link = path;
@@ -2386,12 +2419,13 @@ creat:
 out:
         if (nd.root.mnt)
                 path_put(&nd.root);
+        if (base)
+                fput(base);
         release_open_intent(&nd);
         return filp;
 
 exit_dput:
         path_put_conditional(&path, &nd);
-out_path:
         path_put(&nd.path);
 out_filp:
         filp = ERR_PTR(error);