new helper: lookup_positive_unlocked()

fix #27211210

commit 6c2d4798a8d16cf4f3a28c3cd4af4f1dcbbb4d04 upstream.

Most of the callers of lookup_one_len_unlocked() treat negatives are
ERR_PTR(-ENOENT).  Provide a helper that would do just that.  Note
that a pinned positive dentry remains positive - it's ->d_inode is
stable, etc.; a pinned _negative_ dentry can become positive at any
point as long as you are not holding its parent at least shared.
So using lookup_one_len_unlocked() needs to be careful;
lookup_positive_unlocked() is safer and that's what the callers
end up open-coding anyway.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NJeffle Xu <jefflexu@linux.alibaba.com>
Acked-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

fs/cifs/cifsfs.c

@@ -663,11 +663,6 @@ cifs_get_root(struct smb_vol *vol, struct super_block *sb)
 		struct inode *dir = d_inode(dentry);
 		struct dentry *child;
 
-		if (!dir) {
-			dput(dentry);
-			dentry = ERR_PTR(-ENOENT);
-			break;
-		}
 		if (!S_ISDIR(dir->i_mode)) {
 			dput(dentry);
 			dentry = ERR_PTR(-ENOTDIR);
@@ -684,7 +679,7 @@ cifs_get_root(struct smb_vol *vol, struct super_block *sb)
 		while (*s && *s != sep)
 			s++;
 
-		child = lookup_one_len_unlocked(p, dentry, s - p);
+		child = lookup_positive_unlocked(p, dentry, s - p);
 		dput(dentry);
 		dentry = child;
 	} while (!IS_ERR(dentry));

fs/debugfs/inode.c

@@ -275,13 +275,9 @@ struct dentry *debugfs_lookup(const char *name, struct dentry *parent)
 	if (!parent)
 		parent = debugfs_mount->mnt_root;
 
-	dentry = lookup_one_len_unlocked(name, parent, strlen(name));
+	dentry = lookup_positive_unlocked(name, parent, strlen(name));
 	if (IS_ERR(dentry))
 		return NULL;
-	if (!d_really_is_positive(dentry)) {
-		dput(dentry);
-		return NULL;
-	}
 	return dentry;
 }
 EXPORT_SYMBOL_GPL(debugfs_lookup);

fs/kernfs/mount.c

@@ -212,7 +212,7 @@ struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
 			dput(dentry);
 			return ERR_PTR(-EINVAL);
 		}
-		dtmp = lookup_one_len_unlocked(kntmp->name, dentry,
+		dtmp = lookup_positive_unlocked(kntmp->name, dentry,
 					       strlen(kntmp->name));
 		dput(dentry);
 		if (IS_ERR(dtmp))

fs/namei.c

@@ -2568,6 +2568,26 @@ struct dentry *lookup_one_len_unlocked(const char *name,
 }
 EXPORT_SYMBOL(lookup_one_len_unlocked);
 
+/*
+ * Like lookup_one_len_unlocked(), except that it yields ERR_PTR(-ENOENT)
+ * on negatives.  Returns known positive or ERR_PTR(); that's what
+ * most of the users want.  Note that pinned negative with unlocked parent
+ * _can_ become positive at any time, so callers of lookup_one_len_unlocked()
+ * need to be very careful; pinned positives have ->d_inode stable, so
+ * this one avoids such problems.
+ */
+struct dentry *lookup_positive_unlocked(const char *name,
+				       struct dentry *base, int len)
+{
+	struct dentry *ret = lookup_one_len_unlocked(name, base, len);
+	if (!IS_ERR(ret) && d_is_negative(ret)) {
+		dput(ret);
+		ret = ERR_PTR(-ENOENT);
+	}
+	return ret;
+}
+EXPORT_SYMBOL(lookup_positive_unlocked);
+
 #ifdef CONFIG_UNIX98_PTYS
 int path_pts(struct path *path)
 {

fs/nfsd/nfs3xdr.c

@@ -850,13 +850,11 @@ compose_entry_fh(struct nfsd3_readdirres *cd, struct svc_fh *fhp,
 		} else
 			dchild = dget(dparent);
 	} else
-		dchild = lookup_one_len_unlocked(name, dparent, namlen);
+		dchild = lookup_positive_unlocked(name, dparent, namlen);
 	if (IS_ERR(dchild))
 		return rv;
 	if (d_mountpoint(dchild))
 		goto out;
-	if (d_really_is_negative(dchild))
-		goto out;
 	if (dchild->d_inode->i_ino != ino)
 		goto out;
 	rv = fh_compose(fhp, exp, dchild, &cd->fh);

fs/nfsd/nfs4xdr.c

@@ -2984,18 +2984,9 @@ nfsd4_encode_dirent_fattr(struct xdr_stream *xdr, struct nfsd4_readdir *cd,
 	__be32 nfserr;
 	int ignore_crossmnt = 0;
 
-	dentry = lookup_one_len_unlocked(name, cd->rd_fhp->fh_dentry, namlen);
+	dentry = lookup_positive_unlocked(name, cd->rd_fhp->fh_dentry, namlen);
 	if (IS_ERR(dentry))
 		return nfserrno(PTR_ERR(dentry));
-	if (d_really_is_negative(dentry)) {
-		/*
-		 * we're not holding the i_mutex here, so there's
-		 * a window where this directory entry could have gone
-		 * away.
-		 */
-		dput(dentry);
-		return nfserr_noent;
-	}
 
 	exp_get(exp);
 	/*

fs/overlayfs/namei.c

@@ -203,7 +203,7 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 	int err;
 	bool last_element = !post[0];
 
-	this = lookup_one_len_unlocked(name, base, namelen);
+	this = lookup_positive_unlocked(name, base, namelen);
 	if (IS_ERR(this)) {
 		err = PTR_ERR(this);
 		this = NULL;
@@ -211,8 +211,6 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 			goto out;
 		goto out_err;
 	}
-	if (!this->d_inode)
-		goto put_and_out;
 
 	if (ovl_dentry_weird(this)) {
 		/* Don't support traversing automounts and other weirdness */
@@ -654,7 +652,7 @@ struct dentry *ovl_get_index_fh(struct ovl_fs *ofs, struct ovl_fh *fh)
 	if (err)
 		return ERR_PTR(err);
 
-	index = lookup_one_len_unlocked(name.name, ofs->indexdir, name.len);
+	index = lookup_positive_unlocked(name.name, ofs->indexdir, name.len);
 	kfree(name.name);
 	if (IS_ERR(index)) {
 		if (PTR_ERR(index) == -ENOENT)
@@ -662,9 +660,7 @@ struct dentry *ovl_get_index_fh(struct ovl_fs *ofs, struct ovl_fh *fh)
 		return index;
 	}
 
-	if (d_is_negative(index))
-		err = 0;
-	else if (ovl_is_whiteout(index))
+	if (ovl_is_whiteout(index))
 		err = -ESTALE;
 	else if (ovl_dentry_weird(index))
 		err = -EIO;
@@ -688,7 +684,7 @@ struct dentry *ovl_lookup_index(struct ovl_fs *ofs, struct dentry *upper,
 	if (err)
 		return ERR_PTR(err);
 
-	index = lookup_one_len_unlocked(name.name, ofs->indexdir, name.len);
+	index = lookup_positive_unlocked(name.name, ofs->indexdir, name.len);
 	if (IS_ERR(index)) {
 		err = PTR_ERR(index);
 		if (err == -ENOENT) {
@@ -703,9 +699,7 @@ struct dentry *ovl_lookup_index(struct ovl_fs *ofs, struct dentry *upper,
 	}
 
 	inode = d_inode(index);
-	if (d_is_negative(index)) {
-		goto out_dput;
-	} else if (ovl_is_whiteout(index) && !verify) {
+	if (ovl_is_whiteout(index) && !verify) {
 		/*
 		 * When index lookup is called with !verify for decoding an
 		 * overlay file handle, a whiteout index implies that decode
@@ -1134,7 +1128,7 @@ bool ovl_lower_positive(struct dentry *dentry)
 		struct dentry *this;
 		struct dentry *lowerdir = poe->lowerstack[i].dentry;
 
-		this = lookup_one_len_unlocked(name->name, lowerdir,
+		this = lookup_positive_unlocked(name->name, lowerdir,
 					       name->len);
 		if (IS_ERR(this)) {
 			switch (PTR_ERR(this)) {
@@ -1151,10 +1145,8 @@ bool ovl_lower_positive(struct dentry *dentry)
 				break;
 			}
 		} else {
-			if (this->d_inode) {
-				positive = !ovl_is_whiteout(this);
-				done = true;
-			}
+			positive = !ovl_is_whiteout(this);
+			done = true;
 			dput(this);
 		}
 	}

fs/quota/dquot.c

@@ -2500,15 +2500,10 @@ int dquot_quota_on_mount(struct super_block *sb, char *qf_name,
 	struct dentry *dentry;
 	int error;
 
-	dentry = lookup_one_len_unlocked(qf_name, sb->s_root, strlen(qf_name));
+	dentry = lookup_positive_unlocked(qf_name, sb->s_root, strlen(qf_name));
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);
 
-	if (d_really_is_negative(dentry)) {
-		error = -ENOENT;
-		goto out;
-	}
-
 	error = security_quota_on(dentry);
 	if (!error)
 		error = vfs_load_quota_inode(d_inode(dentry), type, format_id,

include/linux/namei.h

@@ -84,6 +84,7 @@ extern int kern_path_mountpoint(int, const char *, struct path *, unsigned int);
 extern struct dentry *try_lookup_one_len(const char *, struct dentry *, int);
 extern struct dentry *lookup_one_len(const char *, struct dentry *, int);
 extern struct dentry *lookup_one_len_unlocked(const char *, struct dentry *, int);
+extern struct dentry *lookup_positive_unlocked(const char *, struct dentry *, int);
 
 extern int follow_down_one(struct path *);
 extern int follow_down(struct path *);