linux-kernel - [PATCH 03/11] VFS: add dentry_lookup

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Message-ID: <20250812235228.3072318-4-neil@brown.name>
Date: Tue, 12 Aug 2025 12:25:06 +1000
From: NeilBrown <neil@...wn.name>
To: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>
Cc: David Howells <dhowells@...hat.com>,
	Marc Dionne <marc.dionne@...istor.com>,
	Xiubo Li <xiubli@...hat.com>,
	Ilya Dryomov <idryomov@...il.com>,
	Tyler Hicks <code@...icks.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	Richard Weinberger <richard@....at>,
	Anton Ivanov <anton.ivanov@...bridgegreys.com>,
	Johannes Berg <johannes@...solutions.net>,
	Trond Myklebust <trondmy@...nel.org>,
	Anna Schumaker <anna@...nel.org>,
	Chuck Lever <chuck.lever@...cle.com>,
	Jeff Layton <jlayton@...nel.org>,
	Amir Goldstein <amir73il@...il.com>,
	Steve French <sfrench@...ba.org>,
	Namjae Jeon <linkinjeon@...nel.org>,
	Carlos Maiolino <cem@...nel.org>,
	linux-fsdevel@...r.kernel.org,
	linux-afs@...ts.infradead.org,
	netfs@...ts.linux.dev,
	ceph-devel@...r.kernel.org,
	ecryptfs@...r.kernel.org,
	linux-um@...ts.infradead.org,
	linux-nfs@...r.kernel.org,
	linux-unionfs@...r.kernel.org,
	linux-cifs@...r.kernel.org,
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	linux-kernel@...r.kernel.org
Subject: [PATCH 03/11] VFS: add dentry_lookup_killable()

btrfs/ioctl.c uses a "killable" lock on the directory when creating an
destroying subvols.  overlayfs also does this.

This patch adds dentry_lookup_killable() for these users.

Possibly all dentry_lookup should be killable as there is no down-side,
but that can come in a later patch.

Signed-off-by: NeilBrown <neil@...wn.name>
---
 fs/namei.c            | 37 +++++++++++++++++++++++++++++++++++++
 include/linux/namei.h |  3 +++
 2 files changed, 40 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index 85b981248a90..7af9b464886a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1837,6 +1837,43 @@ struct dentry *dentry_lookup(struct mnt_idmap *idmap,
 }
 EXPORT_SYMBOL(dentry_lookup);
 
+/**
+ * dentry_lookup_killable - lookup and lock a name prior to dir ops
+ * @last: the name in the given directory
+ * @base: the directory in which the name is to be found
+ * @lookup_flags: %LOOKUP_xxx flags
+ *
+ * The name is looked up and necessary locks are taken so that
+ * the name can be created or removed.
+ * The "necessary locks" are currently the inode lock on @base.
+ * If a fatal signal arrives, or is already pending, the operation is aborted.
+ * The name @last is NOT expected to already have the hash calculated.
+ * Permission checks are performed to ensure %MAY_EXEC access to @base.
+ * Returns: the dentry, suitably locked, or an ERR_PTR().
+ */
+struct dentry *dentry_lookup_killable(struct mnt_idmap *idmap,
+				      struct qstr *last,
+				      struct dentry *base,
+				      unsigned int lookup_flags)
+{
+	struct dentry *dentry;
+	int err;
+
+	err = lookup_one_common(idmap, last, base);
+	if (err < 0)
+		return ERR_PTR(err);
+
+	err = down_write_killable_nested(&base->d_inode->i_rwsem, I_MUTEX_PARENT);
+	if (err)
+		return ERR_PTR(err);
+
+	dentry = lookup_one_qstr_excl(last, base, lookup_flags);
+	if (IS_ERR(dentry))
+		inode_unlock(base->d_inode);
+	return dentry;
+}
+EXPORT_SYMBOL(dentry_lookup_killable);
+
 /**
  * done_dentry_lookup - finish a lookup used for create/delete
  * @dentry:  the target dentry
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 932cb94c3538..facb5852afa9 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -83,6 +83,9 @@ struct dentry *lookup_one_positive_unlocked(struct mnt_idmap *idmap,
 struct dentry *dentry_lookup(struct mnt_idmap *idmap,
 			       struct qstr *last, struct dentry *base,
 			       unsigned int lookup_flags);
+struct dentry *dentry_lookup_killable(struct mnt_idmap *idmap,
+				      struct qstr *last, struct dentry *base,
+				      unsigned int lookup_flags);
 struct dentry *dentry_lookup_noperm(struct qstr *name, struct dentry *base,
 				      unsigned int lookup_flags);
 void done_dentry_lookup(struct dentry *dentry);
-- 
2.50.0.107.gf914562f5916.dirty