[PATCH 1/2] new helper: user_path_locked_at()

[Al Viro <viro@zeniv.linux.org.uk>]

(in #work.namei; used for bcachefs locking fix)
From 74d016ecc1a7974664e98d1afbf649cd4e0e0423 Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Wed, 15 Nov 2023 22:41:27 -0500
Subject: [PATCH 1/2] new helper: user_path_locked_at()

Equivalent of kern_path_locked() taking dfd/userland name.
User introduced in the next commit.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/namei.c            | 16 +++++++++++++---
 include/linux/namei.h |  1 +
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 71c13b2990b4..3ffbe268d52c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2573,13 +2573,13 @@ static int filename_parentat(int dfd, struct filename *name,
 }
 
 /* does lookup, returns the object with parent locked */
-static struct dentry *__kern_path_locked(struct filename *name, struct path *path)
+static struct dentry *__kern_path_locked(int dfd, struct filename *name, struct path *path)
 {
 	struct dentry *d;
 	struct qstr last;
 	int type, error;
 
-	error = filename_parentat(AT_FDCWD, name, 0, path, &last, &type);
+	error = filename_parentat(dfd, name, 0, path, &last, &type);
 	if (error)
 		return ERR_PTR(error);
 	if (unlikely(type != LAST_NORM)) {
@@ -2598,12 +2598,22 @@ static struct dentry *__kern_path_locked(struct filename *name, struct path *pat
 struct dentry *kern_path_locked(const char *name, struct path *path)
 {
 	struct filename *filename = getname_kernel(name);
-	struct dentry *res = __kern_path_locked(filename, path);
+	struct dentry *res = __kern_path_locked(AT_FDCWD, filename, path);
 
 	putname(filename);
 	return res;
 }
 
+struct dentry *user_path_locked_at(int dfd, const char __user *name, struct path *path)
+{
+	struct filename *filename = getname(name);
+	struct dentry *res = __kern_path_locked(dfd, filename, path);
+
+	putname(filename);
+	return res;
+}
+EXPORT_SYMBOL(user_path_locked_at);
+
 int kern_path(const char *name, unsigned int flags, struct path *path)
 {
 	struct filename *filename = getname_kernel(name);
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 3100371b5e32..74e0cc14ebf8 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -66,6 +66,7 @@ extern struct dentry *kern_path_create(int, const char *, struct path *, unsigne
 extern struct dentry *user_path_create(int, const char __user *, struct path *, unsigned int);
 extern void done_path_create(struct path *, struct dentry *);
 extern struct dentry *kern_path_locked(const char *, struct path *);
+extern struct dentry *user_path_locked_at(int , const char __user *, struct path *);
 int vfs_path_parent_lookup(struct filename *filename, unsigned int flags,
 			   struct path *parent, struct qstr *last, int *type,
 			   const struct path *root);
-- 
2.39.2