[PATCH 1/3] fs: introduce fsuidgid_has_mapping() helper

[Christian Brauner <christian.brauner@ubuntu.com>]

Don't open-code the checks and instead move them into a clean little
helper we can call. This also reduces the risk that if we ever change
something we forget to change all locations.

Inspired-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 fs/namei.c         | 11 +++--------
 include/linux/fs.h | 11 +++++++++++
 2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 216f16e74351..bc03cbc37ba7 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2823,16 +2823,14 @@ static int may_delete(struct user_namespace *mnt_userns, struct inode *dir,
 static inline int may_create(struct user_namespace *mnt_userns,
 			     struct inode *dir, struct dentry *child)
 {
-	struct user_namespace *s_user_ns;
 	audit_inode_child(dir, child, AUDIT_TYPE_CHILD_CREATE);
 	if (child->d_inode)
 		return -EEXIST;
 	if (IS_DEADDIR(dir))
 		return -ENOENT;
-	s_user_ns = dir->i_sb->s_user_ns;
-	if (!kuid_has_mapping(s_user_ns, fsuid_into_mnt(mnt_userns)) ||
-	    !kgid_has_mapping(s_user_ns, fsgid_into_mnt(mnt_userns)))
+	if (!fsuidgid_has_mapping(dir->i_sb, mnt_userns))
 		return -EOVERFLOW;
+
 	return inode_permission(mnt_userns, dir, MAY_WRITE | MAY_EXEC);
 }
 
@@ -3034,14 +3032,11 @@ static int may_o_create(struct user_namespace *mnt_userns,
 			const struct path *dir, struct dentry *dentry,
 			umode_t mode)
 {
-	struct user_namespace *s_user_ns;
 	int error = security_path_mknod(dir, dentry, mode, 0);
 	if (error)
 		return error;
 
-	s_user_ns = dir->dentry->d_sb->s_user_ns;
-	if (!kuid_has_mapping(s_user_ns, fsuid_into_mnt(mnt_userns)) ||
-	    !kgid_has_mapping(s_user_ns, fsgid_into_mnt(mnt_userns)))
+	if (!fsuidgid_has_mapping(dir->dentry->d_sb, mnt_userns))
 		return -EOVERFLOW;
 
 	error = inode_permission(mnt_userns, dir->dentry->d_inode,
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ec8f3ddf4a6a..edcb1aa99fd6 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1620,6 +1620,17 @@ static inline kgid_t fsgid_into_mnt(struct user_namespace *mnt_userns)
 	return kgid_from_mnt(mnt_userns, current_fsgid());
 }
 
+static inline bool fsuidgid_has_mapping(struct super_block *sb,
+					struct user_namespace *mnt_userns)
+{
+	struct user_namespace *s_user_ns = sb->s_user_ns;
+
+	return kuid_has_mapping(s_user_ns,
+				kuid_from_mnt(mnt_userns, current_fsuid())) &&
+	       kgid_has_mapping(s_user_ns,
+				kgid_from_mnt(mnt_userns, current_fsgid()));
+}
+
 extern struct timespec64 current_time(struct inode *inode);
 
 /*
-- 
2.27.0