vfs: export check_sticky()

It's already duplicated in btrfs and about to be used in overlayfs too.

Move the sticky bit check to an inline helper and call the out-of-line
helper only in the unlikly case of the sticky bit being set.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/btrfs/ioctl.c

@@ -765,23 +765,6 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 	return ret;
 }
 
-/*  copy of check_sticky in fs/namei.c()
-* It's inline, so penalty for filesystems that don't use sticky bit is
-* minimal.
-*/
-static inline int btrfs_check_sticky(struct inode *dir, struct inode *inode)
-{
-	kuid_t fsuid = current_fsuid();
-
-	if (!(dir->i_mode & S_ISVTX))
-		return 0;
-	if (uid_eq(inode->i_uid, fsuid))
-		return 0;
-	if (uid_eq(dir->i_uid, fsuid))
-		return 0;
-	return !capable(CAP_FOWNER);
-}
-
 /*  copy of may_delete in fs/namei.c()
  *	Check whether we can remove a link victim from directory dir, check
  *  whether the type of victim is right.
@@ -817,8 +800,7 @@ static int btrfs_may_delete(struct inode *dir, struct dentry *victim, int isdir)
 		return error;
 	if (IS_APPEND(dir))
 		return -EPERM;
-	if (btrfs_check_sticky(dir, victim->d_inode)||
-		IS_APPEND(victim->d_inode)||
+	if (check_sticky(dir, victim->d_inode) || IS_APPEND(victim->d_inode) ||
 	    IS_IMMUTABLE(victim->d_inode) || IS_SWAPFILE(victim->d_inode))
 		return -EPERM;
 	if (isdir) {

fs/namei.c

@@ -2384,22 +2384,17 @@ kern_path_mountpoint(int dfd, const char *name, struct path *path,
 }
 EXPORT_SYMBOL(kern_path_mountpoint);
 
-/*
- * It's inline, so penalty for filesystems that don't use sticky bit is
- * minimal.
- */
-static inline int check_sticky(struct inode *dir, struct inode *inode)
+int __check_sticky(struct inode *dir, struct inode *inode)
 {
 	kuid_t fsuid = current_fsuid();
 
-	if (!(dir->i_mode & S_ISVTX))
-		return 0;
 	if (uid_eq(inode->i_uid, fsuid))
 		return 0;
 	if (uid_eq(dir->i_uid, fsuid))
 		return 0;
 	return !capable_wrt_inode_uidgid(inode, CAP_FOWNER);
 }
+EXPORT_SYMBOL(__check_sticky);
 
 /*
  *	Check whether we can remove a link victim from directory dir, check

include/linux/fs.h

@@ -2259,6 +2259,7 @@ extern int notify_change(struct dentry *, struct iattr *, struct inode **);
 extern int inode_permission(struct inode *, int);
 extern int __inode_permission(struct inode *, int);
 extern int generic_permission(struct inode *, int);
+extern int __check_sticky(struct inode *dir, struct inode *inode);
 
 static inline bool execute_ok(struct inode *inode)
 {
@@ -2745,6 +2746,14 @@ static inline int is_sxid(umode_t mode)
 	return (mode & S_ISUID) || ((mode & S_ISGID) && (mode & S_IXGRP));
 }
 
+static inline int check_sticky(struct inode *dir, struct inode *inode)
+{
+	if (!(dir->i_mode & S_ISVTX))
+		return 0;
+
+	return __check_sticky(dir, inode);
+}
+
 static inline void inode_has_no_xattr(struct inode *inode)
 {
 	if (!is_sxid(inode->i_mode) && (inode->i_sb->s_flags & MS_NOSEC))