Make 'check_acl()' a first-class filesystem op

This is stage one in flattening out the callchains for the common
permission testing. Rather than have most filesystem implement their
own inode->i_op->permission function that just calls back down to the
VFS layers 'generic_permission()' with the per-filesystem ACL checking
function, the filesystem can just expose its 'check_acl' function
directly, and let the VFS layer do everything for it.

This is all just preparatory - no filesystem actually enables this yet.

Reviewed-by: James Morris <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/namei.c

···
 EXPORT_SYMBOL(putname);
 #endif
 
+/*
+ * This does basic POSIX ACL permission checking
+ */
+static int acl_permission_check(struct inode *inode, int mask,
+		int (*check_acl)(struct inode *inode, int mask))
+{
+	umode_t			mode = inode->i_mode;
+
+	mask &= MAY_READ | MAY_WRITE | MAY_EXEC;
+
+	if (current_fsuid() == inode->i_uid)
+		mode >>= 6;
+	else {
+		if (IS_POSIXACL(inode) && (mode & S_IRWXG) && check_acl) {
+			int error = check_acl(inode, mask);
+			if (error != -EAGAIN)
+				return error;
+		}
+
+		if (in_group_p(inode->i_gid))
+			mode >>= 3;
+	}
+
+	/*
+	 * If the DACs are ok we don't need any capability check.
+	 */
+	if ((mask & ~mode) == 0)
+		return 0;
+	return -EACCES;
+}
 
 /**
  * generic_permission  -  check for access rights on a Posix-like filesystem
···
 int generic_permission(struct inode *inode, int mask,
 		int (*check_acl)(struct inode *inode, int mask))
 {
-	umode_t			mode = inode->i_mode;
-
-	mask &= MAY_READ | MAY_WRITE | MAY_EXEC;
-
-	if (current_fsuid() == inode->i_uid)
-		mode >>= 6;
-	else {
-		if (IS_POSIXACL(inode) && (mode & S_IRWXG) && check_acl) {
-			int error = check_acl(inode, mask);
-			if (error == -EACCES)
-				goto check_capabilities;
-			else if (error != -EAGAIN)
-				return error;
-		}
-
-		if (in_group_p(inode->i_gid))
-			mode >>= 3;
-	}
+	int ret;
 
 	/*
-	 * If the DACs are ok we don't need any capability check.
+	 * Do the basic POSIX ACL permission checks.
 	 */
-	if ((mask & ~mode) == 0)
-		return 0;
+	ret = acl_permission_check(inode, mask, check_acl);
+	if (ret != -EACCES)
+		return ret;
 
- check_capabilities:
 	/*
 	 * Read/write DACs are always overridable.
 	 * Executable DACs are overridable if at least one exec bit is set.
···
 	if (inode->i_op->permission)
 		retval = inode->i_op->permission(inode, mask);
 	else
-		retval = generic_permission(inode, mask, NULL);
+		retval = generic_permission(inode, mask, inode->i_op->check_acl);
 
 	if (retval)
 		return retval;
···
  */
 static int exec_permission_lite(struct inode *inode)
 {
-	umode_t	mode = inode->i_mode;
+	int ret;
 
 	if (inode->i_op->permission) {
-		int ret = inode->i_op->permission(inode, MAY_EXEC);
+		ret = inode->i_op->permission(inode, MAY_EXEC);
 		if (!ret)
 			goto ok;
 		return ret;
 	}
-
-	if (current_fsuid() == inode->i_uid)
-		mode >>= 6;
-	else if (in_group_p(inode->i_gid))
-		mode >>= 3;
-
-	if (mode & MAY_EXEC)
+	ret = acl_permission_check(inode, MAY_EXEC, inode->i_op->check_acl);
+	if (!ret)
 		goto ok;
 
 	if (capable(CAP_DAC_OVERRIDE) || capable(CAP_DAC_READ_SEARCH))
 		goto ok;
 
-	return -EACCES;
+	return ret;
 ok:
 	return security_inode_permission(inode, MAY_EXEC);
 }

include/linux/fs.h

···
 	void (*put_link) (struct dentry *, struct nameidata *, void *);
 	void (*truncate) (struct inode *);
 	int (*permission) (struct inode *, int);
+	int (*check_acl)(struct inode *, int);
 	int (*setattr) (struct dentry *, struct iattr *);
 	int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
 	int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);