From 2f221d6f7b881d95de1f356a3097d755ab1e47d4 Mon Sep 17 00:00:00 2001
From: Christian Brauner <christian.brauner@ubuntu.com>
Date: Thu, 21 Jan 2021 14:19:26 +0100
Subject: [Backport 2f221d6f7b88] attr: handle idmapped mounts

When file attributes are changed most filesystems rely on the
setattr_prepare(), setattr_copy(), and notify_change() helpers for
initialization and permission checking. Let them handle idmapped mounts.
If the inode is accessed through an idmapped mount map it into the
mount's user namespace. Afterwards the checks are identical to
non-idmapped mounts. If the initial user namespace is passed nothing
changes so non-idmapped mounts will see identical behavior as before.

Helpers that perform checks on the ia_uid and ia_gid fields in struct
iattr assume that ia_uid and ia_gid are intended values and have already
been mapped correctly at the userspace-kernelspace boundary as we
already do today. If the initial user namespace is passed nothing
changes so non-idmapped mounts will see identical behavior as before.

Link: https://lore.kernel.org/r/20210121131959.646623-8-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 src/file.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/file.c b/src/file.c
index a92478eabfa4e43f2198ccf9b62b8632b697c8dc..ace35aa8e64b7f416f26cfa194253d27389dfc7f 100644
--- a/src/file.c
+++ b/src/file.c
@@ -305,7 +305,7 @@ int exfat_setattr(struct dentry *dentry, struct iattr *attr)
 				ATTR_TIMES_SET);
 	}
 
-	error = setattr_prepare(dentry, attr);
+	error = setattr_prepare(&init_user_ns, dentry, attr);
 	attr->ia_valid = ia_valid;
 	if (error)
 		goto out;
@@ -340,7 +340,7 @@ int exfat_setattr(struct dentry *dentry, struct iattr *attr)
 		up_write(&EXFAT_I(inode)->truncate_lock);
 	}
 
-	setattr_copy(inode, attr);
+	setattr_copy(&init_user_ns, inode, attr);
 	exfat_truncate_atime(&inode->i_atime);
 	mark_inode_dirty(inode);
 
-- 
2.31.1