linux-kernel - [PATCH] overlayfs: add FS_ALLOW_IDMAP flag to enable idmapped mounts

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Message-ID: <20250815150629.2097562-1-sohank2602@gmail.com>
Date: Fri, 15 Aug 2025 11:06:29 -0400
From: Sohan Kunkerkar <sohank2602@...il.com>
To: miklos@...redi.hu,
	amir73il@...il.com
Cc: linux-unionfs@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Sohan Kunkerkar <sohank2602@...il.com>
Subject: [PATCH] overlayfs: add FS_ALLOW_IDMAP flag to enable idmapped mounts

OverlayFS already has comprehensive support for idmapped mounts through
its ovl_copyattr() function and proper mnt_idmap() handling throughout
the codebase. The infrastructure correctly maps UIDs/GIDs from idmapped
upper and lower layers.

However, the filesystem was missing the FS_ALLOW_IDMAP flag, which
caused mount_setattr() calls with MOUNT_ATTR_IDMAP to fail with -EINVAL.

This change enables idmapped mount support by adding the FS_ALLOW_IDMAP
flag to the overlayfs file_system_type, allowing containers and other
applications to use idmapped mounts with overlay filesystems.

Signed-off-by: Sohan Kunkerkar <sohank2602@...il.com>
---
 fs/overlayfs/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index e19940d64..c628f9179 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1516,7 +1516,7 @@ struct file_system_type ovl_fs_type = {
 	.name			= "overlay",
 	.init_fs_context	= ovl_init_fs_context,
 	.parameters		= ovl_parameter_spec,
-	.fs_flags		= FS_USERNS_MOUNT,
+	.fs_flags		= FS_USERNS_MOUNT | FS_ALLOW_IDMAP,
 	.kill_sb		= kill_anon_super,
 };
 MODULE_ALIAS_FS("overlay");
-- 
2.50.1