Re: [PATCH v2] btrfs: allow idmapped DEFRAG ioctls

[Christian Brauner]

On Sat, Aug 15, 2026 at 09:22:59PM +0800, Tao Cui wrote:
> From: Tao Cui <cuitao@xxxxxxxxxx>
> 
> btrfs_ioctl_defrag() checks MAY_WRITE against nop_mnt_idmap, so on an
> idmapped mount the owner comparison uses the caller's fsuid against the
> raw on-disk uid and the ioctl fails with -EPERM even for the file's
> owner.  Pass the mount idmap down so defrag works on idmapped mounts.
> 
> The check, added in 616d374efa23 ("btrfs: allow defrag on a file opened
> read-only that has rw permissions"), is not a privilege gate.  It only
> tests whether the file could have been opened for writing, and a user
> who owns the file on the idmapped mount can already open it O_RDWR,
> write() to it or fallocate() it.  Defragmenting a regular file only
> rearranges the caller's own extents; there is nothing it can rewrite
> that the caller could not rewrite anyway.  Whole-subvolume defrag on a
> directory keeps its capable(CAP_SYS_ADMIN) requirement, and the
> !capable() guard around this check is left as is.
> 
> FIDEDUPERANGE already works this way for unprivileged callers:
> may_dedupe_file() in fs/remap_range.c compares the inode owner through
> file_mnt_idmap(file) and falls back to inode_permission() with the same
> idmap, and dedupe can rewrite one file's extents from another file's
> contents, which is a stronger operation than defrag.
> 
> On regular mounts file_mnt_idmap() is nop_mnt_idmap and nothing
> changes.
> 
> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
> ---

Reviewed-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>