[PATCH v2] btrfs: allow idmapped DEFRAG ioctls

[PATCH v2] btrfs: allow idmapped DEFRAG ioctls

[Tao Cui]

From: Tao Cui <cuitao@kylinos.cn>

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@kylinos.cn>
---
v2: rewrite the commit message to explain why defrag is safe for
idmapped users (Seth Forshee); the code is unchanged.
---
 fs/btrfs/ioctl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 72bc9d4f7708..607329dedd50 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2458,7 +2458,7 @@ static int btrfs_ioctl_defrag(struct file *file, void __user *argp)
 		 * running and allows defrag on files open in read-only mode.
 		 */
 		if (!capable(CAP_SYS_ADMIN) &&
-		    inode_permission(&nop_mnt_idmap, inode, MAY_WRITE)) {
+		    inode_permission(file_mnt_idmap(file), inode, MAY_WRITE)) {
 			ret = -EPERM;
 			goto out;
 		}
-- 
2.43.0

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

[Seth Forshee]

On Sat, Aug 15, 2026 at 09:22:59PM +0800, Tao Cui wrote:
> From: Tao Cui <cuitao@kylinos.cn>
> 
> 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@kylinos.cn>

Thanks for the updated commit message!

Reviewed-by: Seth Forshee <sforshee@kernel.org>