Re: [PATCH] btrfs: use mount idmap for defrag permission check

[Seth Forshee <sforshee@kernel.org>]

On Thu, Aug 13, 2026 at 04:04:05PM +0930, Qu Wenruo wrote:
> 
> 
> 在 2026/8/13 13:11, Tao Cui 写道:
> > From: Tao Cui <cuitao@kylinos.cn>
> > 
> > btrfs_ioctl_defrag() checks MAY_WRITE with nop_mnt_idmap, which skips
> > the mount idmap.  On an idmapped mount the owner comparison then uses
> > the caller's fsuid against the raw on-disk uid, dropping the mapping.
> > Every other permission/owner check in btrfs ioctl uses
> > file_mnt_idmap(file) (e.g. :1152, :1310, :1946); this one missed it.
> > 
> > Switch to file_mnt_idmap(file).  It equals nop_mnt_idmap on a normal
> > mount, and the check stays behind !capable(CAP_SYS_ADMIN), so only
> > unprivileged callers on idmapped btrfs change.  The RO-fd note in the
> > comment above is about the file descriptor, not this inode check, and
> > is unaffected.
> > 
> > Signed-off-by: Tao Cui <cuitao@kylinos.cn>
> 
> Fixes: 4609e1f18e19 ("fs: port ->permission() to pass mnt_idmap")

This is a misattribution. Using nop_mnt_idmap there keeps the behavior
the same as it was before the commit. Switching to the mount idmap opens
up BTRFS_IOC_DEFRAG* to idmapped users when they weren't before, which
is a separate policy decision that didn't belong in that change.

Saying that these ioctls should be allowed for idmapped users just
because others are isn't a good justification. Why are these ioctls safe
for idmapped users?  Do they actually require this capability?

Thanks,
Seth