From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta1.migadu.com (out-117.mta1.migadu.com [95.215.58.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 76FAE2405EB for ; Fri, 14 Aug 2026 05:16:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.117 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786684568; cv=none; b=dj1MlNxjGrk2AccCUNCubjzjhkp9Zoyke3yQFGRBqA2Nnhjz+zycmhgumv4R+XyEvtnSn7hQxw0baBnwOtkzXE22v5+WUuycm0k5VE9KAPC/q3L/3ty06PlNvXLAKbT47TDC2AWaVeBTvsKyUS5zJAW/5GP6L87Iz00A+NQUs64= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786684568; c=relaxed/simple; bh=j4GJRfc1aJDPjMxnoYFQ35bEfPIds5rkg8YABFOZbLg=; h=Message-ID:Date:MIME-Version:Cc:Subject:To:References:From: In-Reply-To:Content-Type; b=um+pedAiFpNIZyWoQbnpK9x0Ra8DVO9RkOr8MxfjogNMQH/0LpRXqvQ93LjjOAaYwKi6c3DLsm96tt6ib4oY5nd4yI4ZxUpYcxx1IQPiDF2RD75sS7zPwvxWWEvy49AhYV99PxQ6pR6ftINO7/A0ubNGrFoHtrhmsi0I6zx5FS0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=O9SRypj/; arc=none smtp.client-ip=95.215.58.117 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="O9SRypj/" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=j4GJRfc1aJDPjMxnoYFQ35bEfPIds5rkg8YABFOZbLg=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1786684562; v=1; x=1787289362; b=O9SRypj/X89XlsDtx1DKfx2uA4FMDERI7LIVgFlKsDXMqXHM/mb8pEZLwLKEYEesmHDRsQLB eJu4mMAVLtL8Ie7AixZp9ys7Ipmvy2+AANEXUbAuODfEUVAN/zO4Iqsijml/azlijFLF7fONl7R Q8AS/jA/anmeY/6IRu1zfMO4= X-Envelope-To: linux-kernel@vger.kernel.org Received: from [192.168.1.116] (111.162.215.50) by smtp.migadu.com with ESMTPS id 83b9bc82b1922ad3; Fri, 14 Aug 2026 05:16:02 +0000 X-Migadu-Flow: FLOW_OUT Message-ID: <1168774a-74d3-4e77-98e3-cd688a2d7e0f@linux.dev> Date: Fri, 14 Aug 2026 13:15:51 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: cui.tao@linux.dev, clm@fb.com, dsterba@suse.com, josef@toxicpanda.com, brauner@kernel.org, linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Tao Cui Subject: Re: [PATCH] btrfs: use mount idmap for defrag permission check To: Seth Forshee , Qu Wenruo References: <20260813034146.1207640-1-cui.tao@linux.dev> From: Tao Cui In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 在 2026/8/14 01:11, Seth Forshee 写道: > 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 >>> >>> 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 >> >> 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. > Agreed, I'll resend it as a behavior change with no Fixes tag. > 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? > The check isn't a privilege gate. 616d374efa23 added it to test "whether the file could have been opened rw", and the caller can already do that on the idmapped mount -- open O_RDWR, write(), fallocate(). Only the regular-file branch changes, and defrag only reorganizes the caller's own extents; the S_IFDIR branch (whole-subvolume defrag) keeps its capable(CAP_SYS_ADMIN). Nothing is weakened: !capable(CAP_SYS_ADMIN) stays exactly as is. may_dedupe_file() in fs/remap_range.c already gates FIDEDUPERANGE the same way -- capable(CAP_SYS_ADMIN), then i_uid_into_vfsuid( file_mnt_idmap(file)) / inode_permission(idmap, MAY_WRITE) -- and dedupe is stronger, since it can rewrite one file's extents from another file's contents. Idmapped users can already dedupe on btrfs. Thanks, Tao > Thanks, > Seth