From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: [PATCH 7/9 updated] vfs: protect remounting superblock read-only Date: Sat, 23 Oct 2010 17:19:48 +0100 Message-ID: <20101023161948.GP19804@ZenIV.linux.org.uk> References: <20101005103108.288743609@szeredi.hu> <20101005103144.051231469@szeredi.hu> <20101022064627.GA21607@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: dave@linux.vnet.ibm.com, akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org To: Miklos Szeredi Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Fri, Oct 22, 2010 at 06:14:01PM +0200, Miklos Szeredi wrote: > @@ -1782,6 +1844,14 @@ int do_add_mount(struct vfsmount *newmnt > > mnt_flags &= ~(MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL); Obviously not enough - you've just added a new flag that needs to be trimmed from mnt_flags. > + /* Locking is necessary to prevent racing with remount r/o */ > + down_read(&newmnt->mnt_sb->s_umount); > + if (newmnt->mnt_sb->s_flags & MS_RDONLY) > + mnt_flags |= MNT_READONLY; > + > + newmnt->mnt_flags = mnt_flags; > + up_read(&newmnt->mnt_sb->s_umount); FWIW, I really don't like the way you are doing that; what we really need there is a per-sb analog of mnt_want_write()/mnt_drop_write(). With mnt_want_write() bumping per-sb write count, which would solve all these problems quite nicely. NOTE: vfsmount being ro and sb being ro are *independent* things; either is enough to deny writes. Having remount ro + remount rw lose the state of other vfsmounts is a Bad Thing(tm). Another thing: "If clone_mnt() happens while mnt_make_readonly() is running, the cloned mount might have MNT_WRITE_HOLD flag set, which results in mnt_want_write() spinning forever on this mount." actually means "neither clone_mnt() nor remounts should ever be done without namespace_sem held exclusive; if that ever happens, we have a serious bug that can lead to any number of bad things happening". Do you actually see such places? If so, that's what needs fixing.