From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:47350 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752819AbeCQT2N (ORCPT ); Sat, 17 Mar 2018 15:28:13 -0400 Message-ID: <1521314890.4064.12.camel@kernel.org> Subject: Re: [PATCH] locks: change POSIX lock ownership on execve when files_struct is displaced From: Jeff Layton To: Al Viro Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, "J. Bruce Fields" , Thomas Gleixner , "Daniel P ." =?ISO-8859-1?Q?Berrang=E9?= , Kate Stewart , Dan Williams , Philippe Ombredanne , Greg Kroah-Hartman Date: Sat, 17 Mar 2018 15:28:10 -0400 In-Reply-To: <20180317155228.GN30522@ZenIV.linux.org.uk> References: <20180317142520.30520-1-jlayton@kernel.org> <20180317150533.GM30522@ZenIV.linux.org.uk> <1521301408.4064.4.camel@kernel.org> <20180317155228.GN30522@ZenIV.linux.org.uk> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Sat, 2018-03-17 at 15:52 +0000, Al Viro wrote: > On Sat, Mar 17, 2018 at 11:43:28AM -0400, Jeff Layton wrote: > > On Sat, 2018-03-17 at 15:05 +0000, Al Viro wrote: > > > On Sat, Mar 17, 2018 at 10:25:20AM -0400, Jeff Layton wrote: > > > > From: Jeff Layton > > > > > > > > POSIX mandates that open fds and their associated file locks should be > > > > preserved across an execve. This works, unless the process is > > > > multithreaded at the time that execve is called. > > > > > > > > In that case, we'll end up unsharing the files_struct but the locks will > > > > still have their fl_owner set to the address of the old one. Eventually, > > > > when the other threads die and the last reference to the old > > > > files_struct is put, any POSIX locks get torn down since it looks like > > > > a close occurred on them. > > > > > > > > The result is that all of your open files will be intact with none of > > > > the locks you held before execve. The simple answer to this is "use OFD > > > > locks", but this is a nasty surprise and it violates the spec. > > > > > > > > On a successful execve, change ownership of any POSIX file_locks > > > > associated with the old files_struct to the new one, if we ended up > > > > swapping it out. > > > > > > TBH, I don't like the way you implement that. Why not simply use > > > iterate_fd()? > > > > Ahh, I wasn't aware of it. I copied the loop in change_lock_owners from > > close_files. I'll have a look at iterate_fd(). > > BTW, if iterate_fd() turns out to be slower, it might make sense to have it > look at the bitmap to skip unpopulated parts of descriptor table faster - > other users might also benefit from that. Thanks, I'll keep that in mind. Full disclosure: I haven't done any performance testing with this. My assumption is that threaded programs that execve without forking first are rather rare. I don't know of a great way to confirm that though. I made a small change to the v2 patch as well to use struct files_struct * instead of fl_owner_t here. That gives us more type safety and should prevent any problems if Bruce's patch to remove fl_owner_t gets merged. Thanks, -- Jeff Layton