From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id F2063C0015E for ; Thu, 13 Jul 2023 15:15:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232592AbjGMPPB (ORCPT ); Thu, 13 Jul 2023 11:15:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49114 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232618AbjGMPPA (ORCPT ); Thu, 13 Jul 2023 11:15:00 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2FE4AC1; Thu, 13 Jul 2023 08:14:59 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id C146F6169A; Thu, 13 Jul 2023 15:14:58 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D42BCC433C8; Thu, 13 Jul 2023 15:14:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1689261298; bh=cuphCuKw9YA4JZPmXVooU1lbc4+mnudUODRU3BSMBMA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=UtpR9POBdAYRSnYKPl/Fp2tSHRA/2T/A5hw9q/4wIg09jEf5+KoRsaKSa8R6mKxSo j2gx6T3Ur+PGlZkvgCTdF0C2FPQkQP5eB6XaNEvmCKmPTmCSKnFU17Kv+Cx1aOOBGb jQ5RY3DRLF1FUotERqo73tKM9nE7TD3EqJcC9Jo36OpESKPdcipEKbYcqTxqNLT/bp iOowRwxWQaIsgB0det7IbX+vAmOiMlgJRseJZ0oTgp0ve7f5WQyxD4yu1KwR81ASS7 F4/DfPwMyIvL46dlO7mO281PLfmPVPpSo+/OvhugrB+a+oXME7QHr0ws1uLyVgBbI0 5gO8s15zQECEA== Date: Thu, 13 Jul 2023 17:14:53 +0200 From: Christian Brauner To: Hao Xu Cc: io-uring@vger.kernel.org, Jens Axboe , Dominique Martinet , Pavel Begunkov , Alexander Viro , Stefan Roesch , Clay Harris , Dave Chinner , linux-fsdevel@vger.kernel.org, Wanpeng Li Subject: Re: [PATCH 3/3] io_uring: add support for getdents Message-ID: <20230713-verglast-pfuschen-50197f8be98b@brauner> References: <20230711114027.59945-1-hao.xu@linux.dev> <20230711114027.59945-4-hao.xu@linux.dev> <20230712-alltag-abberufen-67a615152bee@brauner> <20230713-sitzt-zudem-67bc5d860cb4@brauner> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Thu, Jul 13, 2023 at 05:06:32PM +0800, Hao Xu wrote: > Hi Christian, > > On 7/13/23 15:10, Christian Brauner wrote: > > On Thu, Jul 13, 2023 at 12:35:07PM +0800, Hao Xu wrote: > > > On 7/12/23 23:27, Christian Brauner wrote: > > > > On Tue, Jul 11, 2023 at 07:40:27PM +0800, Hao Xu wrote: > > > > > From: Hao Xu > > > > > > > > > > This add support for getdents64 to io_uring, acting exactly like the > > > > > syscall: the directory is iterated from it's current's position as > > > > > stored in the file struct, and the file's position is updated exactly as > > > > > if getdents64 had been called. > > > > > > > > > > For filesystems that support NOWAIT in iterate_shared(), try to use it > > > > > first; if a user already knows the filesystem they use do not support > > > > > nowait they can force async through IOSQE_ASYNC in the sqe flags, > > > > > avoiding the need to bounce back through a useless EAGAIN return. > > > > > > > > > > Co-developed-by: Dominique Martinet > > > > > Signed-off-by: Dominique Martinet > > > > > Signed-off-by: Hao Xu > > > > > --- > > > > > include/uapi/linux/io_uring.h | 7 ++++ > > > > > io_uring/fs.c | 60 +++++++++++++++++++++++++++++++++++ > > > > > io_uring/fs.h | 3 ++ > > > > > io_uring/opdef.c | 8 +++++ > > > > > 4 files changed, 78 insertions(+) > > > > > > > > > > diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h > > > > > index 08720c7bd92f..6c0d521135a6 100644 > > > > > --- a/include/uapi/linux/io_uring.h > > > > > +++ b/include/uapi/linux/io_uring.h > > > > > @@ -65,6 +65,7 @@ struct io_uring_sqe { > > > > > __u32 xattr_flags; > > > > > __u32 msg_ring_flags; > > > > > __u32 uring_cmd_flags; > > > > > + __u32 getdents_flags; > > > > > }; > > > > > __u64 user_data; /* data to be passed back at completion time */ > > > > > /* pack this to avoid bogus arm OABI complaints */ > > > > > @@ -235,6 +236,7 @@ enum io_uring_op { > > > > > IORING_OP_URING_CMD, > > > > > IORING_OP_SEND_ZC, > > > > > IORING_OP_SENDMSG_ZC, > > > > > + IORING_OP_GETDENTS, > > > > > /* this goes last, obviously */ > > > > > IORING_OP_LAST, > > > > > @@ -273,6 +275,11 @@ enum io_uring_op { > > > > > */ > > > > > #define SPLICE_F_FD_IN_FIXED (1U << 31) /* the last bit of __u32 */ > > > > > +/* > > > > > + * sqe->getdents_flags > > > > > + */ > > > > > +#define IORING_GETDENTS_REWIND (1U << 0) > > > > > + > > > > > /* > > > > > * POLL_ADD flags. Note that since sqe->poll_events is the flag space, the > > > > > * command flags for POLL_ADD are stored in sqe->len. > > > > > diff --git a/io_uring/fs.c b/io_uring/fs.c > > > > > index f6a69a549fd4..77f00577e09c 100644 > > > > > --- a/io_uring/fs.c > > > > > +++ b/io_uring/fs.c > > > > > @@ -47,6 +47,13 @@ struct io_link { > > > > > int flags; > > > > > }; > > > > > +struct io_getdents { > > > > > + struct file *file; > > > > > + struct linux_dirent64 __user *dirent; > > > > > + unsigned int count; > > > > > + int flags; > > > > > +}; > > > > > + > > > > > int io_renameat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe) > > > > > { > > > > > struct io_rename *ren = io_kiocb_to_cmd(req, struct io_rename); > > > > > @@ -291,3 +298,56 @@ void io_link_cleanup(struct io_kiocb *req) > > > > > putname(sl->oldpath); > > > > > putname(sl->newpath); > > > > > } > > > > > + > > > > > +int io_getdents_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe) > > > > > +{ > > > > > + struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents); > > > > > + > > > > > + if (READ_ONCE(sqe->off) != 0) > > > > > + return -EINVAL; > > > > > + > > > > > + gd->dirent = u64_to_user_ptr(READ_ONCE(sqe->addr)); > > > > > + gd->count = READ_ONCE(sqe->len); > > > > > + > > > > > + return 0; > > > > > +} > > > > > + > > > > > +int io_getdents(struct io_kiocb *req, unsigned int issue_flags) > > > > > +{ > > > > > + struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents); > > > > > + struct file *file; > > > > > + unsigned long getdents_flags = 0; > > > > > + bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK; > > > > > + bool should_lock = false; > > > > > + int ret; > > > > > + > > > > > + if (force_nonblock) { > > > > > + if (!(req->file->f_mode & FMODE_NOWAIT)) > > > > > + return -EAGAIN; > > > > > + > > > > > + getdents_flags = DIR_CONTEXT_F_NOWAIT; > > > > > > > > I mentioned this on the other patch but it seems really pointless to > > > > have that extra flag. I would really like to hear a good reason for > > > > this. > > > > > > > > > + } > > > > > + > > > > > + file = req->file; > > > > > + if (file && (file->f_mode & FMODE_ATOMIC_POS)) { > > > > > + if (file_count(file) > 1) > > > > > > > > Assume we have a regular non-threaded process that just opens an fd to a > > > > file. The process registers an async readdir request via that fd for the > > > > file with io_uring and goes to do other stuff while waiting for the > > > > result. > > > > > > > > Some time later, io_uring gets to io_getdents() and the task is still > > > > single threaded and the file hasn't been shared in the meantime. So > > > > io_getdents() doesn't take the lock and starts the readdir() call. > > > > > > > > Concurrently, the process that registered the io_uring request was free > > > > to do other stuff and issued a synchronous readdir() system call which > > > > calls fdget_pos(). Since the fdtable still isn't shared it doesn't > > > > increment f_count and doesn't acquire the mutex. Now there's another > > > > concurrent readdir() going on. > > > > > > > > (Similar thing can happen if the process creates a thread for example.) > > > > > > > > Two readdir() requests now proceed concurrently which is not intended. > > > > Now to verify that this race can't happen with io_uring: > > > > > > > > * regular fds: > > > > It seems that io_uring calls fget() on each regular file descriptor > > > > when an async request is registered. So that means that io_uring > > > > always hold its own explicit reference here. > > > > So as long as the original task is alive or another thread is alive > > > > f_count is guaranteed to be > 1 and so the mutex would always be > > > > acquired. > > > > > > > > If the registering process dies right before io_uring gets to the > > > > io_getdents() request no other process can steal the fd anymore and in > > > > that case the readdir call would not lock. But that's fine. > > > > > > > > * fixed fds: > > > > I don't know the reference counting rules here. io_uring would need to > > > > ensure that it's impossible for two async readdir requests via a fixed > > > > fd to race because f_count is == 1. > > > > > > > > Iiuc, if a process registers a file it opened as a fixed file and > > > > immediately closes the fd afterwards - without anyone else holding a > > > > reference to that file - and only uses the fixed fd going forward, the > > > > f_count of that file in io_uring's fixed file table is always 1. > > > > > > > > So one could issue any number of concurrent readdir requests with no > > > > mutual exclusion. So for fixed files there definitely is a race, no? > > > > > > Hi Christian, > > > The ref logic for fixed file is that it does fdget() when registering > > > > It absolutely can't be the case that io_uring uses fdget()/fdput() for > > long-term file references. fdget() internally use __fget_light() which > > avoids taking a reference on the file if the file table isn't shared. So > > should that file be stashed anywhere for async work its a UAF waiting to > > happen. > > > > Yes, I typed the wrong name, should be fget() not fdget(). > > > > the file, and fdput() when unregistering it. So the ref in between is > > > always > 1. The fixed file feature is to reduce frequent fdget/fdput, > > > but it does call them at the register/unregister time. > > > > So consider: > > > > // Caller opens some file. > > fd_register = open("/some/file", ...); // f_count == 1 > > > > // Caller registers that file as a fixed file > > IORING_REGISTER_FILES > > -> io_sqe_files_register() > > -> fget(fd_register) // f_count == 2 > > -> io_fixed_file_set() > > > > // Caller trades regular fd reference for fixed file reference completely. > > close(fd_register); > > -> close_fd(fd_register) > > -> file = pick_file() > > -> filp_close(file) > > -> fput(file) // f_count == 1 > > > > > > // Caller spawns a second thread. Both treads issue async getdents via > > // fixed file. > > T1 T2 > > IORING_OP_GETDENTS IORING_OP_GETDENTS > > > > // At some point io_assign_file() must be called which has: > > > > if (req->flags & REQ_F_FIXED_FILE) > > req->file = io_file_get_fixed(req, req->cqe.fd, issue_flags); > > else > > req->file = io_file_get_normal(req, req->cqe.fd); > > > > // Since this is REQ_F_FIXED_FILE f_count == 1 > > > > if (file && (file->f_mode & FMODE_ATOMIC_POS)) { > > if (file_count(file) > 1) > > > > // No lock is taken; T1 and T2 issue getdents concurrently without any > > // locking. -> race on f_pos > > > > I'm happy to be convinced that this is safe, but please someone explain > > in detail why this can't happen and where that extra f_count reference > > for fixed files that this code wants to rely on is coming from. > > > > Afaik, the whole point is that fixed files don't ever call fget()/fput() > > after having been registered anymore. Consequently, f_count should be 1 > > once io_uring has taken full ownership of the file and the file can only > > be addressed via a fixed file reference. > > Thanks for explanation, I now realize it's an issue, even for non-fixed > files when io_uring takes full ownership. for example: > > io_uring submit a getdents --> f_count == 2, get the lock > nowait submission fails --> f_count == 2, release the lock > punt it to io-wq thread and return to userspace > close(fd) --> f_count == 1 > call sync getdents64 --> doing getdents without lock > the io-wq thread begins to run --> f_count == 1, doing getdents > without lock. > > Though this looks like a silly use case but users can do that anyway. > > How about remove this f_count > 1 small optimization in io_uring and always > get the lock, looks like it makes big trouble for async > situation. and there may often be parallel io_uring getdents in the > same time for a file [1], it may be not very meaningful to do this > file count optimization. > > [1] I believe users will issue multiple async getdents at same time rather > than issue them one by one to get better performance. Could someone with perf experience try and remove that f_count == 1 optimization from __fdget_pos() completely and make it always acquire the mutex? I wonder what the performance impact of that is.