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 D0D31EB64DD for ; Thu, 13 Jul 2023 07:10:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231555AbjGMHKJ (ORCPT ); Thu, 13 Jul 2023 03:10:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36212 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230281AbjGMHKI (ORCPT ); Thu, 13 Jul 2023 03:10:08 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CEF8D119; Thu, 13 Jul 2023 00:10:06 -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 6A44861A32; Thu, 13 Jul 2023 07:10:06 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 80AB0C433C7; Thu, 13 Jul 2023 07:10:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1689232205; bh=RdIQiT2khPXP/ZV8I8C1KhIFPpD6kEkdeGs7E9vow6M=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=AlkISGn4AVDhzQEbV/iMmIX1OWP82xTmrRzfje2dEKwnbm7RGHbUCcNtl0f08mPZk bjBPNITTikVDboOkAnaOL6/Dauvun9Of6D+48ECFH6q32PVJeXjrj9EVBn2HeF+/Nw KUEvKwpMAnTIi0zY0hXUXY4SwTv5IFIwb//8J/sy3ki0cRlxaAu6b7xZWJjuZjbzj7 SSLHLCSlFH1Jbz6XFHbKLUTDIhVKf3anDoNNr8COuKaXzt6oboQyiNq6b7p+tYJXO9 xIWU52KB8NsKZV5AZfjU5RVgky/xPz5xTzivSUEPkwQ0winHzRZGUHQBeelSIGmmcq UtZGnp74bvs5w== Date: Thu, 13 Jul 2023 09:10:00 +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-sitzt-zudem-67bc5d860cb4@brauner> References: <20230711114027.59945-1-hao.xu@linux.dev> <20230711114027.59945-4-hao.xu@linux.dev> <20230712-alltag-abberufen-67a615152bee@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 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. > 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. > > > > > > All of that could ofc be simplified if we could just always acquire the > > mutex in fdget_pos() and other places and drop that file_count(file) > 1 > > optimization everywhere. But I have no idea if the optimization for not > > acquiring the mutex if f_count == 1 is worth it? > > > > I hope I didn't confuse myself here. > > > > Jens, do yo have any input here? > > > > > + should_lock = true; > > > + } > > > + if (should_lock) { > > > + if (!force_nonblock) > > > + mutex_lock(&file->f_pos_lock); > > > + else if (!mutex_trylock(&file->f_pos_lock)) > > > + return -EAGAIN; > > > + } > > > > Open-coding this seems extremely brittle with an invitation for subtle > > bugs. > > Could you elaborate on this, I'm not sure that I understand it quite > well. Sorry for my poor English. No need to apologize. I'm wondering whether this should be moved into a tiny helper and actually be exposed via a vfs header if we go this route is all.