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Fri, 29 May 2020 01:10:33 -0700 (PDT) To: Jann Horn Cc: io-uring , Jens Axboe References: <94f75705-3506-4c58-c1ff-cced9c045956@gmail.com> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: [RFC] .flush and io_uring_cancel_files Message-ID: Date: Fri, 29 May 2020 11:09:18 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 27/05/2020 21:04, Jann Horn wrote: > On Wed, May 27, 2020 at 12:14 PM Pavel Begunkov wrote: >> On 27/05/2020 01:04, Jann Horn wrote: >>> On Tue, May 26, 2020 at 8:11 PM Pavel Begunkov wrote: >>>> It looks like taking ->uring_lock should work like kind of grace >>>> period for struct files_struct and io_uring_flush(), and that would >>>> solve the race with "fcheck(ctx->ring_fd) == ctx->ring_file". >>>> >>>> Can you take a look? If you like it, I'll send a proper patch >>>> and a bunch of cleanups on top. >>>> >>>> >>>> diff --git a/fs/io_uring.c b/fs/io_uring.c >>>> index a3dbd5f40391..012af200dc72 100644 >>>> --- a/fs/io_uring.c >>>> +++ b/fs/io_uring.c >>>> @@ -5557,12 +5557,11 @@ static int io_grab_files(struct io_kiocb *req) >>>> * the fd has changed since we started down this path, and disallow >>>> * this operation if it has. >>>> */ >>>> - if (fcheck(ctx->ring_fd) == ctx->ring_file) { >>>> - list_add(&req->inflight_entry, &ctx->inflight_list); >>>> - req->flags |= REQ_F_INFLIGHT; >>>> - req->work.files = current->files; >>>> - ret = 0; >>>> - } >>>> + list_add(&req->inflight_entry, &ctx->inflight_list); >>>> + req->flags |= REQ_F_INFLIGHT; >>>> + req->work.files = current->files; >>>> + ret = 0; >>>> + >>>> spin_unlock_irq(&ctx->inflight_lock); >>>> rcu_read_unlock(); >>>> >>>> @@ -7479,6 +7478,10 @@ static int io_uring_release(struct inode *inode, struct >>>> file *file) >>>> static void io_uring_cancel_files(struct io_ring_ctx *ctx, >>>> struct files_struct *files) >>>> { >>>> + /* wait all submitters that can race for @files */ >>>> + mutex_lock(&ctx->uring_lock); >>>> + mutex_unlock(&ctx->uring_lock); >>>> + >>>> while (!list_empty_careful(&ctx->inflight_list)) { >>>> struct io_kiocb *cancel_req = NULL, *req; >>>> DEFINE_WAIT(wait); >>> >>> First off: You're removing a check in io_grab_files() without changing >>> the comment that describes the check; and the new comment you're >>> adding in io_uring_cancel_files() is IMO too short to be useful. >> >> Obviously, it was stripped down to show the idea, nobody is talking about >> commiting it as is. I hoped Jens remembers it well enough to understand. >> Let me describe it in more details then: >> >>> >>> I'm trying to figure out how your change is supposed to work, and I >>> don't get it. If a submitter is just past fdget() (at which point no >>> locks are held), the ->flush() caller can instantly take and drop the >>> ->uring_lock, and then later the rest of the submission path will grab >>> an unprotected pointer to the files_struct. Am I missing something? >> >> old = tsk->files; >> task_lock(tsk); >> tsk->files = files; >> task_unlock(tsk); >> put_files_struct(old); (i.e. ->flush(old)) >> >> It's from reset_files_struct(), and I presume the whole idea of >> io_uring->flush() is to protect against racing for similarly going away @old >> files. I.e. ensuring of not having io_uring requests holding @old files. > > Kind of. We use the ->flush() handler to be notified when the > files_struct goes away, so that instead of holding a reference to the > files_struct (which would cause a reference loop), we can clean up our > references when it goes away. > >> The only place, where current->files are accessed and copied by io_uring, is >> io_grab_files(), which is called in the submission path. And the whole >> submission path is done under @uring_mtx. > > No it isn't. We do fdget(fd) at the start of the io_uring_enter > syscall, and at that point we obviously can't hold the uring_mtx yet. __directly__ accessing ->files, or hand-coded. fdget() by itself shouldn't be a problem. > >> For your case, the submitter will take @uring_mtx only after this lock/unlock >> happened, so it won't see old files (happens-before by locking mutex). > > No, it will see the old files. The concurrent operation we're worried >From what you wrote below, it will see the old files just because nobody would try to replace them. Is that it? > about is not that the files_struct goes away somehow (that can't > happen); what we want to guard against is a concurrent close() or > dup2() or so removing the uring fd from the files_struct, because if > someone calls close() before we stash a pointer to current->files, > that pointer isn't protected anymore. Let me guess, you mean maintenance of ->files itself, such as expand_files()? > > >> The thing I don't know is why current->files is originally accessed without >> protection in io_grab_files(), but presumably rcu_read_lock() there is for that >> reason. > > No, it's because current->files can never change under you; pretty > much the only places where current->files can change are unshare() and > execve(). I see, good to know -- Pavel Begunkov