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Wed, 27 May 2020 03:14:29 -0700 (PDT) From: Pavel Begunkov To: Jann Horn Cc: io-uring , Jens Axboe References: <94f75705-3506-4c58-c1ff-cced9c045956@gmail.com> 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: Wed, 27 May 2020 13:13:15 +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 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. 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. 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). The idea behind lock/unlock is that - submitters already locked @uring_mtx (i.e. started submission) before the lock/unlock, are waited for in the flush. These can potentially access @old. - submitters, that came after the lock/unlock, won't see @old files. So, no new request associated with @old can appear after that. All's left is to deal with already submitted requests, that's done by the rest of io_uring_cancel_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. Do you see anything suspicious from the description? -- Pavel Begunkov