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 X-Spam-Level: X-Spam-Status: No, score=-6.5 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 426EBC433DF for ; Sat, 4 Jul 2020 19:41:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1A38A20870 for ; Sat, 4 Jul 2020 19:41:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="key not found in DNS" (0-bit key) header.d=szeredi.hu header.i=@szeredi.hu header.b="ANB3jbE5" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727849AbgGDTl0 (ORCPT ); Sat, 4 Jul 2020 15:41:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57844 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727816AbgGDTlW (ORCPT ); Sat, 4 Jul 2020 15:41:22 -0400 Received: from mail-ej1-x644.google.com (mail-ej1-x644.google.com [IPv6:2a00:1450:4864:20::644]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D951FC08C5DF for ; Sat, 4 Jul 2020 12:41:21 -0700 (PDT) Received: by mail-ej1-x644.google.com with SMTP id w16so37903882ejj.5 for ; Sat, 04 Jul 2020 12:41:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=szeredi.hu; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=oYoqd/CUPo684lZ33nBT5H+rI2BWES+cDScLFKGmu2Y=; b=ANB3jbE5Aespe1bV8GUSrufR+UgUOcLLAce0mIM6RWiw6ddEBfK4M4umN+gDfubuLQ Nkv6ZOnUULIggH34MXijLri4E67UdDtVYgNoi6Xp6EHGQgwkj0MrpFEGNmKuMCE7oaXy sVM/9FM1+ZIbdrCOCvBtEzrAS0Mh9st+EXtgg= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=oYoqd/CUPo684lZ33nBT5H+rI2BWES+cDScLFKGmu2Y=; b=sNPPWOS+N/o1YsrizmKk6Bw5N1/K0tVA5TOmMC13xZz3xS6v1XHsQXMjoCzE5xZHf/ IbYYN1OfR1tTjywGyU3cauxa3iBnqN/nmrGo3QfQHz0I7n38VBj2yJraQq+7xWL+sVb4 7pVsgPfndPYgZqMpK7EJPbR+xGpOqm9qGuWhKWhnvAuzye3JEFudwmPPST70tYitgWQq O8nTYrG3As+y6i6cIUo+pp5PrQI58M01atdskUt5+EGh8ykVXN4NFA92OgeiiEmvMXld oIE7S8uB5koo79SYOm8piRg+i+muUv+fFzXhjLoKZXLjoB4kRgKRPPlO/tqylra/Pora WGOw== X-Gm-Message-State: AOAM532wxYl0LyQlJ2vjMSqTqGsoKbBAjYhkXGqDbaHXNKaWF6N6qo+v 0S+YpbYHKaEPYO4XoLbqbjZxRx+22anVdnUAZ/H6MA== X-Google-Smtp-Source: ABdhPJyBxGUwYEhvhTUGxE8h2XDZCFFswvZ1ByHXCuGt9UuHeYQFizfvUsknnM+fvByRLxVRrSZ1+AkntCkhvZDN10Q= X-Received: by 2002:a17:906:1c05:: with SMTP id k5mr36355926ejg.320.1593891680498; Sat, 04 Jul 2020 12:41:20 -0700 (PDT) MIME-Version: 1.0 References: <20200704140250.423345-1-gregkh@linuxfoundation.org> <20200704140250.423345-2-gregkh@linuxfoundation.org> In-Reply-To: <20200704140250.423345-2-gregkh@linuxfoundation.org> From: Miklos Szeredi Date: Sat, 4 Jul 2020 21:41:09 +0200 Message-ID: Subject: Re: [PATCH 1/3] readfile: implement readfile syscall To: Greg Kroah-Hartman Cc: Al Viro , Michael Kerrisk , shuah@kernel.org, Linux API , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-man , linux-kselftest@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Sender: linux-man-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org On Sat, Jul 4, 2020 at 4:03 PM Greg Kroah-Hartman wrote: > > It's a tiny syscall, meant to allow a user to do a single "open this > file, read into this buffer, and close the file" all in a single shot. > > Should be good for reading "tiny" files like sysfs, procfs, and other > "small" files. > > There is no restarting the syscall, this is a "simple" syscall, with the > attempt to make reading "simple" files easier with less syscall > overhead. > > Cc: Alexander Viro > Signed-off-by: Greg Kroah-Hartman > --- > fs/open.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 50 insertions(+) > > diff --git a/fs/open.c b/fs/open.c > index 6cd48a61cda3..4469faa9379c 100644 > --- a/fs/open.c > +++ b/fs/open.c > @@ -1370,3 +1370,53 @@ int stream_open(struct inode *inode, struct file *filp) > } > > EXPORT_SYMBOL(stream_open); > + > +static struct file *readfile_open(int dfd, const char __user *filename, > + struct open_flags *op) > +{ > + struct filename *tmp; > + struct file *f; > + > + tmp = getname(filename); > + if (IS_ERR(tmp)) > + return (struct file *)tmp; > + > + f = do_filp_open(dfd, tmp, op); > + if (!IS_ERR(f)) > + fsnotify_open(f); > + > + putname(tmp); > + return f; > +} > + > +SYSCALL_DEFINE5(readfile, int, dfd, const char __user *, filename, > + char __user *, buffer, size_t, bufsize, int, flags) > +{ > + struct open_flags op; > + struct open_how how; > + struct file *file; > + loff_t pos = 0; > + int retval; > + > + /* only accept a small subset of O_ flags that make sense */ > + if ((flags & (O_NOFOLLOW | O_NOATIME)) != flags) > + return -EINVAL; > + > + /* add some needed flags to be able to open the file properly */ > + flags |= O_RDONLY | O_LARGEFILE; > + > + how = build_open_how(flags, 0000); > + retval = build_open_flags(&how, &op); > + if (retval) > + return retval; > + > + file = readfile_open(dfd, filename, &op); > + if (IS_ERR(file)) > + return PTR_ERR(file); > + > + retval = vfs_read(file, buffer, bufsize, &pos); > + > + filp_close(file, NULL); > + > + return retval; Manpage says: "doing the sequence of open() and then read() and then close()", which is exactly what it does. But then it goes on to say: "If the file is larger than the value provided in count then only count number of bytes will be copied into buf", which is only half true, it should be: "If the file is larger than the value provided in count then at most count number of bytes will be copied into buf", which is not a lot of information. And "If the size of file is smaller than the value provided in count then the whole file will be copied into buf", which is simply a lie; for example seq_file will happily return a smaller-than-PAGE_SIZE chunk if at least one record fits in there. You'll have a very hard time explaining that in the man page. So I think there are two possible ways forward: 1) just leave the first explanation (it's an open + read + close equivalent) and leave out the rest 2) add a loop around the vfs_read() in the code. I'd strongly prefer #2 because with the non-looping read it's impossible to detect whether the file was completely read or not, and that's just going to lead to surprises and bugs in userspace code. Thanks, Miklos