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From: Ian Lance Taylor <iant@golang.org>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: Nicolas Boichat <drinkcat@chromium.org>,
	"Darrick J . Wong" <djwong@kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Luis Lozano <llozano@chromium.org>,
	Dave Chinner <david@fromorbit.com>,
	linux-fsdevel@vger.kernel.org,
	lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated
Date: Fri, 12 Feb 2021 12:22:33 -0800	[thread overview]
Message-ID: <CAOyqgcXV96OeHg8OkzP-EYV7WCn+sJNGsy1RXBP86wYAYv+g3Q@mail.gmail.com> (raw)
In-Reply-To: <YCaswkcgM2NMxiMh@kroah.com>

[-- Attachment #1: Type: text/plain, Size: 3550 bytes --]

On Fri, Feb 12, 2021 at 8:28 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Fri, Feb 12, 2021 at 07:59:04AM -0800, Ian Lance Taylor wrote:
> > On Fri, Feb 12, 2021 at 7:45 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> > >
> > > On Fri, Feb 12, 2021 at 07:33:57AM -0800, Ian Lance Taylor wrote:
> > > > On Fri, Feb 12, 2021 at 12:38 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> > > > >
> > > > > Why are people trying to use copy_file_range on simple /proc and /sys
> > > > > files in the first place?  They can not seek (well most can not), so
> > > > > that feels like a "oh look, a new syscall, let's use it everywhere!"
> > > > > problem that userspace should not do.
> > > >
> > > > This may have been covered elsewhere, but it's not that people are
> > > > saying "let's use copy_file_range on files in /proc."  It's that the
> > > > Go language standard library provides an interface to operating system
> > > > files.  When Go code uses the standard library function io.Copy to
> > > > copy the contents of one open file to another open file, then on Linux
> > > > kernels 5.3 and greater the Go standard library will use the
> > > > copy_file_range system call.  That seems to be exactly what
> > > > copy_file_range is intended for.  Unfortunately it appears that when
> > > > people writing Go code open a file in /proc and use io.Copy the
> > > > contents to another open file, copy_file_range does nothing and
> > > > reports success.  There isn't anything on the copy_file_range man page
> > > > explaining this limitation, and there isn't any documented way to know
> > > > that the Go standard library should not use copy_file_range on certain
> > > > files.
> > >
> > > But, is this a bug in the kernel in that the syscall being made is not
> > > working properly, or a bug in that Go decided to do this for all types
> > > of files not knowing that some types of files can not handle this?
> > >
> > > If the kernel has always worked this way, I would say that Go is doing
> > > the wrong thing here.  If the kernel used to work properly, and then
> > > changed, then it's a regression on the kernel side.
> > >
> > > So which is it?
> >
> > I don't work on the kernel, so I can't tell you which it is.  You will
> > have to decide.
>
> As you have the userspace code, it should be easier for you to test this
> on an older kernel.  I don't have your userspace code...

Sorry, I'm not sure what you are asking.

I've attached a sample Go program.  On kernel version 2.6.32 this
program exits 0.  On kernel version 5.7.17 it prints

got "" want "./foo\x00"

and exits with status 1.

This program hardcodes the string "/proc/self/cmdline" for
convenience, but of course the same results would happen if this were
a generic copy program that somebody invoked with /proc/self/cmdline
as a command line option.

I could write the same program in C easily enough, by explicitly
calling copy_file_range.  Would it help to see a sample C program?


> > From my perspective, as a kernel user rather than a kernel developer,
> > a system call that silently fails for certain files and that provides
> > no way to determine either 1) ahead of time that the system call will
> > fail, or 2) after the call that the system call did fail, is a useless
> > system call.
>
> Great, then don't use copy_file_range() yet as it seems like it fits
> that category at the moment :)

That seems like an unfortunate result, but if that is the determining
opinion then I guess that is what we will have to do in the Go
standard library.

Ian

[-- Attachment #2: foo7.go --]
[-- Type: text/x-go, Size: 972 bytes --]

package main

import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"os"
)

func main() {
	tmpfile, err := ioutil.TempFile("", "copy_file_range")
	if err != nil {
		fmt.Fprint(os.Stderr, err)
		os.Exit(1)
	}
	status := copy(tmpfile)
	os.Remove(tmpfile.Name())
	os.Exit(status)
}

func copy(tmpfile *os.File) int {
	cmdline, err := os.Open("/proc/self/cmdline")
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		return 1
	}
	defer cmdline.Close()
	if _, err := io.Copy(tmpfile, cmdline); err != nil {
		fmt.Fprintf(os.Stderr, "copy failed: %v\n", err)
		return 1
	}
	if err := tmpfile.Close(); err != nil {
		fmt.Fprintln(os.Stderr, err)
		return 1
	}
	old, err := ioutil.ReadFile("/proc/self/cmdline")
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		return 1
	}
	new, err := ioutil.ReadFile(tmpfile.Name())
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		return 1
	}
	if !bytes.Equal(old, new) {
		fmt.Fprintf(os.Stderr, "got %q want %q\n", new, old)
		return 1
	}
	return 0
}

  reply	other threads:[~2021-02-12 20:23 UTC|newest]

Thread overview: 136+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-12  4:43 [PATCH 0/6] Add generated flag to filesystem struct to block copy_file_range Nicolas Boichat
2021-02-12  4:44 ` [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated Nicolas Boichat
2021-02-12  7:46   ` Greg KH
2021-02-12  8:20     ` Nicolas Boichat
2021-02-12  8:37       ` Greg KH
2021-02-12 15:33         ` Ian Lance Taylor
2021-02-12 15:45           ` Greg KH
2021-02-12 15:59             ` Ian Lance Taylor
2021-02-12 16:28               ` Greg KH
2021-02-12 20:22                 ` Ian Lance Taylor [this message]
2021-02-12 23:03             ` Dave Chinner
2021-02-12 23:07               ` Ian Lance Taylor
2021-02-12 23:27                 ` Dave Chinner
2021-02-12 23:54                   ` Darrick J. Wong
2021-02-15  0:38                     ` Dave Chinner
2021-02-15  1:12                       ` Ian Lance Taylor
2021-02-15  1:25                         ` Nicolas Boichat
2021-02-15  5:56                           ` Amir Goldstein
2021-02-15  8:30                           ` Greg KH
2021-02-12  8:22     ` Amir Goldstein
2021-02-12  8:39       ` Greg KH
2021-02-12 12:05         ` Luis Henriques
2021-02-12 12:18           ` Greg KH
2021-02-12 12:41             ` Luis Henriques
2021-02-12 14:11               ` Greg KH
2021-02-12 15:01                 ` Luis Henriques
2021-02-15  6:12               ` Amir Goldstein
2021-02-15 10:39                 ` Luis Henriques
2021-02-15 12:22                   ` Luis Henriques
2021-02-15 14:23                     ` Amir Goldstein
2021-02-15 14:51                       ` Luis Henriques
2021-02-15 15:43                       ` [PATCH v2] vfs: prevent copy_file_range to copy across devices Luis Henriques
2021-02-15 16:02                         ` Trond Myklebust
2021-02-16  0:25                           ` Steve French
2021-02-15 16:34                         ` Amir Goldstein
2021-02-15 16:53                           ` Trond Myklebust
2021-02-15 17:24                             ` Amir Goldstein
2021-02-15 18:57                               ` Trond Myklebust
2021-02-15 19:43                                 ` Amir Goldstein
2021-02-16 11:17                                   ` Luis Henriques
2021-02-16 11:28                                     ` gregkh
2021-02-16 12:01                                       ` Luis Henriques
2021-02-16 12:08                                         ` Greg KH
2021-02-16 13:51                                     ` Amir Goldstein
2021-02-16 16:42                                       ` Luis Henriques
2021-02-16 17:44                                         ` Amir Goldstein
2021-02-16 18:55                                           ` Luis Henriques
2021-02-16 19:20                                             ` Amir Goldstein
2021-02-16 19:27                                               ` Anna Schumaker
2021-02-16 19:31                                                 ` Steve French
2021-02-16 19:40                                                   ` Amir Goldstein
2021-02-16 21:15                                                     ` Steve French
2021-02-17  8:08                                                       ` Amir Goldstein
2021-02-17 17:26                                                         ` [PATCH v3] vfs: fix copy_file_range regression in cross-fs copies Luis Henriques
2021-02-17 20:47                                                           ` Amir Goldstein
2021-02-18  0:56                                                           ` Nicolas Boichat
2021-02-18  5:32                                                           ` Olga Kornievskaia
2021-02-18  6:47                                                             ` Amir Goldstein
2021-02-18 16:28                                                               ` Olga Kornievskaia
2021-02-18  7:43                                                           ` Christoph Hellwig
2021-02-18  0:50                                                         ` [PATCH v2] vfs: prevent copy_file_range to copy across devices Andreas Dilger
2021-02-18  7:34                                                           ` gregkh
2021-02-16 18:54                                       ` Andreas Dilger
2021-02-17  4:45                         ` Nicolas Boichat
2021-02-18  7:42                         ` Christoph Hellwig
2021-02-18  9:10                           ` Amir Goldstein
2021-02-18 10:29                             ` Luis Henriques
2021-02-18 12:15                               ` Luis Henriques
2021-02-18 12:49                                 ` Amir Goldstein
2021-02-18 14:36                                   ` [PATCH v4] vfs: fix copy_file_range regression in cross-fs copies Luis Henriques
2021-02-18 14:58                                     ` Amir Goldstein
2021-02-18 15:17                                       ` [PATCH v5] " Luis Henriques
2021-02-18 15:53                                         ` Amir Goldstein
2021-02-18 16:35                                           ` Luis Henriques
2021-02-18 17:18                                             ` [PATCH v6] " Luis Henriques
2021-02-19 21:18                                               ` Olga Kornievskaia
2021-02-19 21:52                                                 ` Amir Goldstein
2021-02-21 19:58                                                 ` [PATCH v7] " Luis Henriques
2021-02-22  3:00                                                   ` Nicolas Boichat
2021-02-22 10:24                                                   ` [PATCH v8] " Luis Henriques
2021-02-22 10:46                                                     ` Amir Goldstein
2021-02-22 16:25                                                     ` dai.ngo
2021-02-23 10:32                                                       ` Luis Henriques
2021-02-23 15:28                                                         ` Amir Goldstein
2021-02-23 15:29                                                         ` dai.ngo
2021-02-23 16:02                                                           ` dai.ngo
2021-02-23 16:47                                                             ` Amir Goldstein
2021-02-23 16:57                                                               ` dai.ngo
     [not found]                                                                 ` <e3eed18b-fc7e-e687-608b-7f662017329c@oracle.com>
2021-02-23 17:33                                                                   ` Amir Goldstein
2021-02-24  0:13                                                                     ` dai.ngo
2021-02-23 17:56                                                                 ` Luis Henriques
2021-02-23 17:13                                                             ` Olga Kornievskaia
2021-02-24  1:00                                                     ` Olga Kornievskaia
2021-02-24 10:23                                                       ` Luis Henriques
2021-02-24 10:44                                                         ` Nicolas Boichat
2021-04-09  5:23                                                           ` Nicolas Boichat
2021-04-09 13:39                                                             ` Luis Henriques
2021-04-09 13:50                                                               ` Amir Goldstein
2021-04-23  4:40                                                                 ` Nicolas Boichat
2021-05-03  8:54                                                                   ` Luis Henriques
2021-05-10  4:59                                                                     ` Amir Goldstein
2021-05-10  9:10                                                                       ` Luis Henriques
2021-02-24 14:23                                                     ` [PATCH] copy_file_range.2: Kernel v5.12 updates Luis Henriques
2021-02-24 16:10                                                       ` Amir Goldstein
2021-02-25 10:21                                                         ` Luis Henriques
2021-02-26 10:13                                                           ` Alejandro Colomar (man-pages)
2021-02-26 10:34                                                             ` Amir Goldstein
2021-02-26 11:15                                                               ` Alejandro Colomar (man-pages)
2021-02-26 13:59                                                                 ` Jeff Layton
2021-02-26 21:26                                                                   ` Alejandro Colomar (man-pages)
2021-02-26 22:18                                                         ` Alejandro Colomar (man-pages)
2021-02-27  5:41                                                           ` Amir Goldstein
2021-02-27 12:20                                                             ` Alejandro Colomar (man-pages)
2021-02-27 13:49                                                               ` [RFC v2] copy_file_range.2: Update cross-filesystem support for 5.12 Alejandro Colomar
2021-02-27 16:00                                                                 ` Amir Goldstein
2021-02-27 23:08                                                             ` [PATCH] copy_file_range.2: Kernel v5.12 updates Steve French
2021-02-28  7:35                                                               ` Amir Goldstein
2021-02-28 22:25                                                                 ` Steve French
2021-03-01  6:18                                                                   ` Amir Goldstein
2021-03-01 14:41                                                       ` [RFC v3] copy_file_range.2: Update cross-filesystem support for 5.12 Alejandro Colomar
2021-03-01 14:58                                                         ` Amir Goldstein
2021-03-04  9:38                                                       ` [RFC v4] " Alejandro Colomar
2021-03-04 17:13                                                         ` Darrick J. Wong
2021-03-04 18:24                                                           ` Alejandro Colomar (man-pages)
2021-03-04 23:50                                                             ` Darrick J. Wong
2021-02-24  7:15                                     ` [PATCH v4] vfs: fix copy_file_range regression in cross-fs copies Amir Goldstein
2021-02-24  8:30                                       ` Petr Vorel
2021-02-18 20:41                             ` [PATCH v2] vfs: prevent copy_file_range to copy across devices Steve French
2021-02-12 23:15       ` [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated Dave Chinner
2021-02-12  7:54   ` Amir Goldstein
2021-02-12  4:44 ` [PATCH 2/6] proc: Add FS_GENERATED_CONTENT to filesystem flags Nicolas Boichat
2021-02-12  4:44 ` [PATCH 6/6] vfs: Disallow copy_file_range on generated file systems Nicolas Boichat
2021-02-12  4:53   ` Darrick J. Wong
2021-02-12  4:59     ` Darrick J. Wong
2021-02-12  5:24       ` Nicolas Boichat
2021-02-14 23:09 ` [PATCH 0/6] Add generated flag to filesystem struct to block copy_file_range Al Viro

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