linux-kernel - Re: [PATCH v2] vfs: prevent copy_file

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Date:   Mon, 15 Feb 2021 18:25:11 -0600
From:   Steve French <smfrench@...il.com>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     "drinkcat@...omium.org" <drinkcat@...omium.org>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>,
        "iant@...gle.com" <iant@...gle.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "dchinner@...hat.com" <dchinner@...hat.com>,
        "llozano@...omium.org" <llozano@...omium.org>,
        "lhenriques@...e.de" <lhenriques@...e.de>,
        "sfrench@...ba.org" <sfrench@...ba.org>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "amir73il@...il.com" <amir73il@...il.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "miklos@...redi.hu" <miklos@...redi.hu>,
        "samba-technical@...ts.samba.org" <samba-technical@...ts.samba.org>,
        "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>
Subject: Re: [PATCH v2] vfs: prevent copy_file_range to copy across devices

On Mon, Feb 15, 2021 at 10:11 AM Trond Myklebust
<trondmy@...merspace.com> wrote:
>
> On Mon, 2021-02-15 at 15:43 +0000, Luis Henriques wrote:
> > Nicolas Boichat reported an issue when trying to use the
> > copy_file_range
> > syscall on a tracefs file.  It failed silently because the file
> > content is
> > generated on-the-fly (reporting a size of zero) and copy_file_range
> > needs
> > to know in advance how much data is present.
>
> That explanation makes no sense whatsoever. copy_file_range is a non-
> atomic operation and so the file can change while being copied. Any
> determination of 'how much data is present' that is made in advance
> would therefore be a flaw in the copy process being used (i.e.
> do_splice_direct()). Does sendfile() also 'issue' in the same way?

I agree that the explanation of the tracefs problem motivating this
patch doesn't make sense.


-- 
Thanks,

Steve