linux-kernel - Re: [PATCH 02/10] fs: don't allow splice read/write without explicit ops

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Message-ID: <3088368.1603790984@warthog.procyon.org.uk>
Date:   Tue, 27 Oct 2020 09:29:44 +0000
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     dhowells@...hat.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>, x86@...nel.org,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 02/10] fs: don't allow splice read/write without explicit ops

Christoph Hellwig <hch@....de> wrote:

> default_file_splice_write is the last piece of generic code that uses
> set_fs to make the uaccess routines operate on kernel pointers.  It
> implements a "fallback loop" for splicing from files that do not actually
> provide a proper splice_read method.  The usual file systems and other
> high bandwith instances all provide a ->splice_read, so this just removes
> support for various device drivers and procfs/debugfs files.  If splice
> support for any of those turns out to be important it can be added back
> by switching them to the iter ops and using generic_file_splice_read.

Hmmm...  this causes the copy_file_range() syscall to fail with EINVAL in some
places where before it used to work.

For my part, it causes the generic/112 xfstest to fail with afs, but there may
be other places.

Is this a regression we need to fix in the VFS core?  Or is it something we
need to fix in xfstests and assume userspace will fallback to doing it itself?

David