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

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Message-ID: <3155818.1603792294@warthog.procyon.org.uk>
Date:   Tue, 27 Oct 2020 09:51:34 +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

David Howells <dhowells@...hat.com> 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?

That said, for afs at least, the fix seems to be just this:

diff --git a/fs/afs/file.c b/fs/afs/file.c
index 395075d7fe02..2bc6adfe351a 100644
--- a/fs/afs/file.c
+++ b/fs/afs/file.c
@@ -33,6 +33,7 @@ const struct file_operations afs_file_operations = {
 	.write_iter	= afs_file_write,
 	.mmap		= afs_file_mmap,
 	.splice_read	= generic_file_splice_read,
+	.splice_write	= iter_file_splice_write,
 	.fsync		= afs_fsync,
 	.lock		= afs_lock,
 	.flock		= afs_flock,

David