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Date:   Fri, 8 Jan 2021 15:44:59 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Siddharth Gupta <sidgup@...eaurora.org>
Cc:     mcgrof@...nel.org, rafael@...nel.org, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "psodagud@...eaurora.org" <psodagud@...eaurora.org>
Subject: Re: PROBLEM: Firmware loader fallback mechanism no longer works with
 sendfile

On Thu, Jan 07, 2021 at 02:03:47PM -0800, Siddharth Gupta wrote:
> On 1/6/2021 2:33 AM, Greg KH wrote:
> > > > > Since the binary attributes don't support splice_{read,write} functions the
> > > > > calls to splice_{read,write} used the default kernel_{read,write} functions.
> > > > > With the above change this results in an -EINVAL return from
> > > > > do_splice_from[4].
> > > > > 
> > > > > This essentially means that sendfile will not work for any binary attribute
> > > > > in the sysfs.
> > > > Have you tried fixing this with a patch much like what we did for the
> > > > proc files that needed this?  If not, can you?
> > > I am not aware of this fix, could you provide me a link for reference? I
> > > will try it out.
> > Look at the series of commits starting at fe33850ff798 ("proc: wire up
> > generic_file_splice_read for iter ops") for how this was fixed in procfs
> > as an example of what also needs to be done for binary sysfs files.
> I tried to follow these fixes, but I am unfamiliar with fs code. I don't see
> the generic_file_splice_write function anymore on newer kernels, also AFAICT
> kernfs_ops does not define {read,write}_iter operations. If the solution is
> simple and someone could provide the patches I would be happy to test them
> out. If not, some more information about how to proceed would be nice.

Can you try this tiny patch out below?

thanks,

greg k-h

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index f277d023ebcd..113bc816d430 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -968,6 +968,8 @@ const struct file_operations kernfs_file_fops = {
 	.release	= kernfs_fop_release,
 	.poll		= kernfs_fop_poll,
 	.fsync		= noop_fsync,
+	.splice_read	= generic_file_splice_read,
+	.splice_write	= iter_file_splice_write,
 };
 
 /**

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