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Date:   Wed, 6 Jan 2021 11:33:39 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Siddharth Gupta <sidgup@codeaurora.org>
Cc:     mcgrof@kernel.org, rafael@kernel.org, viro@zeniv.linux.org.uk,
        linux-fsdevel@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "psodagud@codeaurora.org" <psodagud@codeaurora.org>
Subject: Re: PROBLEM: Firmware loader fallback mechanism no longer works with
 sendfile
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On Tue, Jan 05, 2021 at 05:00:58PM -0800, Siddharth Gupta wrote:
> 
> On 1/4/2021 10:36 PM, Greg KH wrote:
> > On Mon, Jan 04, 2021 at 02:43:45PM -0800, Siddharth Gupta wrote:
> > > Hi all,
> > > 
> > > With the introduction of the filesystem change "fs: don't allow splice
> > > read/write without explicit ops"[1] the fallback mechanism of the firmware
> > > loader[2] no longer works when using sendfile[3] from the userspace.
> > What userspace program are you using to load firmware?
> The userspace program is in the android userspace which listens to a uevent
> from the firmware loader and then loads the firmware using sendfile[1].
> >   Are you not using the in-kernel firmware loader for some reason?
> We have certain non-standard firmware paths that should not be added to the
> linux kernel, and the firmware_class.path only supports a single path.

That option is just for a single override, which should be all that you
need if the other paths that are built into the kernel do not work.
Surely one of the 5 different paths here are acceptable?

If not, how many more do you need?

And last I looked, Android wants you to use the built-in kernel firmware
loader, and NOT an external firmware binary anymore.  So this shouldn't
be an issue for your newer systems anyway :)

> > > 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.

thanks,

greg k-h