linux-kernel - Re: Splicing to/from a tty

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Message-ID: <9d3baff9-5d4a-4308-9556-8dd037c2fe4b@phytec.de>
Date:   Tue, 19 Jan 2021 17:56:45 +0100
From:   Robert Karszniewicz <r.karszniewicz@...tec.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Christoph Hellwig <hch@....de>, Oliver Giles <ohw.giles@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jiri Slaby <jirislaby@...nel.org>
Subject: Re: Splicing to/from a tty

On 1/19/21 12:53 PM, Greg Kroah-Hartman wrote:
> This looks sane, but I'm still missing what the goal of this is here.
> It's nice from a "don't make the ldisc do the userspace copy", point of
> view, but what is the next step in order to tie that into splice?
> 
> I ask as I also have reports that sysfs binary files are now failing for
> this same reason, so I need to make the same change for them and it's
> not excatly obvious what to do:
> 	https://lore.kernel.org/r/1adf9aa4-ed7e-8f05-a354-57419d61ec18@codeaurora.org

I would like to confirm this. We are using firmwared and it returns EINVAL on
sendfile(), too. I have tried setting the .splice_write callback as in the
linked thread, but it didn't help, it just EINVAL'd in a different place.

I have bisected this issue down to this commit:
4d03e3cc5982 ("fs: don't allow kernel reads and writes without iter ops")

Another case I've also noticed is writing to a serial connection:
kernel write not supported for file /ttymxc0 (pid: 252 comm: cat)

(Which still prints, though, because cat falls back to write(2), I suppose.)

Thank you,
Robert