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

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Message-ID: <20210121060522.GA23430@1wt.eu>
Date:   Thu, 21 Jan 2021 07:05:22 +0100
From:   Willy Tarreau <w@....eu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@....de>,
        Johannes Berg <johannes@...solutions.net>,
        Oliver Giles <ohw.giles@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: Splicing to/from a tty

On Wed, Jan 20, 2021 at 07:38:38PM -0800, Linus Torvalds wrote:
> On Wed, Jan 20, 2021 at 5:45 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > splice() triggers an error for seekable destination with O_APPEND and
> > with NULL off_out.
> 
> Ok, that's just broken.
> 
> > Same for splice() to socket with
> >         fcntl(sock_fd, F_SETFL, O_APPEND);
> > done first.
> 
> Same.
> 
> As long as you don't pass a position pointer, I think both should just work.
> 
> Not that I imagine it matters for a lot of people..

I think that most users of splice() on sockets got used to falling back
to recv/send on splice failure due to various cases not being supported
historically (UNIX family sockets immediately come to my mind but I seem
to remember other combinations). Thus I guess that most users of splice()
detect that it doesn't work either due to lower than expected performance
or while running strace.

Willy