linux-kernel - Re: Bug in short splice to socket?

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Message-ID: <1227123.1685706296@warthog.procyon.org.uk>
Date:   Fri, 02 Jun 2023 12:44:56 +0100
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, Jakub Kicinski <kuba@...nel.org>,
        netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Chuck Lever <chuck.lever@...cle.com>,
        Boris Pismenny <borisp@...dia.com>,
        John Fastabend <john.fastabend@...il.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: Bug in short splice to socket?

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> Do what I already suggested: making SPLICE_F_MORE reflect reality.

I'm trying to.  I need MSG_MORE to behave sensibly for what I want.

What I have signals SPLICE_F_MORE (and thus MSG_MORE) as long as we haven't
yet read enough data to fulfill the request - and will break out of the loop
if we get a zero-length read.

But this causes a change in behaviour because we then leave the protocol
having seen MSG_MORE set where it didn't previously see that.

This causes "tls -r tls.12_aes_gcm.multi_chunk_sendfile" on the TLS kselftest
to fail.

Now, if we're fine with the change in behaviour, I can make the selftest
observe the short sendfile() and cancel MSG_MORE itself - but that's just a
test program.

So that's the question: Do I have to maintain the current behaviour for the
short-splice case?

David