netdev - Re: [PATCH net-next v4 03/11] splice, net: Use sendmsg(MSG_SPLICE

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Message-ID: <1729074.1685977857@warthog.procyon.org.uk>
Date: Mon, 05 Jun 2023 16:10:57 +0100
From: David Howells <dhowells@...hat.com>
To: Simon Horman <simon.horman@...igine.com>
Cc: dhowells@...hat.com, netdev@...r.kernel.org,
    Linus Torvalds <torvalds@...ux-foundation.org>,
    Chuck Lever <chuck.lever@...cle.com>,
    Boris Pismenny <borisp@...dia.com>,
    John Fastabend <john.fastabend@...il.com>,
    Jakub Kicinski <kuba@...nel.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
Subject: Re: [PATCH net-next v4 03/11] splice, net: Use sendmsg(MSG_SPLICE_PAGES) rather than ->sendpage()

Simon Horman <simon.horman@...igine.com> wrote:

> I'm assuming the answer is that this cannot occur,
> but I thought I should mention this anyway.
> 
> If the initial value of len is 0 (or less).
> ...
> > +	return spliced ?: ret;
> 
> Then ret will be used uninitialised here.

len shouldn't be <0 as it's size_t.

I don't think it should be possible to get there with len==0 - at least from
userspace.  sys_splice() returns immediately and sys_sendfile() either splices
to a pipe or goes via splice_direct_to_actor() will just drop straight out.
But there are kernel users - nfsd for example - but I don't know if they would
splice directly to a socket.

That said, it's probably worth preclearing ret just to be sure.

David