linux-kernel - 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