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From:   David Howells <dhowells@redhat.com>
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References: <ZH32Jp1Iop8FaDtC@corigine.com> <20230605124600.1722160-1-dhowells@redhat.com> <20230605124600.1722160-4-dhowells@redhat.com>
To:     Simon Horman <simon.horman@corigine.com>
Cc:     dhowells@redhat.com, netdev@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Chuck Lever <chuck.lever@oracle.com>,
        Boris Pismenny <borisp@nvidia.com>,
        John Fastabend <john.fastabend@gmail.com>,
        Jakub Kicinski <kuba@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        David Ahern <dsahern@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH net-next v4 03/11] splice, net: Use sendmsg(MSG_SPLICE_PAGES) rather than ->sendpage()
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Date:   Mon, 05 Jun 2023 16:10:57 +0100
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Simon Horman <simon.horman@corigine.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