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From:   Kal Cutter Conley <kal.conley@dectris.com>
Date:   Mon, 3 Apr 2023 14:50:34 +0200
Message-ID: <CAHApi-mCm1hWyc1jfB3isPqWqaJUuqn7vN1hfSgPb741ngJK9g@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2 06/10] xsk: Add check for unaligned
 descriptors that overrun UMEM
To:     Magnus Karlsson <magnus.karlsson@gmail.com>
Cc:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        Maxim Mikityanskiy <maximmi@mellanox.com>,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org
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>
> Let me just check that I understand the conditions under which this
> occurs. When selecting unaligned mode, there is no check that the size
> is divisible by the chunk_size as is the case in aligned mode. So we
> can register a umem that is for example 15 4K pages plus 100 bytes and
> in this case the second to last page will be marked as contiguous
> (with the CONTIG_MASK) and a packet of length 300 starting at 15*4K -
> 100 will be marked as valid even though it extends 100 bytes outside
> the umem which ends at 15*4K + 100. Did I get this correctly? If so,
> some more color in the commit message would be highly appreciated.

Yes. You don't even need to cross the page. For example, if you have a
packet length of 300 _within_ the final page then it could go past the
end of the umem. In this case, the CONTIG_MASK would not even be
looked at. The explanation is in the next commit message with the
test. I will improve the commit message here though.