Date: Mon, 30 Nov 2015 15:30:34 -0800
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: davem@davemloft.net, Alexei Starovoitov <ast@kernel.org>,
        netdev <netdev@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>,
        Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Sasha Levin <sasha.levin@oracle.com>
Subject: Re: [PATCH net] bpf: fix allocation warnings in bpf maps and integer
 overflow
Message-ID: <20151130233033.GA67392@ast-mbp.thefacebook.com>
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 <565C549C.5080408@iogearbox.net>
 <565C5E7B.2080602@iogearbox.net>
 <20151130181310.GA30762@ast-mbp.thefacebook.com>
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On Mon, Nov 30, 2015 at 11:16:46PM +0100, Daniel Borkmann wrote:
> 
> So, when creating a sufficiently large map where map->key_size + map->value_size
> would be > MAX_BPF_STACK (but map->key_size still <= MAX_BPF_STACK), we can only
> read the map from an eBPF program, but not update it. In such cases, updates could
> only happen from user space application.

yes and no.
If both key_size + value_size > MAX_BPF_STACK, the program cannot technically
call bpf_map_update_elem() helper, but the user space can still populate large map
elements and the program can update it, since it can have a pointer via
bpf_map_lookup_elem(). So depends on definition of 'update'.

btw, the large-ish key support is actually needed too, since on tracing side
we need to be able to do map[kernel_stack_and_user_stack]++ and key is multipage long.
On userspace side it will be consumed by flamegraphs.

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