Re: [PATCH net] bpf: fix allocation warnings in bpf maps and integer overflow

[Alexei Starovoitov]

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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