netdev - Re: [PATCH] Fix values type used in test

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Message-ID: <58F92725.5050304@iogearbox.net>
Date:   Thu, 20 Apr 2017 23:24:53 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     David Miller <davem@...emloft.net>, ast@...com
CC:     borkmann@...earbox.net, netdev@...r.kernel.org
Subject: Re: [PATCH] Fix values type used in test_maps

On 04/20/2017 09:20 PM, David Miller wrote:
>
> Maps of per-cpu type have their value element size adjusted to 8 if it
> is specified smaller during various map operations.
>
> This makes test_maps as a 32-bit binary fail, in fact the kernel
> writes past the end of the value's array on the user's stack.
>
> To be quite honest, I think the kernel should reject creation of a
> per-cpu map that doesn't have a value size of at least 8 if that's
> what the kernel is going to silently adjust to later.

It's unintuitive, agree, and it's in fact a round_up(value_size, 8),
so rejecting a value size smaller than 8 doesn't really help; commit
15a07b33814d ("bpf: add lookup/update support for per-cpu hash and
array maps") explained the rationale a bit. Hmm, we should probably
move at least the bpf_num_possible_cpus() and round_up(val_size, 8)
calculation as a single wrapper function to be used for determining
the array size into bpf_util.h, so that it's slightly easier to deal
with.

> If the user passed something smaller, it is a sizeof() calcualtion
> based upon the type they will actually use (just like in this testcase
> code) in later calls to the map operations.

Fixes: df570f577231 ("samples/bpf: unit test for BPF_MAP_TYPE_PERCPU_ARRAY")

> Signed-off-by: David S. Miller <davem@...emloft.net>

Acked-by: Daniel Borkmann <daniel@...earbox.net>

Thanks!