linux-kernel - Re: [PATCH] libbpf hashmap: fix undefined behavior in hash

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Date:   Fri, 8 May 2020 00:11:58 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] libbpf hashmap: fix undefined behavior in hash_bits

On Thu, May 7, 2020 at 11:40 PM Ian Rogers <irogers@...gle.com> wrote:
>
> If bits is 0, the case when the map is empty, then the >> is the size of
> the register which is undefined behavior - on x86 it is the same as a
> shift by 0. Fix by handling the 0 case explicitly.
>
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---

No need. The only case when bits can be 0 is when hashmap is
completely empty (no elements have ever been added yet). In that case,
it doesn't matter what value hash_bits() returns,
hashmap__for_each_key_entry/hashmap__for_each_key_entry_safe will
behave correctly, because map->buckets will be NULL.

>  tools/lib/bpf/hashmap.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/tools/lib/bpf/hashmap.h b/tools/lib/bpf/hashmap.h
> index d5ef212a55ba..781db653d16c 100644
> --- a/tools/lib/bpf/hashmap.h
> +++ b/tools/lib/bpf/hashmap.h
> @@ -19,6 +19,8 @@
>  static inline size_t hash_bits(size_t h, int bits)
>  {
>         /* shuffle bits and return requested number of upper bits */
> +       if (bits == 0)
> +               return 0;
>         return (h * 11400714819323198485llu) >> (__WORDSIZE - bits);
>  }
>
> --
> 2.26.2.645.ge9eca65c58-goog
>