From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Ian Rogers <irogers@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@chromium.org>,
Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] libbpf hashmap: fix undefined behavior in hash_bits
Date: Fri, 8 May 2020 00:11:58 -0700 [thread overview]
Message-ID: <CAEf4BzYT5FfDt2oqctHC6dXNmwg5gaaNcFu1StObuYk-jKocLQ@mail.gmail.com> (raw)
In-Reply-To: <20200508063954.256593-1-irogers@google.com>
On Thu, May 7, 2020 at 11:40 PM Ian Rogers <irogers@google.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@google.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
>
next prev parent reply other threads:[~2020-05-08 7:12 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-08 6:39 [PATCH] libbpf hashmap: fix undefined behavior in hash_bits Ian Rogers
2020-05-08 7:11 ` Andrii Nakryiko [this message]
2020-05-08 7:21 ` Ian Rogers
2020-05-08 18:04 ` Andrii Nakryiko
2020-10-29 16:09 [PATCH] libbpf hashmap: Fix " Ian Rogers
2020-10-29 17:45 ` Song Liu
2020-10-29 19:37 ` Ian Rogers
2020-10-29 20:16 ` Andrii Nakryiko
2020-10-29 20:58 ` Ian Rogers
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