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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 >
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