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

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Date:   Fri, 8 May 2020 11:04:29 -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 Fri, May 8, 2020 at 12:21 AM Ian Rogers <irogers@...gle.com> wrote:
>
> On Fri, May 8, 2020 at 12:12 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > 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.
>
> Agreed. Unfortunately the LLVM undefined behavior sanitizer (I've not
> tested with GCC to the same extent) will cause an exit when it sees >>
> 64 regardless of whether the value is used or not. It'd be possible to
> #ifdef this code on whether a sanitizer was present.

Yeah, let's do that rather than slowing down hashing function.

>
> Thanks,
> Ian
>
> > >  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
> > >