Re: [PATCH] fix libbpf hashmap with size_t shorter than long long

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Jakub Bogusz <qboosh@pld-linux.org>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [PATCH] fix libbpf hashmap with size_t shorter than long long
Date: Sat, 27 Jun 2020 13:25:01 -0700	[thread overview]
Message-ID: <CAEf4BzbL0LTf9tsBAfyvLho5195a1Kwya8zw3r1_Gc3XMEr54g@mail.gmail.com> (raw)
In-Reply-To: <20200627090713.GA9141@mail>

On Sat, Jun 27, 2020 at 2:07 AM Jakub Bogusz <qboosh@pld-linux.org> wrote:
>
> On Tue, Jun 23, 2020 at 12:40:02PM -0700, Andrii Nakryiko wrote:
> > On Tue, Jun 23, 2020 at 12:29 PM Jakub Bogusz <qboosh@pld-linux.org> wrote:
> > >
> > > On Mon, Jun 22, 2020 at 10:44:56PM -0700, Andrii Nakryiko wrote:
> > > > On Sun, Jun 21, 2020 at 7:34 AM Jakub Bogusz <qboosh@pld-linux.org> wrote:
> > > > >
> > > > > Hello,
> > > > >
> > > > > I noticed that _bpftool crashes when building kernel tools (5.7.x) for
> > > > > 32-bit targets because in libbpf hashmap implementation hash_bits()
> > > > > function returning numbers exceeding hashmap buckets capacity.
> > > > >
> > > > > Attached patch fixes this problem.
> > > > >
> > > >
> > > > Thanks! But this was already fixed by Arnaldo Carvalho de Melo <acme@kernel.org>
> > > > in 8ca8d4a84173 ("libbpf: Define __WORDSIZE if not available").
> > >
> > > No, it's not:
> > > This change worked around __WORDSIZE not always being available.
> > >
> > > But the issue on (I)LP32 platforms is that 64-bit value is shifted by
> > > (32-bits) instead of (64-bits).
> > >
> > > (__SIZEOF_LONG__ * 8) is 32 on such architectures (i686, arm).
> > > I used __SIZEOF_LONG_LONG__ to get proper bit shift both on (I)LP32 and
> > > LP64 architectures.
> > >
> >
> > Ah, I see. I actually mentioned __SIZEOF_ constants on the original
> > fix patch. But I think in this case it has to use __SIZEOF_SIZE_T,
> > which on 32-bit should be 4, right?
>
> After changing constant to 32-bit, yes (to be precise, it should use maximum
> of __SIZEOF_SIZE_T__ and __SIZEOF_LONG__ if constant is specified with
> UL suffix; there is no constant suffix available for size_t).
>
> > > Should I provide an updated patch to apply on top of acme change?
> >
> > Yes, that would be good. But I think there is no need to penalize
> > 32-bit arches with use of 64-bit long longs, and instead it's better
> > to use #ifdef for 32-bit case vs 64-bit case. The multiplication
> > constant will change, of course, should be 2654435769. I'd appreciate
> > it if you can do the patch, thanks!
>
> OK, so now the patch provides two variants:
> - "long long" case for LP64 architectures
> - "long" case for (I)LP32 architectures
> (selected basing of __SIZEOF_ constants)
> matter)
>
>

Change looks good, thanks! But it would be more convenient for
everyone if you submitted it not as an attachment, but as a proper
patch email message.

Acked-by: Andrii Nakryiko <andriin@fb.com>

> Regards,
>
> --
> Jakub Bogusz    http://qboosh.pl/