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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Lorenz Bauer <lmb@cloudflare.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	"David S . Miller" <davem@davemloft.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	mcroce@microsoft.com, bpf <bpf@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH RFC bpf-next 03/10] bpf: Add proto of bpf_core_apply_relo()
Date: Thu, 23 Sep 2021 11:48:22 -0700	[thread overview]
Message-ID: <CAEf4BzZWFRcc6fwqPYZDCex0E_70oWMB-Ce7bV2SX-HC3GmSEg@mail.gmail.com> (raw)
In-Reply-To: <CACAyw992kSRHmHky+S03TdOcwDLCAsqK9quoy-p3vQ9DjCdyKA@mail.gmail.com>

On Thu, Sep 23, 2021 at 4:22 AM Lorenz Bauer <lmb@cloudflare.com> wrote:
>
> On Fri, 17 Sept 2021 at 22:57, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > From: Alexei Starovoitov <ast@kernel.org>
> >
> > Prototype of bpf_core_apply_relo() helper.
> >
> > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
>
> ...
>
> > @@ -6313,4 +6321,10 @@ enum bpf_core_relo_kind {
> >         BPF_CORE_ENUMVAL_VALUE = 11,         /* enum value integer value */
> >  };
> >
> > +struct bpf_core_relo_desc {
> > +       __u32 type_id;
> > +       __u32 access_str_off;
> > +       enum bpf_core_relo_kind kind;
>
> Not a C expert, I thought enums don't have a fixed size?

They are backed by int (4 bytes) in C, unless values don't fit into
int, then it's long long (or unsigned variants of int or long long).
The only exception is when it's used in a packed struct, then field
will get allocated only as little space as necessary to keep all
values (so could be byte or two bytes). You can also have a
bitfield-backed enum field in a struct, but enum itself is still going
to be of whole and power-of-two of number of bytes.

So long story short, here it's 4 bytes and will stay 4 bytes.

C++ gives more flexibility and you can actually specify backing
integer type. Not the case for C, though.

>
> --
> Lorenz Bauer  |  Systems Engineer
> 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK
>
> www.cloudflare.com

  reply	other threads:[~2021-09-23 18:48 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-17 21:57 [PATCH RFC bpf-next 00/10] bpf: CO-RE support in the kernel Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 01/10] bpf: Prepare relo_core.c for kernel duty Alexei Starovoitov
2021-09-21 21:25   ` Andrii Nakryiko
2021-09-28 14:45   ` Matteo Croce
2021-09-28 16:37     ` Alexei Starovoitov
2021-09-28 17:11       ` Matteo Croce
2021-09-28 20:34         ` Alexei Starovoitov
2021-09-29 12:32           ` Matteo Croce
2021-09-29 17:38             ` Matteo Croce
2021-09-29 23:00               ` Alexei Starovoitov
2021-09-29 23:49                 ` Matteo Croce
2021-10-22  0:48                   ` Matteo Croce
2021-10-22  0:51                     ` Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 02/10] bpf: Define enum bpf_core_relo_kind as uapi Alexei Starovoitov
2021-09-21 21:27   ` Andrii Nakryiko
2021-09-17 21:57 ` [PATCH RFC bpf-next 03/10] bpf: Add proto of bpf_core_apply_relo() Alexei Starovoitov
2021-09-23 11:21   ` Lorenz Bauer
2021-09-23 18:48     ` Andrii Nakryiko [this message]
2021-09-17 21:57 ` [PATCH RFC bpf-next 04/10] bpf: Add bpf_core_add_cands() and wire it into bpf_core_apply_relo_insn() Alexei Starovoitov
2021-09-21 21:34   ` Andrii Nakryiko
2021-09-27 18:04   ` Matteo Croce
2021-09-17 21:57 ` [PATCH RFC bpf-next 05/10] libbpf: Use CO-RE in the kernel in light skeleton Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 06/10] libbpf: Make gen_loader data aligned Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 07/10] libbpf: Support init of inner maps in light skeleton Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 08/10] selftests/bpf: Convert kfunc test with CO-RE to lskel Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 09/10] selftests/bpf: Improve inner_map test coverage Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 10/10] selftests/bpf: Convert map_ptr_kern test to use light skeleton Alexei Starovoitov
2021-09-23 11:33 ` [PATCH RFC bpf-next 00/10] bpf: CO-RE support in the kernel Lorenz Bauer
2021-09-23 18:52   ` Andrii Nakryiko
2021-09-24 23:13   ` Alexei Starovoitov
2021-09-27 16:12     ` Lorenz Bauer
2021-09-27 16:50       ` Alexei Starovoitov
2021-09-28  8:30         ` Lorenz Bauer
2021-09-28 16:35           ` Alexei Starovoitov

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