netdev - Re: [PATCH bpf-next v3 3/3] bpf: Use 64-bit return value for bpf_prog

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Message-ID: <CAADnVQJjxdUAE6NHNtbbqVj3p3=8KsKrvRb3ShdYb9CcfVY8Ow@mail.gmail.com>
Date:   Tue, 15 Nov 2022 09:09:04 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 3/3] bpf: Use 64-bit return value for bpf_prog_run

On Tue, Nov 8, 2022 at 6:07 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> From: Kumar Kartikeya Dwivedi <memxor@...il.com>
>
> BPF ABI always uses 64-bit return value, but so far __bpf_prog_run and
> higher level wrappers always truncated the return value to 32-bit. We
> want to be able to introduce a new BPF program type that returns a
> PTR_TO_BTF_ID or NULL from the BPF program to the caller context in the
> kernel. To be able to use this returned pointer value, the bpf_prog_run
> invocation needs to be able to return a 64-bit value, so update the
> definitions to allow this.

...

> -static __always_inline u32 __bpf_prog_run(const struct bpf_prog *prog,
> +static __always_inline u64 __bpf_prog_run(const struct bpf_prog *prog,
>                                           const void *ctx,
>                                           bpf_dispatcher_fn dfunc)
>  {
> -       u32 ret;
> +       u64 ret;
>
>         cant_migrate();
>         if (static_branch_unlikely(&bpf_stats_enabled_key)) {
> @@ -602,7 +602,7 @@ static __always_inline u32 __bpf_prog_run(const struct bpf_prog *prog,
>         return ret;
>  }
>
> -static __always_inline u32 bpf_prog_run(const struct bpf_prog *prog, const void *ctx)
> +static __always_inline u64 bpf_prog_run(const struct bpf_prog *prog, const void *ctx)
>  {
>         return __bpf_prog_run(prog, ctx, bpf_dispatcher_nop_func);
>  }

I suspect 32-bit archs with JITs are partially broken with this change.
As long as progs want to return pointers it's ok, but actual
64-bit values will be return garbage in upper 32-bit, since 32-bit
JITs won't populate the upper bits.
I don't think changing u32->long retval is a good idea either,
since BPF ISA has to be stable regardless of underlying arch.
The u32->u64 transition is good long term, but let's add
full 64-bit tests to lib/test_bpf and fix JITs.

I've applied the first two patches of the series.