netdev - Re: [PATCH bpf-next v3 4/6] libbpf: Unify memory address casting operation style

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Message-ID: <CAEf4BzaDQ+cPh8pLGqg-GSM+ryZz3vvDtUy=o2u19KM0CTrewg@mail.gmail.com>
Date:   Fri, 8 Jul 2022 15:30:53 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Pu Lehui <pulehui@...wei.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, bpf <bpf@...r.kernel.org>,
        linux-riscv@...ts.infradead.org,
        Networking <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Björn Töpel <bjorn@...nel.org>,
        Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>
Subject: Re: [PATCH bpf-next v3 4/6] libbpf: Unify memory address casting
 operation style

On Thu, Jul 7, 2022 at 5:23 AM Pu Lehui <pulehui@...wei.com> wrote:
>
>
>
> On 2022/6/4 5:03, Andrii Nakryiko wrote:
> > On Mon, May 30, 2022 at 2:03 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> >>
> >> On 5/30/22 11:28 AM, Pu Lehui wrote:
> >>> The members of bpf_prog_info, which are line_info, jited_line_info,
> >>> jited_ksyms and jited_func_lens, store u64 address pointed to the
> >>> corresponding memory regions. Memory addresses are conceptually
> >>> unsigned, (unsigned long) casting makes more sense, so let's make
> >>> a change for conceptual uniformity.
> >>>
> >>> Signed-off-by: Pu Lehui <pulehui@...wei.com>
> >>> ---
> >>>    tools/lib/bpf/bpf_prog_linfo.c | 9 +++++----
> >>>    1 file changed, 5 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/tools/lib/bpf/bpf_prog_linfo.c b/tools/lib/bpf/bpf_prog_linfo.c
> >>> index 5c503096ef43..7beb060d0671 100644
> >>> --- a/tools/lib/bpf/bpf_prog_linfo.c
> >>> +++ b/tools/lib/bpf/bpf_prog_linfo.c
> >>> @@ -127,7 +127,8 @@ struct bpf_prog_linfo *bpf_prog_linfo__new(const struct bpf_prog_info *info)
> >>>        prog_linfo->raw_linfo = malloc(data_sz);
> >>>        if (!prog_linfo->raw_linfo)
> >>>                goto err_free;
> >>> -     memcpy(prog_linfo->raw_linfo, (void *)(long)info->line_info, data_sz);
> >>> +     memcpy(prog_linfo->raw_linfo, (void *)(unsigned long)info->line_info,
> >>> +            data_sz);
> >>
> >> Took in patch 1-3, lgtm, thanks! My question around the cleanups in patch 4-6 ...
> >> there are various other such cases e.g. in libbpf, perhaps makes sense to clean all
> >> of them up at once and not just the 4 locations in here.
> >
> > if (void *)(long) pattern is wrong, then I guess the best replacement
> > should be (void *)(uintptr_t) ?
> >
>
> I also think that (void *)(uintptr_t) would be the best replacement. I
> applied the changes to kernel/bpf and samples/bpf, and it worked fine.
> But in selftests/bpf, the following similar error occur at compile time:
>
> progs/test_cls_redirect.c:504:11: error: cast to 'uint8_t *' (aka
> 'unsigned char *') from smaller integer type 'uintptr_t' (aka 'unsigned
> int') [-Werror,-Wint-to-pointer-cast]
>         .head = (uint8_t *)(uintptr_t)skb->data,

this is BPF-side code so using system's uintptr_t definition won't
work correctly here. Just do (unsigned long) instead?

>
> I take clang to compile with the front and back end separation, like
> samples/bpf, and it works. It seems that the all-in-one clang has
> problems handling the uintptr_t.
>
> >>
> >> Thanks,
> >> Daniel
> > .
> >