netdev - Re: [RFC PATCH bpf-next 1/2] bpf: add bpf

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Date:   Sat, 6 Nov 2021 13:07:00 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Hou Tao <houtao1@...wei.com>, Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        Song Liu <songliubraving@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [RFC PATCH bpf-next 1/2] bpf: add bpf_strncmp helper

On Sat, Nov 6, 2021 at 12:26 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Sat, Nov 06, 2021 at 09:28:21PM +0800, Hou Tao wrote:
> > The helper compares two strings: one string is a null-terminated
> > read-only string, and another one has const max storage size. And
> > it can be used to compare file name in tracing or LSM program.
> >
> > We don't check whether or not s2 in bpf_strncmp() is null-terminated,
> > because its content may be changed by malicous program, and we only
> > ensure the memory accessed is bounded by s2_sz.
>
> I think "malicous" adjective is unnecessary and misleading.
> It's also misspelled.
> Just mention that 2nd argument doesn't have to be null terminated.
>
> > + * long bpf_strncmp(const char *s1, const char *s2, u32 s2_sz)
> ...
> > +BPF_CALL_3(bpf_strncmp, const char *, s1, const char *, s2, size_t, s2_sz)
>
> probably should match u32 instead of size_t.
>
> > @@ -1210,6 +1210,8 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >               return &bpf_get_branch_snapshot_proto;
> >       case BPF_FUNC_trace_vprintk:
> >               return bpf_get_trace_vprintk_proto();
> > +     case BPF_FUNC_strncmp:
> > +             return &bpf_strncmp_proto;
>
> why tracing only?
> Should probably be in bpf_base_func_proto.
>
> I was thinking whether the proto could be:
> long bpf_strncmp(const char *s1, u32 s1_sz, const char *s2)
> but I think your version is better though having const string as 1st arg
> is a bit odd in normal C.

Why do you think it's better? This is equivalent to `123 == x` if it
was integer comparison, so it feels like bpf_strncmp(s, sz, "blah") is
indeed more natural. No big deal, just curious what's better about it.

>
> Would it make sense to add bpf_memchr as well while we are at it?
> And
> static inline bpf_strnlen(const char *s, u32 sz)
> {
>   return bpf_memchr(s, sz, 0);
> }
> to bpf_helpers.h ?