Re: [PATCH v6 bpf-next 4/6] selftests/bpf: add bpf_snprintf_btf helper tests

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Sep 24, 2020 at 5:51 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Wed, Sep 23, 2020 at 06:46:26PM +0100, Alan Maguire wrote:
> > +static int __strncmp(const void *m1, const void *m2, size_t len)
> > +{
> > +     const unsigned char *s1 = m1;
> > +     const unsigned char *s2 = m2;
> > +     int i, delta = 0;
> > +
> > +#pragma clang loop unroll(full)
>
> Shouldn't be needed?
> The verifier supports bounded loops.
>
> > +     for (i = 0; i < len; i++) {
> > +             delta = s1[i] - s2[i];
> > +             if (delta || s1[i] == 0 || s2[i] == 0)
> > +                     break;
> > +     }
> > +     return delta;
> > +}
> > +
> > +/* Use __builtin_btf_type_id to test snprintf_btf by type id instead of name */
> > +#if __has_builtin(__builtin_btf_type_id)
> > +#define TEST_BTF_BY_ID(_str, _typestr, _ptr, _hflags)                        \
> > +     do {                                                            \
> > +             int _expected_ret = ret;                                \
> > +             _ptr.type = 0;                                          \
> > +             _ptr.type_id = __builtin_btf_type_id(_typestr, 0);      \
>
> The test is passing for me, but I don't understand why :)
> __builtin_btf_type_id(, 0); means btf_id of the bpf program.
> While bpf_snprintf_btf() is treating it as btf_id of vmlinux_btf.
> So it really should have been __builtin_btf_type_id(,1);

Better still to use bpf_core_type_id_kernel() macro from bpf_core_read.h.

>
> The following diff works:
> diff --git a/tools/testing/selftests/bpf/progs/netif_receive_skb.c b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
> index b4f96f1f6830..bffa786e3b03 100644
> --- a/tools/testing/selftests/bpf/progs/netif_receive_skb.c
> +++ b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
> @@ -45,7 +45,7 @@ static int __strncmp(const void *m1, const void *m2, size_t len)
>         do {                                                            \
>                 int _expected_ret = ret;                                \
>                 _ptr.type = 0;                                          \
> -               _ptr.type_id = __builtin_btf_type_id(_typestr, 0);      \
> +               _ptr.type_id = __builtin_btf_type_id(_typestr, 1);      \
>                 ret = bpf_snprintf_btf(_str, STRSIZE, &_ptr,            \
>                                        sizeof(_ptr), _hflags);          \
>                 if (ret != _expected_ret) {                             \
> @@ -88,7 +88,7 @@ static int __strncmp(const void *m1, const void *m2, size_t len)
>                         ret = -EBADMSG;                                 \
>                         break;                                          \
>                 }                                                       \
> -               TEST_BTF_BY_ID(_str, #_type, _ptr, _hflags);            \
> +               TEST_BTF_BY_ID(_str, _ptr, _ptr, _hflags);              \
>
> But still makes me suspicious of the test. I haven't debugged further.