Re: [PATCH bpf-next v2 13/15] selftests/bpf: Add test cases for narrowing fill

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

On Mon, Jan 8, 2024 at 12:53 PM Maxim Mikityanskiy <maxtram95@gmail.com> wrote:
>
> From: Maxim Mikityanskiy <maxim@isovalent.com>
>
> The previous commit allowed to preserve boundaries and track IDs of
> scalars on narrowing fills. Add test cases for that pattern.
>
> Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
> Acked-by: Eduard Zingerman <eddyz87@gmail.com>
> ---
>  .../selftests/bpf/progs/verifier_spill_fill.c | 108 ++++++++++++++++++
>  1 file changed, 108 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> index fab8ae9fe947..3764111d190d 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> @@ -936,4 +936,112 @@ l0_%=:    r0 = 0;                                         \
>         : __clobber_all);
>  }
>
> +SEC("xdp")
> +__description("32-bit fill after 64-bit spill")
> +__success __retval(0)
> +__naked void fill_32bit_after_spill_64bit(void)

I guess these tests are an answer for my question about mixing
spill/fill sizes on earlier patch (so disregard those)

> +{
> +       asm volatile("                                  \
> +       /* Randomize the upper 32 bits. */              \
> +       call %[bpf_get_prandom_u32];                    \
> +       r0 <<= 32;                                      \
> +       /* 64-bit spill r0 to stack. */                 \
> +       *(u64*)(r10 - 8) = r0;                          \
> +       /* 32-bit fill r0 from stack. */                \
> +       r0 = *(u32*)(r10 - %[offset]);                          \

have you considered doing the BYTE_ORDER check right here and have
offset embedded in assembly instruction directly:

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
    r0 = *(u32*)(r10 - 8);
#else
    r0 = *(u32*)(r10 - 4);
#endif

It's a bit less jumping around the code when reading. And it's kind of
obviously that this is endianness-dependent without jumping to
definition of %[offset]?


> +       /* Boundary check on r0 with predetermined result. */\
> +       if r0 == 0 goto l0_%=;                          \
> +       /* Dead branch: the verifier should prune it. Do an invalid memory\
> +        * access if the verifier follows it.           \
> +        */                                             \
> +       r0 = *(u64*)(r9 + 0);                           \
> +l0_%=: exit;                                           \
> +"      :
> +       : __imm(bpf_get_prandom_u32),
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +         __imm_const(offset, 8)
> +#else
> +         __imm_const(offset, 4)
> +#endif
> +       : __clobber_all);
> +}
> +

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