Re: [PATCH bpf v3] bpf: fix narrower loads on s390

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Jul 19, 2019 at 2:18 AM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> The very first check in test_pkt_md_access is failing on s390, which
> happens because loading a part of a struct __sk_buff field produces
> an incorrect result.
>
> The preprocessed code of the check is:
>
> {
>         __u8 tmp = *((volatile __u8 *)&skb->len +
>                 ((sizeof(skb->len) - sizeof(__u8)) / sizeof(__u8)));
>         if (tmp != ((*(volatile __u32 *)&skb->len) & 0xFF)) return 2;
> };
>
> clang generates the following code for it:
>
>       0:        71 21 00 03 00 00 00 00 r2 = *(u8 *)(r1 + 3)
>       1:        61 31 00 00 00 00 00 00 r3 = *(u32 *)(r1 + 0)
>       2:        57 30 00 00 00 00 00 ff r3 &= 255
>       3:        5d 23 00 1d 00 00 00 00 if r2 != r3 goto +29 <LBB0_10>
>
> Finally, verifier transforms it to:
>
>   0: (61) r2 = *(u32 *)(r1 +104)
>   1: (bc) w2 = w2
>   2: (74) w2 >>= 24
>   3: (bc) w2 = w2
>   4: (54) w2 &= 255
>   5: (bc) w2 = w2
>
> The problem is that when verifier emits the code to replace a partial
> load of a struct __sk_buff field (*(u8 *)(r1 + 3)) with a full load of
> struct sk_buff field (*(u32 *)(r1 + 104)), an optional shift and a
> bitwise AND, it assumes that the machine is little endian and
> incorrectly decides to use a shift.
>
> Adjust shift count calculation to account for endianness.
>
> Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Applied to bpf tree. Thanks