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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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
d9b8aadaff