[PATCH bpf] bpf: fix narrower loads on s390

[Ilya Leoshkevich <iii@linux.ibm.com>]

test_pkt_md_access is failing on s390, since the associated eBPF prog
returns TC_ACT_SHOT, which in turn happens because loading a part of a
struct __sk_buff field produces an incorrect result.

The problem is that when verifier emits the code to replace partial load
of a field with a full load, a shift and a bitwise AND, it assumes that
the machine is little endian.

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>
---
 kernel/bpf/verifier.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5900cbb966b1..3f9353653558 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8616,8 +8616,12 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		}
 
 		if (is_narrower_load && size < target_size) {
-			u8 shift = (off & (size_default - 1)) * 8;
-
+			u8 load_off = off & (size_default - 1);
+#ifdef __LITTLE_ENDIAN
+			u8 shift = load_off * 8;
+#else
+			u8 shift = (size_default - (load_off + size)) * 8;
+#endif
 			if (ctx_field_size <= 4) {
 				if (shift)
 					insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
-- 
2.21.0