[sorry, resend again as previous one has come text messed out due to networking issues] On Tue, Jul 16, 2019 at 10:08 PM Y Song <ys114321@xxxxxxxxx> wrote: > > On Tue, Jul 16, 2019 at 4:59 AM Ilya Leoshkevich <iii@xxxxxxxxxxxxx> wrote: > > > > 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@xxxxxxxxxxxxx> > > --- > > 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 > All the values are in register. The shifting operations should be the same for big endian and little endian, e.g., value 64 >> 2 = 16 when value "64" is in register. So I did not see a problem here. Could you elaborate which field access in test_pkt_md_access caused problem? It would be good if you can give detailed memory layout and register values to illustrate the problem. > > > if (ctx_field_size <= 4) { > > if (shift) > > insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH, > > -- > > 2.21.0 > >