On Fri, Dec 20, 2019 at 10:26 PM Martin KaFai Lau <kafai@xxxxxx> wrote:
>
> This patch allows bitfield access as a scalar.
>
> Signed-off-by: Martin KaFai Lau <kafai@xxxxxx>
> ---
> kernel/bpf/btf.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 6e652643849b..da73b63acfc5 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3744,10 +3744,6 @@ int btf_struct_access(struct bpf_verifier_log *log,
> }
>
> for_each_member(i, t, member) {
> - if (btf_member_bitfield_size(t, member))
> - /* bitfields are not supported yet */
> - continue;
> -
> /* offset of the field in bytes */
> moff = btf_member_bit_offset(t, member) / 8;
> if (off + size <= moff)
> @@ -3757,6 +3753,12 @@ int btf_struct_access(struct bpf_verifier_log *log,
> if (off < moff)
> continue;
>
> + if (btf_member_bitfield_size(t, member)) {
> + if (off == moff && off + size <= t->size)
> + return SCALAR_VALUE;
> + continue;
> + }
Shouldn't this check be done before (off < moff) above?
Imagine this situation:
struct {
int :16;
int x:8;
};
Compiler will generate 4-byte load with offset 0, and then bit shifts
to extract third byte. From kernel perspective, you'll see that off=0,
but moff=2, which will get skipped.
So there are two problems, I think:
1. if member is bitfield, special handle that before (off < moff) case.
2. off == moff is too precise, I think it should be `off <= moff`, but
also check that it covers entire bitfield, e.g.:
(off + size) * 8 >= btf_member_bit_offset(t, member) +
btf_member_bitfield_size(t, member)
Make sense or am I missing anything?
> +
> /* type of the field */
> mtype = btf_type_by_id(btf_vmlinux, member->type);
> mname = __btf_name_by_offset(btf_vmlinux, member->name_off);
> --
> 2.17.1
>
