Re: [PATCH bpf] selftests/bpf: fix test_align

[Edward Cree <ecree@solarflare.com>]

On 05/01/18 23:02, Alexei Starovoitov wrote:
> since commit 82abbf8d2fc4 the verifier rejects the bit-wise
> arithmetic on pointers earlier.
> The test 'dubious pointer arithmetic' now has less output to match on.
> Adjust it.
>
> Fixes: 82abbf8d2fc4 ("bpf: do not allow root to mangle valid pointers")
> Reported-by: kernel test robot <xiaolong.ye@intel.com>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
>  tools/testing/selftests/bpf/test_align.c | 22 +---------------------
>  1 file changed, 1 insertion(+), 21 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/test_align.c b/tools/testing/selftests/bpf/test_align.c
> index 8591c89c0828..471bbbdb94db 100644
> --- a/tools/testing/selftests/bpf/test_align.c
> +++ b/tools/testing/selftests/bpf/test_align.c
> @@ -474,27 +474,7 @@ static struct bpf_align_test tests[] = {
>  		.result = REJECT,
>  		.matches = {
>  			{4, "R5=pkt(id=0,off=0,r=0,imm=0)"},
> -			/* ptr & 0x40 == either 0 or 0x40 */
> -			{5, "R5=inv(id=0,umax_value=64,var_off=(0x0; 0x40))"},
> -			/* ptr << 2 == unknown, (4n) */
> -			{7, "R5=inv(id=0,smax_value=9223372036854775804,umax_value=18446744073709551612,var_off=(0x0; 0xfffffffffffffffc))"},
> -			/* (4n) + 14 == (4n+2).  We blow our bounds, because
> -			 * the add could overflow.
> -			 */
> -			{8, "R5=inv(id=0,var_off=(0x2; 0xfffffffffffffffc))"},
> -			/* Checked s>=0 */
> -			{10, "R5=inv(id=0,umin_value=2,umax_value=9223372036854775806,var_off=(0x2; 0x7ffffffffffffffc))"},
> -			/* packet pointer + nonnegative (4n+2) */
> -			{12, "R6=pkt(id=1,off=0,r=0,umin_value=2,umax_value=9223372036854775806,var_off=(0x2; 0x7ffffffffffffffc))"},
> -			{14, "R4=pkt(id=1,off=4,r=0,umin_value=2,umax_value=9223372036854775806,var_off=(0x2; 0x7ffffffffffffffc))"},
> -			/* NET_IP_ALIGN + (4n+2) == (4n), alignment is fine.
> -			 * We checked the bounds, but it might have been able
> -			 * to overflow if the packet pointer started in the
> -			 * upper half of the address space.
> -			 * So we did not get a 'range' on R6, and the access
> -			 * attempt will fail.
> -			 */
> -			{16, "R6=pkt(id=1,off=0,r=0,umin_value=2,umax_value=9223372036854775806,var_off=(0x2; 0x7ffffffffffffffc))"},
> +			/* R5 bitwise operator &= on pointer prohibited */
>  		}
>  	},
>  	{
Rather than neutering this test, we should change it to keep the part where
 it tests that a large pkt_ptr offset prevents us getting a reg->range.
Specifically, in this test we have
    r2 = pkt
    r5 = large unknown scalar
    r6 = r2 + r5
    r4 = r6 + 4
Then we check r4 < pkt_end, which normally would give r6->range = 4, but in
 this case must not do so since r6 could be (u64)(-2) in which case r4 = 2
 < pkt_end despite r6 not pointing into the packet.
AFAICT there is not other coverage of this case in test_align, and I don't
 recall such a test being in test_verifier either.  So please instead replace
 the insns that do prohibited ops on pointers with some other way of creating
 a large unknown scalar, and keep the rest of the test case intact.

-Ed