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SubjectRe: [PATCH 0/2] Fold checksum at the end of bpf_csum_diff and fix
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On 07/12/2019 01:50 AM, Andrii Nakryiko wrote:
> On Thu, Jul 11, 2019 at 2:32 AM Paolo Pisati <p.pisati@gmail.com> wrote:
>> From: Paolo Pisati <paolo.pisati@canonical.com>
>>
>> After applying patch 0001, all checksum implementations i could test (x86-64, arm64 and
>> arm), now agree on the return value.
>>
>> Patch 0002 fix the expected return value for test #13: i did the calculation manually,
>> and it correspond.
>>
>> Unfortunately, after applying patch 0001, other test cases now fail in
>> test_verifier:

Thanks for catching, sigh. :/

>> $ sudo ./tools/testing/selftests/bpf/test_verifier
>> ...
>> #417/p helper access to variable memory: size = 0 allowed on NULL (ARG_PTR_TO_MEM_OR_NULL) FAIL retval 65535 != 0
>> #419/p helper access to variable memory: size = 0 allowed on != NULL stack pointer (ARG_PTR_TO_MEM_OR_NULL) FAIL retval 65535 != 0
>> #423/p helper access to variable memory: size possible = 0 allowed on != NULL packet pointer (ARG_PTR_TO_MEM_OR_NULL) FAIL retval 65535 != 0
>
> I'm not entirely sure this fix is correct, given these failures, to be honest.
>
> Let's wait for someone who understands intended semantics for
> bpf_csum_diff, before changing returned value so drastically.
>
> But in any case, fixes for these test failures should be in your patch
> series as well.

Your change would actually break applications. The bpf_csum_diff() helper is
heavily used with cascading so one result can be fed into another bpf_csum_diff()
call as seed. Quick test on x86-64:

static int __init foo(void)
{
u8 data[32 * sizeof(u32)];
u32 res1, res2, res3;
int i;

prandom_bytes(data, sizeof(data));
res1 = csum_fold(csum_partial(data, sizeof(data), 0));
for (i = sizeof(u32); i < sizeof(data); i += sizeof(u32)) {
res2 = csum_fold(csum_partial(data, i, 0));
res2 = csum_fold(csum_partial(data+i, sizeof(data)-i, res2));
res3 = csum_partial(data, i, 0);
res3 = csum_fold(csum_partial(data+i, sizeof(data)-i, res3));
printk("%8d: [%4x (reference), %4x (unfolded), %4x (folded)]\n", i, res1, res3, res2);
}
return -1;
}

Gives for all three:

[19113.233942] 4: [6b70 (reference), 6b70 (unfolded), 223d (folded)]
[19113.233943] 8: [6b70 (reference), 6b70 (unfolded), a812 (folded)]
[19113.233943] 12: [6b70 (reference), 6b70 (unfolded), 1c26 (folded)]
[19113.233944] 16: [6b70 (reference), 6b70 (unfolded), 4f76 (folded)]
[19113.233944] 20: [6b70 (reference), 6b70 (unfolded), 2801 (folded)]
[19113.233945] 24: [6b70 (reference), 6b70 (unfolded), b63 (folded)]
[19113.233945] 28: [6b70 (reference), 6b70 (unfolded), 2fe0 (folded)]
[19113.233946] 32: [6b70 (reference), 6b70 (unfolded), 18a2 (folded)]
[19113.233946] 36: [6b70 (reference), 6b70 (unfolded), 2597 (folded)]
[19113.233947] 40: [6b70 (reference), 6b70 (unfolded), 2f8e (folded)]
[19113.233947] 44: [6b70 (reference), 6b70 (unfolded), b8af (folded)]
[19113.233948] 48: [6b70 (reference), 6b70 (unfolded), fb8b (folded)]
[19113.233948] 52: [6b70 (reference), 6b70 (unfolded), e9c0 (folded)]
[19113.233949] 56: [6b70 (reference), 6b70 (unfolded), 6af1 (folded)]
[19113.233949] 60: [6b70 (reference), 6b70 (unfolded), d7f4 (folded)]
[19113.233949] 64: [6b70 (reference), 6b70 (unfolded), 8bc6 (folded)]
[19113.233950] 68: [6b70 (reference), 6b70 (unfolded), 8718 (folded)]
[19113.233950] 72: [6b70 (reference), 6b70 (unfolded), 27d8 (folded)]
[19113.233951] 76: [6b70 (reference), 6b70 (unfolded), a2db (folded)]
[19113.233952] 80: [6b70 (reference), 6b70 (unfolded), 3fd (folded)]
[19113.233952] 84: [6b70 (reference), 6b70 (unfolded), 4be5 (folded)]
[19113.233952] 88: [6b70 (reference), 6b70 (unfolded), 41ad (folded)]
[19113.233953] 92: [6b70 (reference), 6b70 (unfolded), ca9b (folded)]
[19113.233953] 96: [6b70 (reference), 6b70 (unfolded), f8ec (folded)]
[19113.233954] 100: [6b70 (reference), 6b70 (unfolded), 5451 (folded)]
[19113.233954] 104: [6b70 (reference), 6b70 (unfolded), 763 (folded)]
[19113.233955] 108: [6b70 (reference), 6b70 (unfolded), e37c (folded)]
[19113.233955] 112: [6b70 (reference), 6b70 (unfolded), 4ee6 (folded)]
[19113.233956] 116: [6b70 (reference), 6b70 (unfolded), 4f73 (folded)]
[19113.233956] 120: [6b70 (reference), 6b70 (unfolded), 1cfd (folded)]
[19113.233957] 124: [6b70 (reference), 6b70 (unfolded), 7d1a (folded)]

I'll take a look next week wrt fixing this uniformly for all archs.

Thanks,
Daniel

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