Re: [PATCH bpf-next v2] bpf: Do not try bpf_msg_push_data with len 0

[Yonghong Song <yhs@fb.com>]

On 2/10/22 7:45 AM, Felix Maurer wrote:
> On 09.02.22 18:06, Yonghong Song wrote:
>> On 2/9/22 7:55 AM, Felix Maurer wrote:
>>> If bpf_msg_push_data is called with len 0 (as it happens during
>>> selftests/bpf/test_sockmap), we do not need to do anything and can
>>> return early.
>>>
>>> Calling bpf_msg_push_data with len 0 previously lead to a wrong ENOMEM
>>> error: we later called get_order(copy + len); if len was 0, copy + len
>>> was also often 0 and get_order returned some undefined value (at the
>>> moment 52). alloc_pages caught that and failed, but then
>>> bpf_msg_push_data returned ENOMEM. This was wrong because we are most
>>> probably not out of memory and actually do not need any additional
>>> memory.
>>>
>>> v2: Add bug description and Fixes tag
>>>
>>> Fixes: 6fff607e2f14b ("bpf: sk_msg program helper bpf_msg_push_data")
>>> Signed-off-by: Felix Maurer <fmaurer@redhat.com>
>>
>> LGTM. I am wondering why bpf CI didn't catch this problem. Did you
>> modified the test with length 0 in order to trigger that? If this
>> is the case, it would be great you can add such a test to the
>> test_sockmap.
> 
> I did not modify the tests to trigger that. The state of the selftests
> around that is unfortunately not very good. There is no explicit test
> with length 0 but bpf_msg_push_data is still called with length 0,
> because of what I consider to be bugs in the test. On the other hand,
> explicit tests with other lengths are sometimes not called as well. I'll
> elaborate on that in a bit.
> 
> Something easy to fix is that the tests do not check the return value of
> bpf_msg_push_data which they probably should. That may have helped find
> the problem earlier.
> 
> Now to the issue mentioned in the beginning: Only some of the BPF
> programs used in test_sockmap actually call bpf_msg_push_data. However,
> they are not always attached, just for particular scenarios:
> txmsg_pass==1, txmsg_redir==1, or txmsg_drop==1. If none of those apply,
> bpf_msg_push_data is never called. This happens for example in
> test_txmsg_push. Out of the four defined tests only one actually calls
> the helper.
> 
> But after a test, the parameters in the map are reset to 0 (instead of
> being removed). Therefore, when the maps are reused in a subsequent test
> which is one of the scenarios above, the values are present and
> bpf_msg_push_data is called, albeit with the parameters set to 0. This
> is also what triggered the wrong behavior fixed in the patch.
> 
> Unfortunately, I do not have the time to fix these issues in the test at
> the moment.

Thanks for detailed explanation. Maybe for the immediate case, can you
just fix this in the selftest,

   > Something easy to fix is that the tests do not check the return 
value of
   > bpf_msg_push_data which they probably should. That may have helped find
   > the problem earlier.

This will be enough to verify your kernel change as without it the
test will fail.

The rest of test improvements can come later.

> 
>> Acked-by: Yonghong Song <yhs@fb.com>
> 
> Thanks!
>