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

[Felix Maurer <fmaurer@redhat.com>]

On 10.02.22 19:04, Yonghong Song wrote:
> 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.

I just send a patch checking the return values of the bpf_msg_push_data
usages in the test [1]. Passing the errors to userspace by dropping
packets is not very nice, but a straightforward way in the current test
program.

I did try the same checks of the return values of bpf_msg_pull_data, but
then the tests fail. So there might be something hidden here as well.

[1]:https://lore.kernel.org/bpf/89f767bb44005d6b4dd1f42038c438f76b3ebfad.1644601294.git.fmaurer@redhat.com/

> The rest of test improvements can come later.
> 
>>
>>> Acked-by: Yonghong Song <yhs@fb.com>
>>
>> Thanks!
>>
>