On Thu, Feb 10, 2022 at 10:05 AM Yonghong Song <yhs@xxxxxx> 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@xxxxxxxxxx>
> >>
> >> 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.
John,
what is your take on this fix?
bpf tree material?
