On Tue, Jun 28, 2022 at 11:27 PM Yonghong Song [off-list ref] wrote:

On 6/28/22 12:43 AM, Yosry Ahmed wrote:

quoted

On Mon, Jun 27, 2022 at 11:47 PM Yosry Ahmed [off-list ref] wrote:

quoted

On Mon, Jun 27, 2022 at 11:14 PM Yonghong Song [off-list ref] wrote:

[...]

quoted

quoted

quoted

btf_dump_data:FAIL:ensure expected/actual match unexpected ensure
expected/actual match: actual '(union bpf_iter_link_info){.map =
(struct){.map_fd = (__u32)1,},.cgroup '
test_btf_dump_struct_data:PASS:find struct sk_buff 0 nsec

Yeah I see what happened there. bpf_iter_link_info was changed by the
patch that introduced cgroup_iter, and this specific union is used by
the test to test the "union with nested struct" btf dumping. I will
add a patch in the next version that updates the btf_dump_data test
accordingly. Thanks.

So I actually tried the attached diff to updated the expected dump of
bpf_iter_link_info in this test, but the test still failed:

btf_dump_data:FAIL:ensure expected/actual match unexpected ensure
expected/actual match: actual '(union bpf_iter_link_info){.map =
(struct){.map_fd = (__u32)1,},.cgroup = (struct){.cgroup_fd =
(__u32)1,},}'  != expected '(union bpf_iter_link_info){.map =
(struct){.map_fd = (__u32)1,},.cgroup = (struct){.cgroup_fd =
(__u32)1,.traversal_order = (__u32)1},}'

It seems to me that the actual output in this case is not right, it is
missing traversal_order. Did we accidentally find a bug in btf dumping
of unions with nested structs, or am I missing something here?

Probably there is an issue in btf_dump_data() function in
tools/lib/bpf/btf_dump.c. Could you take a look at it?

Regarding this failure of btf_dump_data, the cause seems that:

I added a new struct in 'union bpf_iter_link_info' in this patch
series, which expanded bpf_iter_link_info's size from 32bit to 64bit.
However, the test still used the old struct to initialize, which makes
a temporary stack variable (of type bpf_iter_link_info) partially
initialized. If I initialize the type by the larger new struct only,
btf_dump_data will output the correct content and the said test will
pass.

Yosry, we need to fold the said solution in the patch which introduced
changes to bpf_iter_link_info, so that it won't break the test.

I haven't dug into btf_dump_data() on why partially initialized union
fails. I need to look at the get_cgroup_vmscan_delay selftest in this
patch now.

Hao