Re: [PATCH bpf-next 3/3] libbpf: add non-mmapable data section selftest

[Dave Marchevsky <davemarchevsky@meta.com>]

On 10/17/22 11:56 PM, Andrii Nakryiko wrote:
> Add non-mmapable data section to test_skeleton selftest and make sure it
> really isn't mmapable by trying to mmap() it anyways.
> 
> Also make sure that libbpf doesn't report BPF_F_MMAPABLE flag to users.
> 
> Additional, some more manual testing was performed that this feature
> works as intended.
> 
> Looking at created map through bpftool shows that flags passed to kernel are
> indeed zero:
> 
>   $ bpftool map show
>   ...
>   1782: array  name .data.non_mmapa  flags 0x0
>           key 4B  value 16B  max_entries 1  memlock 4096B
>           btf_id 1169
>           pids test_progs(8311)
>   ...
> 
> Checking BTF uploaded to kernel for this map shows that zero_key and
> zero_value are indeed marked as static, even though zero_key is actually
> original global (but STV_HIDDEN) variable:
> 
>   $ bpftool btf dump id 1169
>   ...
>   [51] VAR 'zero_key' type_id=2, linkage=static
>   [52] VAR 'zero_value' type_id=7, linkage=static
>   ...
>   [62] DATASEC '.data.non_mmapable' size=16 vlen=2
>           type_id=51 offset=0 size=4 (VAR 'zero_key')
>           type_id=52 offset=4 size=12 (VAR 'zero_value')
>   ...
> 
> And original BTF does have zero_key marked as linkage=global:
> 
>   $ bpftool btf dump file test_skeleton.bpf.linked3.o
>   ...
>   [51] VAR 'zero_key' type_id=2, linkage=global
>   [52] VAR 'zero_value' type_id=7, linkage=static
>   ...
>   [62] DATASEC '.data.non_mmapable' size=16 vlen=2
>           type_id=51 offset=0 size=4 (VAR 'zero_key')
>           type_id=52 offset=4 size=12 (VAR 'zero_value')
> 
> Bpftool didn't require any changes at all because it checks whether internal
> map is mmapable already, but just to double-check generated skeleton, we
> see that .data.non_mmapable neither sets mmaped pointer nor has
> a corresponding field in the skeleton:
> 
>   $ grep non_mmapable test_skeleton.skel.h
>                   struct bpf_map *data_non_mmapable;
>           s->maps[7].name = ".data.non_mmapable";
>           s->maps[7].map = &obj->maps.data_non_mmapable;
> 
> But .data.read_mostly has all of those things:
> 
>   $ grep read_mostly test_skeleton.skel.h
>                   struct bpf_map *data_read_mostly;
>           struct test_skeleton__data_read_mostly {
>                   int read_mostly_var;
>           } *data_read_mostly;
>           s->maps[6].name = ".data.read_mostly";
>           s->maps[6].map = &obj->maps.data_read_mostly;
>           s->maps[6].mmaped = (void **)&obj->data_read_mostly;
>           _Static_assert(sizeof(s->data_read_mostly->read_mostly_var) == 4, "unexpected size of 'read_mostly_var'");
> 
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>