Re: [PATCH bpf-next 0/3] bpf: Allow inner map with different max_entries

On Thu, May 21, 2020 at 3:59 PM Martin KaFai Lau [off-list ref] wrote:

On Thu, May 21, 2020 at 03:39:10PM -0700, Andrii Nakryiko wrote:

quoted

On Thu, May 21, 2020 at 12:18 PM Martin KaFai Lau [off-list ref] wrote:

quoted

This series allows the outer map to be updated with inner map in different
size as long as it is safe (meaning the max_entries is not used in the
verification time during prog load).

Please see individual patch for details.

Few thoughts:

1. You describe WHAT, but not necessarily WHY. Can you please
elaborate in descriptions what motivates these changes?

There are cases where people want to update a bigger size
inner map.  I will update the cover letter.

quoted

2. IMO, "capabilities" is word that way too strongly correlates with
Linux capabilities framework, it's just confusing. It's also more of a
property of a map type, than what map is capable of, but it's more
philosophical distinction, of course :)

Sure. I can rename it to "property"

quoted

3. I'm honestly not convinced that patch #1 qualifies as a clean up. I
think one specific check for types of maps that are not compatible
with map-in-map is just fine. Instead you are spreading this bit flags
into a long list of maps, most of which ARE compatible.

but in one place and at the same time a new map type is added to
bpf_types.h

quoted

It's just hard
to even see which ones are not compatible. I like current way better.

There are multiple cases that people forgot to exclude a new map
type from map-in-map in the first attempt and fix it up later.

During the map-in-map implementation, this same concern was raised also
about how to better exclude future map type from map-in-map since
not all people has used map-in-map and it is easy to forget during
review.  Having it in one place in bpf_types.h will make this
more obvious in my opinion.  Patch 1 is an attempt to address
this earlier concern in the map-in-map implementation.

Ok, just invert the condition and list only types that **are** allowed
inside map-in-map. If someone forgot to add it to map-in-map check, it
can be done later when someone needs it. The point is that we have a
check and a list in one place, close to where it matters, instead of
tracing where the value of ->capabilities comes from. Finding that in
bpf_types.h is not easy and not obvious, unfortunately, and is very
distant from where it's actually checked.

quoted

4. Then for size check change, again, it's really much simpler and
cleaner just to have a special case in check in bpf_map_meta_equal for
cases where map size matters.

It may be simpler but not necessary less fragile for future map type.

I am OK for removing patch 1 and just check for a specific
type in patch 2 but I think it is fragile for future map
type IMO.

Well, if we think that the good default needs to be to check size,
then similar to above, explicitly list stuff that *does not* follow
the default, i.e., maps that don't want max_elements verification. My
point still stands.

quoted

5. I also wonder if for those inner maps for which size doesn't
matter, maybe we should set max_elements to zero when setting
inner_meta to show that size doesn't matter? This is minor, though.

quoted

Martin KaFai Lau (3):
  bpf: Clean up inner map type check
  bpf: Relax the max_entries check for inner map
  bpf: selftests: Add test for different inner map size

 include/linux/bpf.h                           | 18 +++++-
 include/linux/bpf_types.h                     | 64 +++++++++++--------
 kernel/bpf/btf.c                              |  2 +-
 kernel/bpf/map_in_map.c                       | 12 ++--
 kernel/bpf/syscall.c                          | 19 +++++-
 kernel/bpf/verifier.c                         |  2 +-
 .../selftests/bpf/prog_tests/btf_map_in_map.c | 12 ++++
 .../selftests/bpf/progs/test_btf_map_in_map.c | 31 +++++++++
 8 files changed, 119 insertions(+), 41 deletions(-)

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2.24.1