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

[Martin KaFai Lau <kafai@fb.com>]

On Thu, May 21, 2020 at 03:39:10PM -0700, Andrii Nakryiko wrote:
> On Thu, May 21, 2020 at 12:18 PM Martin KaFai Lau <kafai@fb.com> wrote:
> >
> > 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.

> 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"

> 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

> 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.

> 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.

> 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.
> 
> 
> > 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(-)
> >
> > --
> > 2.24.1
> >