This patch allows progs to elide a null check on statically known map
lookup keys. In other words, if the verifier can statically prove that
the lookup will be in-bounds, allow the prog to drop the null check.

This is useful for two reasons:

1. Large numbers of nullness checks (especially when they cannot fail)
   unnecessarily pushes prog towards BPF_COMPLEXITY_LIMIT_JMP_SEQ.
2. It forms a tighter contract between programmer and verifier.

For (1), bpftrace is starting to make heavier use of percpu scratch
maps. As a result, for user scripts with large number of unrolled loops,
we are starting to hit jump complexity verification errors.  These
percpu lookups cannot fail anyways, as we only use static key values.
Eliding nullness probably results in less work for verifier as well.

For (2), percpu scratch maps are often used as a larger stack, as the
currrent stack is limited to 512 bytes. In these situations, it is
desirable for the programmer to express: "this lookup should never fail,
and if it does, it means I messed up the code". By omitting the null
check, the programmer can "ask" the verifier to double check the logic.

Changes in v4:
* Only allow for CAP_BPF
* Add test for stack growing upwards
* Improve comment about stack growing upwards

Changes in v3:
* Check if stack is (erroneously) growing upwards
* Mention in commit message why existing tests needed change

Changes in v2:
* Added a check for when R2 is not a ptr to stack
* Added a check for when stack is uninitialized (no stack slot yet)
* Updated existing tests to account for null elision
* Added test case for when R2 can be both const and non-const

Daniel Xu (2):
  bpf: verifier: Support eliding map lookup nullness
  bpf: selftests: verifier: Add nullness elision tests

 kernel/bpf/verifier.c                         |  73 ++++++-
 tools/testing/selftests/bpf/progs/iters.c     |  14 +-
 .../selftests/bpf/progs/map_kptr_fail.c       |   2 +-
 .../bpf/progs/verifier_array_access.c         | 183 ++++++++++++++++++
 .../selftests/bpf/progs/verifier_map_in_map.c |   2 +-
 .../testing/selftests/bpf/verifier/map_kptr.c |   2 +-
 6 files changed, 265 insertions(+), 11 deletions(-)

-- 
2.46.0