[PATCH bpf-next v8 0/4] bpf: Add bpf_rcu_read_lock() support

[Yonghong Song <yhs@fb.com>]

Currently, without rcu attribute info in BTF, the verifier treats
rcu tagged pointer as a normal pointer. This might be a problem
for sleepable program where rcu_read_lock()/unlock() is not available.
For example, for a sleepable fentry program, if rcu protected memory
access is interleaved with a sleepable helper/kfunc, it is possible
the memory access after the sleepable helper/kfunc might be invalid
since the object might have been freed then. Even without
a sleepable helper/kfunc, without rcu_read_lock() protection,
it is possible that the rcu protected object might be release
in the middle of bpf program execution which may cause incorrect
result.

To prevent above cases, enable btf_type_tag("rcu") attributes,
introduce new bpf_rcu_read_lock/unlock() kfuncs and add verifier support.

In the rest of patch set, Patch 1 enabled btf_type_tag for __rcu
attribute. Patche 2 added might_sleep in bpf_func_proto. Patch 3 added new
bpf_rcu_read_lock/unlock() kfuncs and verifier support.
Patch 4 added some tests for these two new kfuncs.

Changelogs:
  v7 -> v8:
    . add might_sleep in bpf_func_proto so we can easily identify whether
      a helper is sleepable or not.
    . do not enforce rcu rules for sleepable, e.g., rcu dereference must
      be in a bpf_rcu_read_lock region. This is to keep old code working
      fine.
    . Mark 'b' in 'b = a->b' (b is tagged with __rcu) as MEM_RCU only if
      'b = a->b' in rcu read region and 'a' is trusted. This adds safety
      guarantee for 'b' inside the rcu read region.
  v6 -> v7:
    . rebase on top of bpf-next.
    . remove the patch which enables sleepable program using
      cgrp_local_storage map. This is orthogonal to this patch set
      and will be addressed separately.
    . mark the rcu pointer dereference result as UNTRUSTED if inside
      a bpf_rcu_read_lock() region.
  v5 -> v6:
    . fix selftest prog miss_unlock which tested nested locking.
    . add comments in selftest prog cgrp_succ to explain how to handle
      nested memory access after rcu memory load.
  v4 -> v5:
    . add new test to aarch64 deny list.
  v3 -> v4:
    . fix selftest failures when built with gcc. gcc doesn't support
      btf_type_tag yet and some tests relies on that. skip these
      tests if vmlinux BTF does not have btf_type_tag("rcu").
  v2 -> v3:
    . went back to MEM_RCU approach with invalidate rcu ptr registers
      at bpf_rcu_read_unlock() place.
    . remove KF_RCU_LOCK/UNLOCK flag and compare btf_id at verification
      time instead.
  v1 -> v2:
    . use kfunc instead of helper for bpf_rcu_read_lock/unlock.
    . not use MEM_RCU bpf_type_flag, instead use active_rcu_lock
      in reg state to identify rcu ptr's.
    . Add more self tests.
    . add new test to s390x deny list.

Yonghong Song (4):
  compiler_types: Define __rcu as __attribute__((btf_type_tag("rcu")))
  bpf: Introduce might_sleep field in bpf_func_proto
  bpf: Add kfunc bpf_rcu_read_lock/unlock()
  selftests/bpf: Add tests for bpf_rcu_read_lock()

 include/linux/bpf.h                           |   4 +
 include/linux/bpf_verifier.h                  |   4 +-
 include/linux/compiler_types.h                |   3 +-
 kernel/bpf/bpf_lsm.c                          |   6 +-
 kernel/bpf/btf.c                              |   3 +
 kernel/bpf/helpers.c                          |  14 +
 kernel/bpf/verifier.c                         | 123 ++++++-
 kernel/trace/bpf_trace.c                      |   4 +-
 tools/testing/selftests/bpf/DENYLIST.aarch64  |   1 +
 tools/testing/selftests/bpf/DENYLIST.s390x    |   1 +
 .../selftests/bpf/prog_tests/rcu_read_lock.c  | 138 ++++++++
 .../selftests/bpf/progs/rcu_read_lock.c       | 306 ++++++++++++++++++
 12 files changed, 591 insertions(+), 16 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/rcu_read_lock.c
 create mode 100644 tools/testing/selftests/bpf/progs/rcu_read_lock.c

-- 
2.30.2