[PATCH bpf-next v2 0/3] Enable RCU semantics for task kptrs

[David Vernet <void@manifault.com>]

In commit 22df776a9a86 ("tasks: Extract rcu_users out of union"), the
'refcount_t rcu_users' field was extracted out of a union with the
'struct rcu_head rcu' field. This allows us to use the field for
refcounting struct task_struct with RCU protection, as the RCU callback
no longer flips rcu_users to be nonzero after the callback is scheduled.

This patch set leverages this to do a few things:

1. Marks struct task_struct as RCU safe in the verifier, allowing
   referenced kptr tasks stored in maps to be accessed in an RCU
   read region without acquiring a reference (with just a NULL check).
2. Makes bpf_task_acquire() a KF_ACQUIRE | KF_RCU | KF_RET_NULL kfunc.
3. Removes bpf_task_kptr_get() and bpf_task_acquire_not_zero(), as
   they're now redundant with the above two changes.
4. Updates selftests and documentation accordingly.

---
Changelog:
v1: https://lore.kernel.org/all/20230331005733.406202-1-void@manifault.com/
v1 -> v2:
- Remove testcases validating nested trust inheritance. The first
  version used 'struct task_struct __rcu *parent', but because that
  field has the __rcu tag it functions differently on gcc and llvm and
  causes gcc selftests to fail. Alexei is reworking nested trust,
  anyways so let's leave it off for now (Alexei).

David Vernet (3):
  bpf: Make struct task_struct an RCU-safe type
  bpf: Remove now-defunct task kfuncs
  bpf,docs: Update documentation to reflect new task kfuncs

 Documentation/bpf/kfuncs.rst                  |  49 ++++++-
 kernel/bpf/helpers.c                          |  78 +---------
 kernel/bpf/verifier.c                         |   1 +
 .../selftests/bpf/prog_tests/task_kfunc.c     |   3 +-
 .../selftests/bpf/progs/rcu_read_lock.c       |   9 +-
 .../selftests/bpf/progs/task_kfunc_common.h   |   6 +-
 .../selftests/bpf/progs/task_kfunc_failure.c  | 135 ++++++++----------
 .../selftests/bpf/progs/task_kfunc_success.c  |  48 +++++--
 8 files changed, 155 insertions(+), 174 deletions(-)

-- 
2.39.0