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

Message

David Vernet March 31, 2023, 12:57 a.m. UTC

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.

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     |   4 +-
 .../selftests/bpf/progs/rcu_read_lock.c       |   9 +-
 .../selftests/bpf/progs/task_kfunc_common.h   |   6 +-
 .../selftests/bpf/progs/task_kfunc_failure.c  | 126 ++++++++----------
 .../selftests/bpf/progs/task_kfunc_success.c  |  76 +++++++++--
 8 files changed, 174 insertions(+), 175 deletions(-)