Now that BPF supports adding new kernel functions with kfuncs, and storing
kernel objects in maps with kptrs, we can add a set of kfuncs which allow
struct task_struct objects to be stored in maps as referenced kptrs.

The possible use-cases for doing this are plentiful.  During tracing,
for example, it would be useful to be able to collect some tasks that
performed a certain operation, and then periodically summarize who they
are, which cgroup they're in, how much CPU time they've spent, etc.
Doing this now would require storing the task's pids along with some
relevant data to be exported to user space, and later associating the
pids to tasks in other event handlers where the data is recorded.
Another useful by-product of this is that it allows a program to pin a
task, and by proxy therefore also pin its task local storage.

This patch set adds this aforementioned set of kfuncs, along with a new
selftest suite for validation.

Signed-off-by: David Vernet <void@manifault.com>
---
v1 -> v2:
- Rename tracing_btf_ids to generic_kfunc_btf_ids, and add the new
  kfuncs to that list instead of making a separate btf id list (Alexei).
- Don't run the new selftest suite on s390x, which doesn't appear to
  support invoking kfuncs.
- Add a missing __diag_ignore block for -Wmissing-prototypes
  (lkp@intel.com).
- Fix formatting on some of the SPDX-License-Identifier tags.
- Clarified the function header comment a bit on bpf_task_kptr_get().

David Vernet (2):
  bpf: Add kfuncs for storing struct task_struct * as a kptr
  bpf/selftests: Add selftests for new task kfuncs

 kernel/bpf/helpers.c                          |  83 ++++++-
 tools/testing/selftests/bpf/DENYLIST.s390x    |   1 +
 .../selftests/bpf/prog_tests/task_kfunc.c     | 155 ++++++++++++
 .../selftests/bpf/progs/task_kfunc_common.h   |  83 +++++++
 .../selftests/bpf/progs/task_kfunc_failure.c  | 225 ++++++++++++++++++
 .../selftests/bpf/progs/task_kfunc_success.c  | 113 +++++++++
 6 files changed, 655 insertions(+), 5 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/task_kfunc.c
 create mode 100644 tools/testing/selftests/bpf/progs/task_kfunc_common.h
 create mode 100644 tools/testing/selftests/bpf/progs/task_kfunc_failure.c
 create mode 100644 tools/testing/selftests/bpf/progs/task_kfunc_success.c

-- 
2.37.3