Merge branch 'Add bpf_copy_from_user_task helper and sleepable bpf iterator programs'

Kenny Yu says:

====================

This patch series makes the following changes:
* Adds a new bpf helper `bpf_copy_from_user_task` to read user space
  memory from a different task.
* Adds the ability to create sleepable bpf iterator programs.

As an example of how this will be used, at Meta we are using bpf task
iterator programs and this new bpf helper to read C++ async stack traces of
a running process for debugging C++ binaries in production.

Changes since v6:
* Split first patch into two patches: first patch to add support
  for bpf iterators to use sleepable helpers, and the second to add
  the new bpf helper.
* Simplify implementation of `bpf_copy_from_user_task` based on feedback.
* Add to docs that the destination buffer will be zero-ed on error.

Changes since v5:
* Rename `bpf_access_process_vm` to `bpf_copy_from_user_task`.
* Change return value to be all-or-nothing. If we get a partial read,
  memset all bytes to 0 and return -EFAULT.
* Add to docs that the helper can only be used by sleepable BPF programs.
* Fix nits in selftests.

Changes since v4:
* Make `flags` into u64.
* Use `user_ptr` arg name to be consistent with `bpf_copy_from_user`.
* Add an extra check in selftests to verify access_process_vm calls
  succeeded.

Changes since v3:
* Check if `flags` is 0 and return -EINVAL if not.
* Rebase on latest bpf-next branch and fix merge conflicts.

Changes since v2:
* Reorder arguments in `bpf_access_process_vm` to match existing related
  bpf helpers (e.g. `bpf_probe_read_kernel`, `bpf_probe_read_user`,
  `bpf_copy_from_user`).
* `flags` argument is provided for future extensibility and is not
  currently used, and we always invoke `access_process_vm` with no flags.
* Merge bpf helper patch and `bpf_iter_run_prog` patch together for better
  bisectability in case of failures.
* Clean up formatting and comments in selftests.

Changes since v1:
* Fixed "Invalid wait context" issue in `bpf_iter_run_prog` by using
  `rcu_read_lock_trace()` for sleepable bpf iterator programs.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>