Merge branch 'Add support for kptrs in more BPF maps'

Kumar Kartikeya Dwivedi says:

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This set adds support for kptrs in percpu hashmaps, percpu LRU hashmaps,
and local storage maps (covering sk, cgrp, task, inode).

Tests are expanded to test more existing maps at runtime and also test
the code path for the local storage maps (which is shared by all
implementations).

A question for reviewers is what the position of the BPF runtime should
be on dealing with reference cycles that can be created by BPF programs
at runtime using this additional support. For instance, one can store
the kptr of the task in its own task local storage, creating a cycle
which prevents destruction of task local storage. Cycles can be formed
using arbitrarily long kptr ownership chains. Therefore, just preventing
storage of such kptrs in some maps is not a sufficient solution, and is
more likely to hurt usability.

There is precedence in existing runtimes which promise memory safety,
like Rust, where reference cycles and memory leaks are permitted.
However, traditionally the safety guarantees of BPF have been stronger.
Thus, more discussion and thought is invited on this topic to ensure we
cover all usage aspects.

Changelog:
----------
v2 -> v3
v2: https://lore.kernel.org/bpf/20230221200646.2500777-1-memxor@gmail.com/

 * Fix a use-after-free bug in local storage patch
 * Fix selftest for aarch64 (don't use fentry/fmod_ret)
 * Wait for RCU Tasks Trace GP along with RCU GP in selftest

v1 -> v2
v1: https://lore.kernel.org/bpf/20230219155249.1755998-1-memxor@gmail.com

 * Simplify selftests, fix a couple of bugs
====================

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

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