Merge branch 'minimize-annotations-for-arena-programs'

Emil Tsalapatis says:

====================
Minimize annotations for arena programs

BPF programs must currently include code to address two limitations
of function signatures that include arena types. First, arena arguments
must be annotated with __arg_arena in the function signature in addition
to __arena. Second, it is currently not allowed to return an arena pointer
from a subprog, even though it is safe to do so. These limitations require
extra annotations and typecasts respectively, and have proven sources of
confusion to programmers.

The patchset improves arena-related function signatures in two ways.
First, it removes the need for __arg_arena in function signatures.
Second, it allows subprogs to directly return arena pointers to their
caller.

To do this we add a new type tag to the existing __arena annotation.
The annotation is currently an alias for __attribute__((address_space(1))),
which is not discoverable from BTF alone and so cannot be used to
determine whether a pointer variable is an arena pointer during
verification. With the new type tag, we can determine whether
either the arguments and or the return value of a function belong
in an arena.

We test the new code by modifying libarena to take advantage of these
relaxed limitations.

CHANGELOG
=========

v2 -> v3 (https://lore.kernel.org/bpf/20260530002259.4505-1-emil@etsalapatis.com/)

- Added Acks by Eduard
- Complete the __arg_arena removal by removing them from htab (Alexei)
- Add a test in verifier_arena_globals1.c to confirm the new __arena attribute
works as expected in function argument and return types
- Reject type tags on non-pointer types, currently only possible in handcrafted
  BTF (Eduard)
- Undo inaccurate change on verifier comment (AI)
- Fix error return value for invalid BTF return types during BTF parsing (Eduard)

v1 -> v2 (lore.kernel.org/bpf/20260527071457.4598-1-emil@etsalapatis.com/)

- Rebased to fix conflict
- Removed the typedef foo * foo_t typedefs. Those were necessary to avoid
annotating each instance of the type with __arena. The new version of the
patch instead removes typedefs and uses __arena everywhere directly (see
patch 4/5 for more details).
- Reorganized the patchset to frontload all kernel-side changes and place
the libarena changes at the end.
====================

Link: https://patch.msgid.link/20260602004120.17087-1-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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