Merge branch 'bpf-allow-void-return-type-for-global-subprogs'

Emil Tsalapatis says:

====================
bpf: Allow void return type for global subprogs

Global subprogs are currently not allowed to return void. Adjust
verifier logic to allow global functions with a void return type.
Exception callbacks are excluded from this change, and still require
a scalar return type.

Adding additional logic for void globals significantly complicates
check_return_code(), mainly because of how the new feature interacts
with bpf_throw(). To avoid complicating the code, refactor the return
value range validation logic into two separate functions, one for
program exits through BPF_EXIT and one for program exits through
bpf_throw(). The changes are non-functional.

Also adjust existing selftests that were ensuring that global subprogs
with void return types fail to verify. These tests should now load
successfully.

Patches 1-3 refactor check_return_code into two separate functions, one
for BPF_EXIT and one for bpf_throw(). No functional changes.

Patches 4-5 enable void globals in the verifier and add additional
selftests.

v5 -> v6
(https://lore.kernel.org/bpf/20260227154616.6846-1-emil@etsalapatis.com/)

Patch 1:

- Removed unused variables (kernel test robot)
- Add break; on empty default to suppress warning (kernel test robot)
- Added explanation on why change in switch statement for
  BPF_PROG_TYPE_TRACING in check_return_code is a no-op

Patches 2/3:

- Added From: tags to Eduard's patches and changed my Reviewed-by to
  Signed-off-by to properly denote patch route (submitting-patches.rst)
- Renamed check_subprogram_return_code for into check_global_subprog_return_code
  (Eduard)

Patch 4:

- Remove unnecessary subprog_is_global() check in check_return_code (Eduard)
- Fix indentation on patch 4 (AI)

Patch 5:

- Fix unused data in selftests (Eduard)

v4 -> v5
(https://lore.kernel.org/bpf/20260225033356.518313-1-emil@etsalapatis.com/)
- Split value validation logic from throw/nonthrow to main prog/subprog (Eduard)
- Turn return_retval_range() to bool and incorporate return_32bit in retval (Eduard)
- Adjust if-else chain in return_retval_range for cgroup programs into switch
  statement to bring it in line with the other program types

v3 -> v4
(https://lore.kernel.org/bpf/20260223215046.1706110-1-emil@etsalapatis.com/)
- Factor out bpf_throw() return value validation logic into its own
  function (Eduard)
- Validate the range of bpf_throw() return values from void globals (Eduard)
- Move new selftest to verifier_global_subprogs.c (Eduard)
- Remove unnecessary verifier_bug_if() statements (Eduard)
- Add new test to ensure void globals can call bpf_throw

v2 -> v3
(https://lore.kernel.org/bpf/20260210183257.217285-1-emil@etsalapatis.com/)
- Allow freplace for global programs that return void and add selftests (Kumar)

v1 -> v2
(https://lore.kernel.org/bpf/20260127231414.359283-1-emil@etsalapatis.com/)

- Only mark R0 as valid for non-void global functions (AI)
- Rename subprog_is_void to subprog_returns_void (Kumar)
- Add test that R0 is invalid after calling a void global (Alexei)
- Add check to ensure exception callbacks do not return void
and associated selftest

Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
====================

Link: https://patch.msgid.link/20260228184759.108145-1-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>