[PATCH v2 0/4] libbpf: move arena variables out of the zero

[Emil Tsalapatis <emil@etsalapatis.com>]

Modify libbpf to place arena globals in a small offset inside the arena
mapping instead of the very beginning. This allows programs to leave
the "zero page" of the arena unmapped, so that NULL arena pointer
dereferences trigger a page fault and associated backtrace in BPF streams.
In contrast, the current policy of placing global data in the zero pages
means that NULL dereferences silently corrupt global data, e.g, arena
qspinlock state. This makes arena bugs more difficult to debug.

The patchset adds code to libbpf to move global arena data to the end of
the arena. At load time, libbpf adjusts each symbol's location within
the arena to point to the right location in the arena. The patchset 
also adjusts the arena skeleton pointer to point to the arena globals,
now that they are not in the beginning of the arena region.

CHANGESET
=========

v1->v2: (https://lore.kernel.org/bpf/20251118030058.162967-1-emil@etsalapatis.com)

- Moved globals to the end of the mapping: (Andrii)
	- Removed extra parameter for offset and parameter picking logic
	- Removed padding in the skeleton
	- Removed additional libbpf call
- Added Reviewed-by from Eduard on patch 1

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>

Emil Tsalapatis (4):
  selftests/bpf: explicitly account for globals in verifier_arena_large
  bpf/verifier: do not limit maximum direct offset into arena map
  libbpf: move arena globals to the end of the arena
  selftests/bpf: add tests for the arena offset of globals

 kernel/bpf/verifier.c                         |  8 +--
 tools/lib/bpf/libbpf.c                        | 19 ++++--
 .../selftests/bpf/prog_tests/verifier.c       |  4 ++
 .../bpf/progs/verifier_arena_globals1.c       | 58 +++++++++++++++++++
 .../bpf/progs/verifier_arena_globals2.c       | 49 ++++++++++++++++
 .../bpf/progs/verifier_arena_large.c          | 21 +++++--
 6 files changed, 147 insertions(+), 12 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_arena_globals1.c
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_arena_globals2.c

-- 
2.49.0