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

[Emil Tsalapatis <emil@etsalapatis.com>]

Modify libbpf to place arena globals in a small offset inside the arena
mapping instead of at 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 16 pages into 
the arena mapping. If this move is impossible, libbpf tries progressively
smaller increments, and finally defaults to 0 if there is not enough
space in the arena. At load time, libbpf adjusts each symbol's location
within the arena by that offset. The patchset also adds padding to the 
BPF skeleton struct arena datasec to ensure the arena's fields are 
pointing in the right locations within the mapping.

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

Emil Tsalapatis (4):
  selftests/bpf: explicitly account for globals in verifier_arena_large
  libbpf: add stub for offset-related skeleton padding
  libbpf: offset global arena data into the arena if possible
  selftests/bpf: add tests for the arena offset of globals

 tools/bpf/bpftool/gen.c                       | 23 ++++++-
 tools/lib/bpf/libbpf.c                        | 36 ++++++++++-
 tools/lib/bpf/libbpf.h                        |  9 +++
 tools/lib/bpf/libbpf.map                      |  1 +
 .../selftests/bpf/prog_tests/verifier.c       |  6 ++
 .../bpf/progs/verifier_arena_globals1.c       | 60 ++++++++++++++++++
 .../bpf/progs/verifier_arena_globals2.c       | 49 +++++++++++++++
 .../bpf/progs/verifier_arena_globals3.c       | 61 +++++++++++++++++++
 .../bpf/progs/verifier_arena_large.c          | 25 ++++++--
 9 files changed, 261 insertions(+), 9 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
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_arena_globals3.c

-- 
2.49.0