[bpf-next,0/2] Fix libbpf BPF skeleton forward/backward compat

Message

Andrii Nakryiko July 4, 2024, 12:15 a.m. UTC

Fix recently identified (but long standing) bug with handling BPF skeleton
forward and backward compatibility. On libbpf side, even though BPF skeleton
was always designed to be forward and backwards compatible through recording
actual size of constrituents of BPF skeleton itself (map/prog/var skeleton
definitions), libbpf implementation did implicitly hard-code those sizes by
virtue of using a trivial array access syntax.

This issue will only affect libbpf used as a shared library. Statically
compiled libbpfs will always be in sync with BPF skeleton, bypassing this
problem altogether.

This patch set fixes libbpf, but also mitigates the problem for old libbpf
versions by teaching bpftool to generate more conservative BPF skeleton,
if possible (i.e., if there are no struct_ops maps defined).

Andrii Nakryiko (2):
  bpftool: improve skeleton backwards compat with old buggy libbpfs
  libbpf: fix BPF skeleton forward/backward compat handling

 tools/bpf/bpftool/gen.c | 46 ++++++++++++++++++++++++++++------------
 tools/lib/bpf/libbpf.c  | 47 +++++++++++++++++++++++------------------
 2 files changed, 59 insertions(+), 34 deletions(-)