[RFC PATCH bpf-next 0/7] bpf: Implement BPF_LINK_UPDATE for tracing links

Implement update_prog for bpf_tracing_link_lops to enable
BPF_LINK_UPDATE for fentry, fexit, fmod_ret, freplace, etc. links.

My initial motivation for this was to enable a use case where one
process creates and owns links pointing to "hooks" within a tc, xdp, ...
attachment and an external "plugin" loads freplace programs and updates
links to these hooks. Aside from that though, it seemed like it could
be useful to be able to atomically swap out the program associated with
an freplace/fentry/fexit/fmod_ret link more generally.

Implementing program updates for freplace links was fairly
straightforward but proved more difficult for the other link types. The
third patch in this series discusses some other approaches I considered
before settling on the current approach, but I'd appreciate others'
input here to see if there is a better way to implement this that
doesn't require architecture-specific changes.

Thanks!

Jordan

Jordan Rife (7):
  bpf: Set up update_prog scaffolding for bpf_tracing_link_lops
  bpf: Enable BPF_LINK_UPDATE for freplace links
  bpf: Enable BPF_LINK_UPDATE for fentry/fexit/fmod_ret links
  bpf, x86: Make program update work for trampoline ops
  bpf, s390: Make program update work for trampoline ops
  bpf, arm64: Make program update work for trampoline ops
  selftests/bpf: Test BPF_LINK_UPDATE behavior for tracing links

 arch/arm64/net/bpf_jit_comp.c                 |  23 +-
 arch/s390/net/bpf_jit_comp.c                  |  24 +-
 arch/x86/net/bpf_jit_comp.c                   |  17 +-
 include/linux/bpf.h                           |  21 +
 kernel/bpf/syscall.c                          |  68 +++
 kernel/bpf/trampoline.c                       |  75 ++-
 .../bpf/prog_tests/prog_update_tracing.c      | 460 ++++++++++++++++++
 .../selftests/bpf/progs/prog_update_tracing.c | 133 +++++
 8 files changed, 796 insertions(+), 25 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/prog_update_tracing.c
 create mode 100644 tools/testing/selftests/bpf/progs/prog_update_tracing.c

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2.43.0