[PATCH bpf-next 0/7] Introduce BPF_MODIFY_RET tracing progs.

[KP Singh]

From: KP Singh <kpsingh@xxxxxxxxxx>

This was brought up in the KRSI v4 discussion and found to be useful
both for security and tracing programs.

  https://lore.kernel.org/bpf/20200225193108.GB22391@xxxxxxxxxxxx/

The modify_return programs are allowed for security hooks (with an
extra CAP_MAC_ADMIN check) and functions whitelisted for error
injection (ALLOW_ERROR_INJECTION).

The "security_" check is expected to be cleaned up with the KRSI patch
series.

Here is an example of how a fmod_ret program behaves:

int func_to_be_attached(int a, int b)
{  <--- do_fentry

do_fmod_ret:
   <update ret by calling fmod_ret>
   if (ret != 0)
        goto do_fexit;

original_function:

    <side_effects_happen_here>

}  <--- do_fexit

ALLOW_ERROR_INJECTION(func_to_be_attached, ERRNO)

The fmod_ret program attached to this function can be defined as:

SEC("fmod_ret/func_to_be_attached")
BPF_PROG(func_name, int a, int b, int ret)
{
        // This will skip the original function logic.
        return -1;
}

KP Singh (7):
  bpf: Refactor trampoline update code
  bpf: JIT helpers for fmod_ret progs
  bpf: Introduce BPF_MODIFY_RETURN
  bpf: Attachment verification for BPF_MODIFY_RETURN
  tools/libbpf: Add support for BPF_MODIFY_RETURN
  bpf: Add test ops for BPF_PROG_TYPE_TRACING
  bpf: Add selftests for BPF_MODIFY_RETURN

 arch/x86/net/bpf_jit_comp.c                   | 261 +++++++++++++-----
 include/linux/bpf.h                           |  24 +-
 include/uapi/linux/bpf.h                      |   1 +
 kernel/bpf/bpf_struct_ops.c                   |  13 +-
 kernel/bpf/btf.c                              |  27 +-
 kernel/bpf/syscall.c                          |   1 +
 kernel/bpf/trampoline.c                       |  66 +++--
 kernel/bpf/verifier.c                         |  32 +++
 kernel/trace/bpf_trace.c                      |   1 +
 net/bpf/test_run.c                            |  57 +++-
 tools/include/uapi/linux/bpf.h                |   1 +
 tools/lib/bpf/libbpf.c                        |   4 +
 .../selftests/bpf/prog_tests/fentry_fexit.c   |  12 +-
 .../selftests/bpf/prog_tests/fentry_test.c    |  14 +-
 .../selftests/bpf/prog_tests/fexit_test.c     |  69 ++---
 .../selftests/bpf/prog_tests/modify_return.c  |  65 +++++
 .../selftests/bpf/progs/modify_return.c       |  49 ++++
 17 files changed, 509 insertions(+), 188 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/modify_return.c
 create mode 100644 tools/testing/selftests/bpf/progs/modify_return.c

-- 
2.20.1