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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Mar 3, 2020 at 6:12 AM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> 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@chromium.org/
>
> 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)

nit: int BPF_PROG(...)


> {
>         // 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
>