[PATCH bpf-next v2 3/7] bpf: Introduce BPF_MODIFY_RETURN

[Andrii Nakryiko]

On Tue, Mar 3, 2020 at 5:56 PM KP Singh <kpsingh at chromium.org> wrote:
>
> From: KP Singh <kpsingh at google.com>
>
> When multiple programs are attached, each program receives the return
> value from the previous program on the stack and the last program
> provides the return value to the attached function.
>
> The fmod_ret bpf programs are run after the fentry programs and before
> the fexit programs. The original function is only called if all the
> fmod_ret programs return 0 to avoid any unintended side-effects. The
> success value, i.e. 0 is not currently configurable but can be made so
> where user-space can specify it at load time.
>
> For example:
>
> 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
>
> The fmod_ret program attached to this function can be defined as:
>
> SEC("fmod_ret/func_to_be_attached")
> int BPF_PROG(func_name, int a, int b, int ret)
> {
>         // This will skip the original function logic.
>         return 1;
> }
>
> The first fmod_ret program is passed 0 in its return argument.
>
> Signed-off-by: KP Singh <kpsingh at google.com>
> ---
>  arch/x86/net/bpf_jit_comp.c    | 130 ++++++++++++++++++++++++++++++---
>  include/linux/bpf.h            |   1 +
>  include/uapi/linux/bpf.h       |   1 +
>  kernel/bpf/btf.c               |   3 +-
>  kernel/bpf/syscall.c           |   1 +
>  kernel/bpf/trampoline.c        |   5 +-
>  kernel/bpf/verifier.c          |   1 +
>  tools/include/uapi/linux/bpf.h |   1 +
>  8 files changed, 130 insertions(+), 13 deletions(-)
>

This looks good, but I'll Alexei check all the assembly generation
logic, not too big of an expert on that.

[...]


>  static int emit_fallback_jump(u8 **pprog)
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 98ec10b23dbb..3cfdc216a2f4 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -473,6 +473,7 @@ void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start);
>
>  enum bpf_tramp_prog_type {
>         BPF_TRAMP_FENTRY,
> +       BPF_TRAMP_MODIFY_RETURN,

This is probably bad idea to re-number BPF_TRAMP_FEXIT for no good
reason. E.g., if there are some drgn scripts that do some internal
state printing, this is major inconvenience, while really providing no
benefit in itself. Consider putting it right before BPF_TRAMP_MAX.

>         BPF_TRAMP_FEXIT,
>         BPF_TRAMP_MAX,
>         BPF_TRAMP_REPLACE, /* more than MAX */

[...]