bpf: Introduce BPF_MODIFY_RETURN

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@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200304191853.1529-4-kpsingh@chromium.org

1 files changed, 1 insertions, 0 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 289383edfc8c..2460c8e6b5be 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9950,6 +9950,7 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
 		if (!prog_extension)
 			return -EINVAL;
 		/* fallthrough */
+	case BPF_MODIFY_RETURN:
 	case BPF_TRACE_FENTRY:
 	case BPF_TRACE_FEXIT:
 		if (!btf_type_is_func(t)) {