• KP Singh's avatar
    bpf: Introduce BPF_MODIFY_RETURN · ae240823
    KP Singh authored
    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: default avatarKP Singh <kpsingh@google.com>
    Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
    Acked-by: default avatarAndrii Nakryiko <andriin@fb.com>
    Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
    Link: https://lore.kernel.org/bpf/20200304191853.1529-4-kpsingh@chromium.org
    ae240823
bpf_jit_comp.c 51.4 KB