Merge branch 'bpf_modify_ret'

KP Singh says:

====================
v3 -> v4:

* Fix a memory leak noticed by Daniel.

v2 -> v3:

* bpf_trampoline_update_progs -> bpf_trampoline_get_progs + const
  qualification.
* Typos in commit messages.
* Added Andrii's Acks.

v1 -> v2:

* Adressed Andrii's feedback.
* Fixed a bug that Alexei noticed about nop generation.
* Rebase.

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

Signed-off-by: Alexei Starovoitov <ast@kernel.org>