Merge branch 'bpfilter'

Alexei Starovoitov says:

====================
bpfilter

v2->v3:
- followed Luis's suggestion and significantly simplied first patch
  with shmem_kernel_file_setup+kernel_write. Added kdoc for new helper
- fixed typos and race to access pipes with mutex
- tested with bpfilter being 'builtin'. CONFIG_BPFILTER_UMH=y|m both work.
  Interesting to see a usermode executable being embedded inside vmlinux.
- it doesn't hurt to enable bpfilter in .config.
  ip_setsockopt commands sent to usermode via pipes and -ENOPROTOOPT is
  returned from userspace, so kernel falls back to original iptables code

v1->v2:
this patch set is almost a full rewrite of the earlier umh modules approach
The v1 of patches and follow up discussion was covered by LWN:
https://lwn.net/Articles/749108/

I believe the v2 addresses all issues brought up by Andy and others.
Mainly there are zero changes to kernel/module.c
Instead of teaching module loading logic to recognize special
umh module, let normal kernel modules execute part of its own
.init.rodata as a new user space process (Andy's idea)
Patch 1 introduces this new helper:
int fork_usermode_blob(void *data, size_t len, struct umh_info *info);
Input:
  data + len == executable file
Output:
  struct umh_info {
       struct file *pipe_to_umh;
       struct file *pipe_from_umh;
       pid_t pid;
  };

Advantages vs v1:
- the embedded user mode executable is stored as .init.rodata inside
  normal kernel module. These pages are freed when .ko finishes loading
- the elf file is copied into tmpfs file. The user mode process is swappable.
- the communication between user mode process and 'parent' kernel module
  is done via two unix pipes, hence protocol is not exposed to
  user space
- impossible to launch umh on its own (that was the main issue of v1)
  and impossible to be man-in-the-middle due to pipes
- bpfilter.ko consists of tiny kernel part that passes the data
  between kernel and umh via pipes and much bigger umh part that
  doing all the work
- 'lsmod' shows bpfilter.ko as usual.
  'rmmod bpfilter' removes kernel module and kills corresponding umh
- signed bpfilter.ko covers the whole image including umh code

Few issues:
- the user can still attach to the process and debug it with
  'gdb /proc/pid/exe pid', but 'gdb -p pid' doesn't work.
  (a bit worse comparing to v1)
- tinyconfig will notice a small increase in .text
  +766 | TEXT | 7c8b94806bec umh: introduce fork_usermode_blob() helper
====================

Signed-off-by: David S. Miller <davem@davemloft.net>