Re: [PATCH bpf-next v4 0/4] bpf: Fix tailcall infinite loop caused by freplace

[Eduard Zingerman <eddyz87@gmail.com>]

On Sun, 2024-09-29 at 21:27 +0800, Leon Hwang wrote:
> Previously, I fixed a tailcall infinite loop issue caused by trampoline[0].
> 
> At this time, I fix a tailcall infinite loop issue caused by tailcall and
> freplace combination by preventing updating extended prog to prog_array map
> and preventing extending tail callee prog with freplace:
> 
> 1. If a prog or its subprog has been extended by freplace prog, the prog
>    can not be updated to prog_array map.
> 2. If a prog has been updated to prog_array map, it or its subprog can not
>    be extended by freplace prog.

So, once this series is applied we essentially have:
- three variables:
  - tgt_prog->aux->is_extended
  - tgt_prog->aux->prog_array_member_cnt
  - trampoline->extension_prog
- four operations:
  - link/attach extension program 'prog' using trampoline 'tr'
  - unlink/detach extension program 'prog' using trampoline 'tr'
  - put program 'tgt_prog' into prog array
  - remove program 'tgt_prog' from prog array

And above four operations have the following pseudo-code with regards
to update of the variables:

- link/attach extension program 'prog' using trampoline 'tr':

    with lock(tgt_prog->ext_mutex):
      if tgt_prog->aux->prog_array_member_cnt:
         return error
      if tr->extension_prog:
         return error
      tr->extension_prog = prog
      tgt_prog->is_extended = true

- unlink/detach extension program 'prog' using trampoline 'tr':

    with lock(tgt_prog->ext_mutex):
      tr->extension_prog = NULL
      tgt_prog->is_extended = false

- put program 'tgt_prog' into prog array:

    with lock(tgt_prog->ext_mutex):
      if tgt_prog->aux->is_extended:
         return error
      tgt_prog->aux->prog_array_member_cnt++

- remove program 'tgt_prog' from prog array:

    with lock(tgt_prog->ext_mutex):
      tgt_prog->aux->prog_array_member_cnt--

I think this is correct, would be great if someone with more
concurrency related experience would take a look.

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