Add initial code emission for *direct* jumps for tail call maps in
order to avoid the retpoline overhead from a493a87f ("bpf, x64:
implement retpoline for tail call") for situations that allow for
it, meaning, for known constant keys at verification time which are
used as index into the tail call map. In case of Cilium which makes
heavy use of tail calls, constant keys are used in the vast majority,
only for a single occurrence we use a dynamic key.

High level outline is that if the target prog is NULL in the map, we
emit a 5-byte nop for the fall-through case and if not, we emit a
5-byte direct relative jmp to the target bpf_func + skipped prologue
offset. Later during runtime, we patch these 5-byte nop/jmps upon
tail call map update or deletions dynamically. Note that on x86-64
the direct jmp works as we reuse the same stack frame and skip
prologue (as opposed to some other JIT implementations).

One of the issues is that the tail call map slots can change at any
given time even during JITing. Therefore, we have two passes: i) emit
nops for all patchable locations during main JITing phase until we
declare prog->jited = 1 eventually. At this point the image is stable,
not public yet and with all jmps disabled. While JITing, we collect
additional info like poke->ip in order to remember the patch location
for later modifications. In ii) bpf_tail_call_direct_fixup() walks
over the progs poke_tab, locks the tail call maps poke_mutex to
prevent from parallel updates and patches in the right locations via
__bpf_arch_text_poke(). Note, the main bpf_arch_text_poke() cannot
be used at this point since we're not yet exposed to kallsyms. For
the update we use plain memcpy() since the image is not public and
still in read-write mode. After patching, we activate that poke entry
through poke->ip_stable. Meaning, at this point any tail call map
updates/deletions are not going to ignore that poke entry anymore.
Then, bpf_arch_text_poke() might still occur on the read-write image
until we finally locked it as read-only. Both modifications on the
given image are under text_mutex to avoid interference with each
other when update requests come in in parallel for different tail
call maps (current one we have locked in JIT and different one where
poke->ip_stable was already set).

Example prog:

  # ./bpftool p d x i 1655
   0: (b7) r3 = 0
   1: (18) r2 = map[id:526]
   3: (85) call bpf_tail_call#12
   4: (b7) r0 = 1
   5: (95) exit

Before:

  # ./bpftool p d j i 1655
  0xffffffffc076e55c:
   0:   nopl   0x0(%rax,%rax,1)
   5:   push   %rbp
   6:   mov    %rsp,%rbp
   9:   sub    $0x200,%rsp
  10:   push   %rbx
  11:   push   %r13
  13:   push   %r14
  15:   push   %r15
  17:   pushq  $0x0                      _
  19:   xor    %edx,%edx                |_ index (arg 3)
  1b:   movabs $0xffff88d95cc82600,%rsi |_ map (arg 2)
  25:   mov    %edx,%edx                |  index >= array->map.max_entries
  27:   cmp    %edx,0x24(%rsi)          |
  2a:   jbe    0x0000000000000066       |_
  2c:   mov    -0x224(%rbp),%eax        |  tail call limit check
  32:   cmp    $0x20,%eax               |
  35:   ja     0x0000000000000066       |
  37:   add    $0x1,%eax                |
  3a:   mov    %eax,-0x224(%rbp)        |_
  40:   mov    0xd0(%rsi,%rdx,8),%rax   |_ prog = array->ptrs[index]
  48:   test   %rax,%rax                |  prog == NULL check
  4b:   je     0x0000000000000066       |_
  4d:   mov    0x30(%rax),%rax          |  goto *(prog->bpf_func + prologue_size)
  51:   add    $0x19,%rax               |
  55:   callq  0x0000000000000061       |  retpoline for indirect jump
  5a:   pause                           |
  5c:   lfence                          |
  5f:   jmp    0x000000000000005a       |
  61:   mov    %rax,(%rsp)              |
  65:   retq                            |_
  66:   mov    $0x1,%eax
  6b:   pop    %rbx
  6c:   pop    %r15
  6e:   pop    %r14
  70:   pop    %r13
  72:   pop    %rbx
  73:   leaveq
  74:   retq

After; state after JIT:

  # ./bpftool p d j i 1655
  0xffffffffc08e8930:
   0:   nopl   0x0(%rax,%rax,1)
   5:   push   %rbp
   6:   mov    %rsp,%rbp
   9:   sub    $0x200,%rsp
  10:   push   %rbx
  11:   push   %r13
  13:   push   %r14
  15:   push   %r15
  17:   pushq  $0x0                      _
  19:   xor    %edx,%edx                |_ index (arg 3)
  1b:   movabs $0xffff9d8afd74c000,%rsi |_ map (arg 2)
  25:   mov    -0x224(%rbp),%eax        |  tail call limit check
  2b:   cmp    $0x20,%eax               |
  2e:   ja     0x000000000000003e       |
  30:   add    $0x1,%eax                |
  33:   mov    %eax,-0x224(%rbp)        |_
  39:   jmpq   0xfffffffffffd1785       |_ [direct] goto *(prog->bpf_func + prologue_size)
  3e:   mov    $0x1,%eax
  43:   pop    %rbx
  44:   pop    %r15
  46:   pop    %r14
  48:   pop    %r13
  4a:   pop    %rbx
  4b:   leaveq
  4c:   retq

After; state after map update (target prog):

  # ./bpftool p d j i 1655
  0xffffffffc08e8930:
   0:   nopl   0x0(%rax,%rax,1)
   5:   push   %rbp
   6:   mov    %rsp,%rbp
   9:   sub    $0x200,%rsp
  10:   push   %rbx
  11:   push   %r13
  13:   push   %r14
  15:   push   %r15
  17:   pushq  $0x0
  19:   xor    %edx,%edx
  1b:   movabs $0xffff9d8afd74c000,%rsi
  25:   mov    -0x224(%rbp),%eax
  2b:   cmp    $0x20,%eax               .
  2e:   ja     0x000000000000003e       .
  30:   add    $0x1,%eax                .
  33:   mov    %eax,-0x224(%rbp)        |_
  39:   jmpq   0xffffffffffb09f55       |_ goto *(prog->bpf_func + prologue_size)
  3e:   mov    $0x1,%eax
  43:   pop    %rbx
  44:   pop    %r15
  46:   pop    %r14
  48:   pop    %r13
  4a:   pop    %rbx
  4b:   leaveq
  4c:   retq

After; state after map update (no prog):

  # ./bpftool p d j i 1655
  0xffffffffc08e8930:
   0:   nopl   0x0(%rax,%rax,1)
   5:   push   %rbp
   6:   mov    %rsp,%rbp
   9:   sub    $0x200,%rsp
  10:   push   %rbx
  11:   push   %r13
  13:   push   %r14
  15:   push   %r15
  17:   pushq  $0x0
  19:   xor    %edx,%edx
  1b:   movabs $0xffff9d8afd74c000,%rsi
  25:   mov    -0x224(%rbp),%eax
  2b:   cmp    $0x20,%eax               .
  2e:   ja     0x000000000000003e       .
  30:   add    $0x1,%eax                .
  33:   mov    %eax,-0x224(%rbp)        |_
  39:   nopl   0x0(%rax,%rax,1)         |_ fall-through nop
  3e:   mov    $0x1,%eax
  43:   pop    %rbx
  44:   pop    %r15
  46:   pop    %r14
  48:   pop    %r13
  4a:   pop    %rbx
  4b:   leaveq
  4c:   retq

Nice bonus is that this also shrinks the code emission quite a bit
for every tail call invocation.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/6ada4c1c9d35eeb5f4ecfab94593dafa6b5c4b09.1574452833.git.daniel@iogearbox.net