Add tests to directly accesse sock_ops sk field. Then use it to
ensure a bad pointer access will fault if something goes wrong.
We do three tests:

The first test ensures when we read sock_ops sk pointer into the
same register that we don't fault as described earlier. Here r9
is chosen as the temp register.  The xlated code is,

  36: (7b) *(u64 *)(r1 +32) = r9
  37: (61) r9 = *(u32 *)(r1 +28)
  38: (15) if r9 == 0x0 goto pc+3
  39: (79) r9 = *(u64 *)(r1 +32)
  40: (79) r1 = *(u64 *)(r1 +0)
  41: (05) goto pc+1
  42: (79) r9 = *(u64 *)(r1 +32)

The second test ensures the temp register selection does not collide
with in-use register r9. Shown here r8 is chosen because r9 is the
sock_ops pointer. The xlated code is as follows,

  46: (7b) *(u64 *)(r9 +32) = r8
  47: (61) r8 = *(u32 *)(r9 +28)
  48: (15) if r8 == 0x0 goto pc+3
  49: (79) r8 = *(u64 *)(r9 +32)
  50: (79) r9 = *(u64 *)(r9 +0)
  51: (05) goto pc+1
  52: (79) r8 = *(u64 *)(r9 +32)

And finally, ensure we didn't break the base case where dst_reg does
not equal the source register,

  56: (61) r2 = *(u32 *)(r1 +28)
  57: (15) if r2 == 0x0 goto pc+1
  58: (79) r2 = *(u64 *)(r1 +0)

Notice it takes us an extra four instructions when src reg is the
same as dst reg. One to save the reg, two to restore depending on
the branch taken and a goto to jump over the second restore.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/159718355325.4728.4163036953345999636.stgit@john-Precision-5820-Tower