bpf: improve verifier ARG_CONST_SIZE_OR_ZERO semantics

For helpers, the argument type ARG_CONST_SIZE_OR_ZERO permits the
access size to be 0 when accessing the previous argument (arg).
Right now, it requires the arg needs to be NULL when size passed
is 0 or could be 0. It also requires a non-NULL arg when the size
is proved to be non-0.

This patch changes verifier ARG_CONST_SIZE_OR_ZERO behavior
such that for size-0 or possible size-0, it is not required
the arg equal to NULL.

There are a couple of reasons for this semantics change, and
all of them intends to simplify user bpf programs which
may improve user experience and/or increase chances of
verifier acceptance. Together with the next patch which
changes bpf_probe_read arg2 type from ARG_CONST_SIZE to
ARG_CONST_SIZE_OR_ZERO, the following two examples, which
fail the verifier currently, are able to get verifier acceptance.

Example 1:
   unsigned long len = pend - pstart;
   len = len > MAX_PAYLOAD_LEN ? MAX_PAYLOAD_LEN : len;
   len &= MAX_PAYLOAD_LEN;
   bpf_probe_read(data->payload, len, pstart);

It does not have test for "len > 0" and it failed the verifier.
Users may not be aware that they have to add this test.
Converting the bpf_probe_read helper to have
ARG_CONST_SIZE_OR_ZERO helps the above code get
verifier acceptance.

Example 2:
  Here is one example where llvm "messed up" the code and
  the verifier fails.

......
   unsigned long len = pend - pstart;
   if (len > 0 && len <= MAX_PAYLOAD_LEN)
     bpf_probe_read(data->payload, len, pstart);
......

The compiler generates the following code and verifier fails:
......
39: (79) r2 = *(u64 *)(r10 -16)
40: (1f) r2 -= r8
41: (bf) r1 = r2
42: (07) r1 += -1
43: (25) if r1 > 0xffe goto pc+3
  R0=inv(id=0) R1=inv(id=0,umax_value=4094,var_off=(0x0; 0xfff))
  R2=inv(id=0) R6=map_value(id=0,off=0,ks=4,vs=4095,imm=0) R7=inv(id=0)
  R8=inv(id=0) R9=inv0 R10=fp0
44: (bf) r1 = r6
45: (bf) r3 = r8
46: (85) call bpf_probe_read#45
R2 min value is negative, either use unsigned or 'var &= const'
......

The compiler optimization is correct. If r1 = 0,
r1 - 1 = 0xffffffffffffffff > 0xffe.  If r1 != 0, r1 - 1 will not wrap.
r1 > 0xffe at insn #43 can actually capture
both "r1 > 0" and "len <= MAX_PAYLOAD_LEN".
This however causes an issue in verifier as the value range of arg2
"r2" does not properly get refined and lead to verification failure.

Relaxing bpf_prog_read arg2 from ARG_CONST_SIZE to ARG_CONST_SIZE_OR_ZERO
allows the following simplied code:
   unsigned long len = pend - pstart;
   if (len <= MAX_PAYLOAD_LEN)
     bpf_probe_read(data->payload, len, pstart);

The llvm compiler will generate less complex code and the
verifier is able to verify that the program is okay.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>