[bpf] Fix memory offset check for loads and stores

[ast]

Summary

If the offset cannot fit into the instruction, an addition to the
pointer is emitted before the actual access. However, BPF offsets are
16-bit but LLVM considers them to be, for the matter of this check,
to be 32-bit long.

This causes the following program:

int bpf_prog1(void *ign)
{

volatile unsigned long t = 0x8983984739ull;
return *(unsigned long *)((0xffffffff8fff0002ull) + t);

}

To generate the following (wrong) code:

0: 18 01 00 00 39 47 98 83 00 00 00 00 89 00 00 00

r1 = 590618314553ll

2: 7b 1a f8 ff 00 00 00 00 *(u64 *)(r10 - 8) = r1
3: 79 a1 f8 ff 00 00 00 00 r1 = *(u64 *)(r10 - 8)
4: 79 10 02 00 00 00 00 00 r0 = *(u64 *)(r1 + 2)
5: 95 00 00 00 00 00 00 00 exit

Fix it by changing the offset check to 16-bit.

lib/Target/BPF/BPFISelDAGToDAG.cpp

bool BPFDAGToDAGISel::SelectAddr(SDValue Addr, SDValue &Base, SDValue &Offset) {
 
   if (Addr.getOpcode() == ISD::TargetExternalSymbol ||
       Addr.getOpcode() == ISD::TargetGlobalAddress)
     return false;
 
   // Addresses of the form Addr+const or Addr|const
   if (CurDAG->isBaseWithConstantOffset(Addr)) {
     ConstantSDNode *CN = dyn_cast<ConstantSDNode>(Addr.getOperand(1));
-    if (isInt<32>(CN->getSExtValue())) {
+    if (isInt<16>(CN->getSExtValue())) {
 
       // If the first operand is a FI, get the TargetFI Node
       if (FrameIndexSDNode *FIN =
               dyn_cast<FrameIndexSDNode>(Addr.getOperand(0)))
         Base = CurDAG->getTargetFrameIndex(FIN->getIndex(), MVT::i64);
       else
         Base = Addr.getOperand(0);
 

 bool BPFDAGToDAGISel::SelectFIAddr(SDValue Addr, SDValue &Base, SDValue &Offset) {
   SDLoc DL(Addr);
 
   if (!CurDAG->isBaseWithConstantOffset(Addr))
     return false;
 
   // Addresses of the form Addr+const or Addr|const
   ConstantSDNode *CN = dyn_cast<ConstantSDNode>(Addr.getOperand(1));
-  if (isInt<32>(CN->getSExtValue())) {
+  if (isInt<16>(CN->getSExtValue())) {
 
     // If the first operand is a FI, get the TargetFI Node
     if (FrameIndexSDNode *FIN =
             dyn_cast<FrameIndexSDNode>(Addr.getOperand(0)))
       Base = CurDAG->getTargetFrameIndex(FIN->getIndex(), MVT::i64);
     else
       return false;