[bpf] Fix memory offset check for loads and stores

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.

Patch by Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Differential Revision: https://reviews.llvm.org/D32055

llvm-svn: 300269

llvm/lib/Target/BPF/BPFISelDAGToDAG.cpp

@@ -71,7 +71,7 @@ bool BPFDAGToDAGISel::SelectAddr(SDValue Addr, SDValue &Base, SDValue &Offset) {
   // 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 =
@@ -99,7 +99,7 @@ bool BPFDAGToDAGISel::SelectFIAddr(SDValue Addr, SDValue &Base, SDValue &Offset)
 
   // 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 =

llvm/test/CodeGen/BPF/mem_offset.ll

@@ -0,0 +1,17 @@
+; RUN: llc -march=bpfel -show-mc-encoding < %s | FileCheck %s
+
+; Function Attrs: nounwind
+define i32 @bpf_prog1(i8* nocapture readnone) local_unnamed_addr #0 {
+; CHECK: r1 += -1879113726 # encoding: [0x07,0x01,0x00,0x00,0x02,0x00,0xff,0x8f]
+; CHECK: r0 = *(u64 *)(r1 + 0) # encoding: [0x79,0x10,0x00,0x00,0x00,0x00,0x00,0x00]
+  %2 = alloca i64, align 8
+  %3 = bitcast i64* %2 to i8*
+  store volatile i64 590618314553, i64* %2, align 8
+  %4 = load volatile i64, i64* %2, align 8
+  %5 = add i64 %4, -1879113726
+  %6 = inttoptr i64 %5 to i64*
+  %7 = load i64, i64* %6, align 8
+  %8 = trunc i64 %7 to i32
+  ret i32 %8
+}
+