[Thumb] Validate branch target for CBZ/CBNZ instructions.

Summary:
The assembler currently does not check the branch target for CBZ/CBNZ
instructions, which only permit branching forwards with a positive offset. This
adds validation for the branch target to ensure negative PC-relative offsets are
not encoded into the instruction, whether specified as a literal or as an
assembler symbol.

Reviewers: rengolin, t.p.northover

Subscribers: llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D23312

llvm-svn: 278659

llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp

@@ -6684,6 +6684,12 @@ bool ARMAsmParser::validateInstruction(MCInst &Inst,
       return Error(Operands[Op]->getStartLoc(), "branch target out of range");
     break;
   }
+  case ARM::tCBZ:
+  case ARM::tCBNZ: {
+    if (!static_cast<ARMOperand &>(*Operands[2]).isUnsignedOffset<6, 1>())
+      return Error(Operands[2]->getStartLoc(), "branch target out of range");
+    break;
+  }
   case ARM::MOVi16:
   case ARM::t2MOVi16:
   case ARM::t2MOVTi16:

llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp

@@ -578,6 +578,11 @@ unsigned ARMAsmBackend::adjustFixupValue(const MCFixup &Fixup, uint64_t Value,
     // Offset by 4, and don't encode the low two bits.
     return ((Value - 4) >> 2) & 0xff;
   case ARM::fixup_arm_thumb_cb: {
+    // CB instructions can only branch to offsets in [0, 126] in multiples of 2
+    if (Ctx && ((int64_t)Value < 0 || Value > 0x3e || Value & 1)) {
+      Ctx->reportError(Fixup.getLoc(), "out of range pc-relative fixup value");
+      return 0;
+    }
     // Offset by 4 and don't encode the lower bit, which is always 0.
     // FIXME: diagnose if no Thumb2
     uint32_t Binary = (Value - 4) >> 1;

llvm/test/MC/ARM/thumb-cb-negative-offsets.s

@@ -0,0 +1,19 @@
+@ RUN: not llvm-mc -triple thumbv7m-none-eabi      -filetype=obj -o /dev/null %s 2>&1 | FileCheck %s
+@ RUN: not llvm-mc -triple thumbv8m.base-none-eabi -filetype=obj -o /dev/null %s 2>&1 | FileCheck %s
+
+label0:
+  .word 4
+
+@ CHECK: out of range pc-relative fixup value
+  cbz r0, label0
+@ CHECK: out of range pc-relative fixup value
+  cbnz r0, label0
+
+@ CHECK: out of range pc-relative fixup value
+  cbz r0, label1
+@ CHECK: out of range pc-relative fixup value
+  cbnz r0, label1
+
+  .space 1000
+label1:
+  .word 4

llvm/test/MC/ARM/thumb-diagnostics.s

@@ -234,6 +234,23 @@ error: invalid operand for instruction
 @ CHECK-ERRORS: error: branch target out of range
 @ CHECK-ERRORS: error: branch target out of range
 
+@------------------------------------------------------------------------------
+@ CBZ/CBNZ - out of range immediates for branches
+@------------------------------------------------------------------------------
+
+        cbz    r0, #-2
+        cbz    r0, #0
+        cbz    r0, #17
+        cbnz   r0, #126
+        cbnz   r0, #128
+
+@ CHECK-ERRORS-V7M: error: branch target out of range
+@ CHECK-ERRORS-V7M: error: invalid operand for instruction
+@ CHECK-ERRORS-V7M: error: branch target out of range
+@ CHECK-ERRORS-V8: error: branch target out of range
+@ CHECK-ERRORS-V8: error: invalid operand for instruction
+@ CHECK-ERRORS-V8: error: branch target out of range
+
 @------------------------------------------------------------------------------
 @ SEV/WFE/WFI/YIELD - are not supported pre v6M or v6T2
 @------------------------------------------------------------------------------