[lldb] Handle jumping to the end in DW_OP_skip/DW_OP_bra

[werat]

Summary

DW_OP_skip/DW_OP_bra can move offset to the end of the data, which means that this was the last instruction to execute and the interpreter should terminate.

lldb/source/Expression/DWARFExpression.cpp

while (opcodes.ValidOffset(offset)) {
     // OPERANDS: int16_t
     // DESCRIPTION:  An unconditional branch. Its single operand is a 2-byte
     // signed integer constant. The 2-byte constant is the number of bytes of
     // the DWARF expression to skip forward or backward from the current
     // operation, beginning after the 2-byte constant.
     case DW_OP_skip: {
       int16_t skip_offset = (int16_t)opcodes.GetU16(&offset);
       lldb::offset_t new_offset = offset + skip_offset;
-      if (opcodes.ValidOffset(new_offset))
+      // New offset can point at the end of the data, in this case we should
+      // terminate the DWARF expression evaluation (will happen in the loop
+      // condition).
+      if (new_offset <= opcodes.GetByteSize())
         offset = new_offset;
       else {
         if (error_ptr)
-          error_ptr->SetErrorString("Invalid opcode offset in DW_OP_skip.");
+          error_ptr->SetErrorStringWithFormatv(
+              "Invalid opcode offset in DW_OP_skip: {0}+({1}) > {2}", offset,
+              skip_offset, opcodes.GetByteSize());
         return false;
       }
     } break;
 
     // OPCODE: DW_OP_bra
     // OPERANDS: int16_t
     // DESCRIPTION: A conditional branch. Its single operand is a 2-byte
     // signed integer constant. This operation pops the top of stack. If the
     // value popped is not the constant 0, the 2-byte constant operand is the
     // number of bytes of the DWARF expression to skip forward or backward from
     // the current operation, beginning after the 2-byte constant.
     case DW_OP_bra:
       if (stack.empty()) {
         if (error_ptr)
           error_ptr->SetErrorString(
               "Expression stack needs at least 1 item for DW_OP_bra.");
         return false;
       } else {
         tmp = stack.back();
         stack.pop_back();
         int16_t bra_offset = (int16_t)opcodes.GetU16(&offset);
         Scalar zero(0);
         if (tmp.ResolveValue(exe_ctx) != zero) {
           lldb::offset_t new_offset = offset + bra_offset;
-          if (opcodes.ValidOffset(new_offset))
+          // New offset can point at the end of the data, in this case we should
+          // terminate the DWARF expression evaluation (will happen in the loop
+          // condition).
+          if (new_offset <= opcodes.GetByteSize())
             offset = new_offset;
           else {
             if (error_ptr)
-              error_ptr->SetErrorString("Invalid opcode offset in DW_OP_bra.");
+              error_ptr->SetErrorStringWithFormatv(
+                  "Invalid opcode offset in DW_OP_bra: {0}+({1}) > {2}", offset,
+                  bra_offset, opcodes.GetByteSize());
             return false;
           }
         }
       }
       break;
 
     // OPCODE: DW_OP_eq
     // OPERANDS: none

lldb/unittests/Expression/DWARFExpressionTest.cpp

TEST(DWARFExpression, DW_OP_const) {
   EXPECT_THAT_EXPECTED(
       Evaluate({DW_OP_constu, 0x81, 0x82, 0x84, 0x88, 0x90, 0xa0, 0x40}),
       llvm::HasValue(0x01010101010101));
   EXPECT_THAT_EXPECTED(
       Evaluate({DW_OP_consts, 0x81, 0x82, 0x84, 0x88, 0x90, 0xa0, 0x40}),
       llvm::HasValue(0xffff010101010101));
 }
 
+TEST(DWARFExpression, DW_OP_skip) {
+  EXPECT_THAT_EXPECTED(Evaluate({DW_OP_const1u, 0x42, DW_OP_skip, 0x02, 0x00,
+                                 DW_OP_const1u, 0xff}),
+                       llvm::HasValue(0x42));
+}
+
+TEST(DWARFExpression, DW_OP_bra) {
+  EXPECT_THAT_EXPECTED(
+      // clang-format off
+      Evaluate({
+        DW_OP_const1u, 0x42,     // push 0x42
+        DW_OP_const1u, 0x1,      // push 0x1
+        DW_OP_bra, 0x02, 0x00,   // if 0x1 > 0, then skip 0x0002 opcodes
+        DW_OP_const1u, 0xff,     // push 0xff
+      }),
+      // clang-format on
+      llvm::HasValue(0x42));
+}
+
 TEST(DWARFExpression, DW_OP_convert) {
   /// Auxiliary debug info.
   const char *yamldata = R"(
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_EXEC