Question

我正在尝试将DW_AT_data_member_location属性值从矮化信息打印到get the member variables offset from a struct name，但似乎没有帮助函数。

以下是来自objdump的dwarf_info：

<1><1bf>: Abbrev Number: 5 (DW_TAG_structure_type)
<1c0>   DW_AT_name        : (indirect string, offset: 0xf0): class  
<1c4>   DW_AT_byte_size   : 208 
<1c5>   DW_AT_decl_file   : 1   
<1c6>   DW_AT_decl_line   : 10  
<1c7>   DW_AT_sibling     : <0x1f8> 
<2><1cb>: Abbrev Number: 6 (DW_TAG_member)
<1cc>   DW_AT_name        : (indirect string, offset: 0xc7): schools    
<1d0>   DW_AT_decl_file   : 1   
<1d1>   DW_AT_decl_line   : 11  
<1d2>   DW_AT_type        : <0x1f8> 
<1d6>   DW_AT_data_member_location: 2 byte block: 23 0  (DW_OP_plus_uconst: 0)
<2><1d9>: Abbrev Number: 6 (DW_TAG_member)
<1da>   DW_AT_name        : (indirect string, offset: 0xd8): size   
<1de>   DW_AT_decl_file   : 1   
<1df>   DW_AT_decl_line   : 12  
<1e0>   DW_AT_type        : <0x159> 
<1e4>   DW_AT_data_member_location: 3 byte block: 23 c8 1   (DW_OP_plus_uconst: 200)
<2><1e8>: Abbrev Number: 6 (DW_TAG_member)
<1e9>   DW_AT_name        : (indirect string, offset: 0xf6): record 
<1ed>   DW_AT_decl_file   : 1   
<1ee>   DW_AT_decl_line   : 13  
<1ef>   DW_AT_type        : <0x1b8> 
<1f3>   DW_AT_data_member_location: 3 byte block: 23 cc 1   (DW_OP_plus_uconst: 204)

我可以获取属性代码但不能获取属性值：

 if(dwarf_whatattr(attrs[i],&attrcode,&error) != DW_DLV_OK)
                      printf("Error");

 printf("Attrcode: %d\n",attrcode); // This one works

 if(attrcode==DW_AT_data_member_location) 
           dwarf_formudata(attrs[i],&offset,0) // This one does not work

此变量的类型是什么？如何获得它的价值？哪个辅助函数可以在这里工作？

Answer 1

DWARF调试格式允许将成员的偏移量表示为简单常量或需要计算的表达式，以便计算偏移量。无论出于何种原因，您尝试处理的调试信息都以表达式的形式表示了一个简单的常量偏移。您需要＆＃34;评估＆＃34;用于确定偏移量的表达式。像这样：

if (attrcode == DW_AT_data_member_location) {
    Dwarf_Half form;
    dwarf_whatform(attrs[i], &form, &error);
    if (form == DW_FORM_data1 || form == DW_FORM_data2
        form == DW_FORM_data2 || form == DW_FORM_data4
        form == DW_FORM_data8 || form == DW_FORM_udata) {
        dwarf_formudata(attrs[i], &offset, 0);
    } else if (form == DW_FORM_sdata) {
        Dwarf_Signed soffset;
        dwarf_formsdata(attrs[i], &soffset, 0);
        if (soffset < 0) {
             printf("unsupported negative offset\n");
             /* FAIL */
        }
        offset = (Dwarf_Unsigned) soffset;
    } else {
        Dwarf_Locdesc **locdescs;
        Dwarf_Signed len;
        if (dwarf_loclist_n(attrs[i], &locdescs, &len,  &error) == DW_DLV_ERROR) {
             printf("unsupported member offset\n");
             /* FAIL */
        }
        if (len != 1
            || locdescs[0]->ld_cents != 1
            || (locdescs[0]->ld_s[0]).lr_atom != DW_OP_plus_uconst) {
             printf("unsupported location expression\n");
             /* FAIL */
        }
        offset = (locdescs[0]->ld_s[0]).lr_number;
    }
}

Answer 2

如果你更喜欢使用JVM，我写了一个简单的library for parsing dwarf来创建一个基于EMF的模型。下面是打印struct member locations的示例代码：

def someFunction() {
  var elf = new Elf32Context(buffer)
  var dwarf = new Dwarf32Context(elf)
  var model = DwarfModelFactory::createModel(dwarf)
  model.eAllContents.filter(StructureType).forEach[it.dumpStruct]
}


def dumpStruct(CompositeType struct) '''
  «switch(struct) {StructureType:'struct' UnionType:'union' default:'composite?'}» «struct.typedef?.name» {
    «FOR m : struct.members»
        «m.type.dumpType» «m.name»  @«m.dataMemberLocation»
    «ENDFOR»
  }
'''

如何从矮化信息中获取struct成员偏移量？

2 个答案: