[dwarf] Handle DWARF forms for address other than DW_FORM_GNU_addr_index and DW_FORM_addr.

[dawn]

Summary

Other compilers (and older Clang compilers) use data4/data8 forms to specify addresses in DWARF. This patch fixes lldb to correctly read the addresses in DWARF generated by those compilers.

[tberghammer]

LGTM

(Next time please upload the diff with full context because it is easier to review that way)

[clayborg]

This is not the right fix. Compilers that emit addresses with DW_FORM_data* forms are saying that this address is an offset from the DW_AT_low_pc so this function will not return a valid address, but it will now return an offset. If you have a compiler that is actually emitting addresses incorrectly using DW_FORM_data* when the values are not offsets, the compiler should be fixed.

There are two solutions to this:
1 - modify DWARFFormValue::Address() to return LLDB_INVALID_ADDRESS if the form is not DW_FORM_addr or DW_FORM_GNU_addr_index.
2 - modify DWARFFormValue::Address() to take an lldb::addr_t that is the base address that the DW_FORM_data* would be relative to. This argument could be defaulted:

dw_addr_t
DWARFFormValue::Address(lldb::addr_t base_addr = LLDB_INVALID_ADDRESS) const

Then in the code we would do:

dw_addr_t
DWARFFormValue::Address(lldb::addr_t base_addr) const
{
    switch (m_form)
    {
    case DW_FORM_addr:
        return Unsigned();
    case DW_FORM_GNU_addr_index:
        if (m_cu)
        {
            SymbolFileDWARF* symbol_file = m_cu->GetSymbolFileDWARF();
            if (symbol_file)
            {
                uint32_t index_size = m_cu->GetAddressByteSize();
                dw_offset_t addr_base = m_cu->GetAddrBase();
                lldb::offset_t offset = addr_base + m_value.value.uval * index_size;
                return symbol_file->get_debug_addr_data().GetMaxU64(&offset, index_size);
            }
        }
        break;
     case DW_FORM_data1:
     case DW_FORM_data2:
     case DW_FORM_data4:
     case DW_FORM_data8:
     case DW_FORM_sdata:
     case DW_FORM_udata:
         if (base_addr != LLDB_INVALID_ADDRESS)
            return base_addr + Unsigned();
    default:
        break;
    }
    return LLDB_INVALID_ADDRESS;
}

[dawn]

If you have a compiler that is actually emitting addresses incorrectly using DW_FORM_data* when the values are not offsets, the compiler should be fixed.

Thanks Greg! In this case it indeed appears to be a compiler bug (we are getting relocations emitted for them). But I like your suggestion of how to rewrite the Address function. If any valid cases arise, I will use your idea, but instead create an overload which takes a parameter (that way only the DWARF contexts in which addresses are represented using data4/data8 will have to pay the cost of the added code).

[tberghammer]

The feature Greg mentioned is already implemented in DWARFDebugInfoEntry::GetAttributeHighPC and we shouldn't move it to DWARFFormValue::Address because it is only needed for DW_AT_high_pc but DWARFFormValue::Address used for other places as well (e.g. DW_AT_low_pc).

I think what we are seeing isn't a compiler bug, but I need a little bit more information to be sure.

How you end up in calling DWARFFormValue::Address() with a value what have a form type other then DW_FORM_addr and DW_FORM_GNU_addr_index? What is the attribute tag and the form type used? Can you post a call stack for the case when you hit this issue?

[dawn]

In D14631#289917, @tberghammer wrote:

How you end up in calling DWARFFormValue::Address() with a value what have a form type other then DW_FORM_addr and DW_FORM_GNU_addr_index? What is the attribute tag and the form type used? Can you post a call stack for the case when you hit this issue?

We see it in the comp unit's low_pc:

0x000b:  DW_TAG_compile_unit    : abbrev_num=1
0x000c:  DW_AT_producer         : DW_FORM_strp       string=Embarcadero Technologies Inc. clang version 3.1 (33931.6d56f68.9eae00b) (based on LLVM 3.3svn)
0x0010:  DW_AT_language         : DW_FORM_data2      uval=0x4
         language=DW_LANG_C_plus_plus
0x0012:  DW_AT_name             : DW_FORM_strp       string=cprops.cpp
0x0016:  DW_AT_low_pc           : DW_FORM_data4      uval=0x0

Address is called at the beginning of reading the CU's DIE.

[clayborg]

So yes this is a compiler bug where the compiler is producing invalid DWARF.

source/Plugins/SymbolFile/DWARF/DWARFFormValue.cpp

 {
     SymbolFileDWARF* symbol_file = m_cu->GetSymbolFileDWARF();
 
-    if (m_form == DW_FORM_addr)
+    if (m_form != DW_FORM_GNU_addr_index)
         return Unsigned();
 
     assert(m_cu);
-    assert(m_form == DW_FORM_GNU_addr_index);
 
     if (!symbol_file)
         return 0;