Fix TypeDesc.__str__ crash on certain DIEs

[celskeggs]

When my team attempted to upgrade from pyelftools==0.32 to pyelftools==0.33, we found that some of our binary inspection tools were broken by the upgrade. In particular, elftools.dwarf.datatype_cpp.describe_cpp_datatype crashes when fed certain inputs, such as:

DIE DW_TAG_variable, size=18, has_children=False
    |DW_AT_specification:  AttributeValue(name='DW_AT_specification', form='DW_FORM_ref4', value=97, raw_value=97, offset=332360, indirection_length=0)
    |DW_AT_location    :  AttributeValue(name='DW_AT_location', form='DW_FORM_exprloc', value=[161, 61, 148, 1, 52, 30, 48, 34, 159], raw_value=[161, 61, 148, 1, 52, 30, 48, 34, 159], offset=332364, indirection_length=0)
    |DW_AT_linkage_name:  AttributeValue(name='DW_AT_linkage_name', form='DW_FORM_strx2', value=b'_ZN9Va416x0Os12TimerRawTime10s_timer_hiE', raw_value=4207, offset=332374, indirection_length=0)

Here is the specific error we encountered:

File /workspaces/<...>/inspection.py:1014, in get_symbol_location_by_linkage(dwarfinfo, link_name, search_tag, verbose, follow_reference)
   1013 file_info_attr = variable_die.attributes.get("DW_AT_decl_file")
-> 1014 ctype = elftools.dwarf.datatype_cpp.describe_cpp_datatype(variable_die)
   1015 if str(ctype) == "None":

File /usr/local/lib/python3.12/site-packages/elftools/dwarf/datatype_cpp.py:27, in describe_cpp_datatype(var_die)
     26 def describe_cpp_datatype(var_die: DIE) -> str:
---> 27     return str(parse_cpp_datatype(var_die))

File /usr/local/lib/python3.12/site-packages/elftools/dwarf/datatype_cpp.py:156, in TypeDesc.__str__(self)
    151 def __str__(self) -> str:
    152     # Some reference points from dwarfdump:
    153     # const->pointer->const->char = const char *const
    154     # const->reference->const->int = const const int &
    155     # const->reference->int = const int &
--> 156     name = str(self.name)
    157     mods = self.modifiers

AttributeError: 'TypeDesc' object has no attribute 'name'

This appears likely to be related to this change:

v0.32...v0.33#diff-266d689a3956c69d57de4c617fb8e426c32469e6e7f4f0459df93d7fc6b5ae65L133-R151

Since TypeDesc.name is no longer being initialized to None, there are code paths where it does not get initialized before __str__ is called, which results in the AttributeError shown above. I propose to resume initializing the variable to None, which fixes the crash for us.

[eliben]

@pmhahn PTAL - this is likely related to the typing changes

[eliben]

The PR fails the type checker

However, simply initializing the name to the empty string should do the trick?

[celskeggs]

I would have to look closer... I can imagine that change would make the type checker pass but I think it would also change the behavior of __str__; it would no longer return the string "None" in this case. We'll probably stick with 0.32 for now as a workaround, though, so I'm not sure when I'll have the chance to dig more into this.

[pmhahn]

My cd4da4b is the relevant change, which no longer initializes self.name = None

Can you please provide a (minimal) sample of the ELF file, so we can add it to the test suite? It's probably enough if you can publish the C-code, which generates such a problematic DIE, so we can add the C code as a minimal example, compile it ourself and add the resulting ELF file to the repository.

[pmhahn]

This is required because of line 204, where the name: str | None are collected:

Suggested change

	self.scopes: tuple[str | None, ...] = ()