[Dwarf-Discuss] Add file attributes to debug-line file table (proposal 130701.1)

Mark Wielaard mjw@redhat.com
Tue Mar 18 14:33:56 GMT 2014


I was reading the suggestion for adding MD5 digests to the .debug_line
program header. http://dwarfstd.org/ShowIssue.php?issue=130701.1

Adding more attributes of files seems like a good thing, but as
specified this isn't extensible without changing the version number
again and defining new formats. Would it be possible to make this a
little more generic and vendor extensible?

The .debug_macro proposal includes some language for defining
extensibility that could possibly be used.

So instead of a fixed file_entry_format (ubyte) field that just allows
one attribute (group) to be defined we could change it to something

file_attributes (ubyte)
  - Possibly zero, number of attributes added to each file in the
    file_names table.
file_attribute_description (sequence of attribute descriptions)
  - Entries in this description describe which attributes and in what
    format those attributes are encoded for each file entry. It has
    file_attributes entries. Each entry consists of:
    - file_attribute (ubyte). One of DW_LNF_timestamp,
      DW_LNF_length, DW_LNF_MD5, ...
    - file_attribute_format. A uleb128 describing the number of
      arguments for the format followed a single byte describing the
      form of the argument. The allowed values are DW_FORM_data1,
      DW_FORM_data2, DW_FORM_data4, DW_FORM_data8, DW_FORM_sdata, DW_FORM_udata,
      DW_FORM_block, DW_FORM_block1, DW_FORM_block2, DW_FORM_block4, DW_FORM_flag,
      DW_FORM_string, DW_FORM_strp and DW_FORM_sec_offset.

file_names (sequence of file entries)
  - Each entry consists of the following values:
    - A null-terminated string containing the full or relative path name
      of a source file.
    - An unsigned LEB128 number representing the directory index of a
      directory in the include_directories section.
    - For each file_attribute_format described in the
      file_attribute_description entries the value encoded in the format
      given by file_attribute_format.

For example DW_LNF_timestamp and DW_LNF_length would have as format 1
DW_FORM_udata. DW_LNF_MD5 could by described by 2 DW_FORM_data8.

That would allow extending the number of attributes in later DWARF
versions or as vendor extensions and give consumers a way to skip over
any unknown attributes.

Would something like the above suggestion be useful? Then I can work it
out a bit more. Did I miss any subtle corner cases? What is the status
of the .debug_macro proposal (110722.1)? It would be good to match the
descriptions of both extension mechanisms as much as possible.



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