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

[Eric Christopher]

FWIW I fully agree with this line of reasoning. I was going to propose
it as well (though not as comperhensively) since we may decide that we
want to use something other than "the low 64-bits of an md5 hash" to
represent the file. Speaking of which, the particular hash and such
should be explicitly changeable in the header.

-eric

On Tue, Mar 18, 2014 at 7:33 AM, Mark Wielaard <mjw at redhat.com> wrote:
> Hi,
>
> 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.
> http://dwarfstd.org/ShowIssue.php?issue=110722.1
>
> 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
> like:
>
> 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.
>
> Thanks,
>
> Mark
>
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