Re: [PATCH v2 3/4] libdw: Apply DWARF package file section offsets where appropriate

[Omar Sandoval <osandov@osandov.com>]

On Fri, Feb 16, 2024 at 04:00:47PM +0100, Mark Wielaard wrote:
> Hi Omar,
> 
> On Wed, 2023-12-06 at 01:22 -0800, Omar Sandoval wrote:
> > The final piece of DWARF package file support is that offsets have to be
> > interpreted relative to the section offset from the package index.
> > .debug_abbrev.dwo is already covered, so sprinkle around calls to
> > dwarf_cu_dwp_section_info for the remaining sections: .debug_line.dwo,
> > .debug_loclists.dwo/.debug_loc.dwo, .debug_str_offsets.dwo,
> > .debug_macro.dwo/.debug_macinfo.dwo, and .debug_rnglists.dwo.  With all
> > of that in place, we can finally test various libdw functions on dwp
> > files.
> 
> So the offsets for DW_SECT_INFO, DW_SECT_TYPES and DW_SECT_ABBREV were
> already taken into account when setting up a cu from a dwp.
> 
> With this patch __libdw_cu_str_off_base/str_offsets_base_off handles
> DW_SECT_STR_OFFSETS which is used in dwarf_formstring and
> dwarf_getmacros.
> 
> __libdw_cu_ranges_base handles DW_SECT_RNGLISTS, which is used by
> dwarf_ranges. And __libdw_formptr has a special case for
> DW_FORM_sec_offset for IDX_debug_ranges && version < 5 && unit_type ==
> DW_UT_split_compile to also uses __libdw_cu_ranges_base.
> 
> __libdw_cu_locs_base handles DW_SECT_LOCLISTS which is used in
> dwarf_getlocation through initial_offset. I do wonder why the special
> case in __libdw_formptr isn't needed here too.
> 
> dwarf_getmacros handles DW_SECT_MACRO through get_offset_from. And when
> the macros need to refer to the line table, it also handles
> DW_SECT_LINE.
> 
> Don't we also need to handle DW_SECT_LINE in dwarf_getsrclines and
> dwarf_next_lines when looking for DW_AT_stmt_list?

.debug_line is the odd one out in split DWARF: the skeleton file
contains the full .debug_line, and the DWO or DWP files have a skeleton
.debug_line.dwo that only contains the directory and file name tables
(for DW_AT_file and macro info to reference). dwarf_getsrclines and co.
read from the skeleton file, not the DWP file, meaning they shouldn't
use DW_SECT_LINE.

> > 	* libdw/dwarf_getmacros.c (get_macinfo_table): Call
> > 	dwarf_cu_dwp_section_info and add offset to line_offset.
> > 	(get_offset_from): Call dwarf_cu_dwp_section_info and add offset
> > 	to *retp.
> > 	* libdw/libdwP.h (str_offsets_base_off): Call
> > 	dwarf_cu_dwp_section_info and add offset.
> > 	(__libdw_cu_ranges_base): Ditto.
> > 	(__libdw_cu_locs_base): Ditto.
> > 	* libdw/dwarf_getlocation.c (initial_offset): Call
> > 	dwarf_cu_dwp_section_info and add offset to start_offset.
> > 	* tests/run-varlocs.sh: Check testfile-dwp-5 and testfile-dwp-4.
> > 	* tests/run-all-dwarf-ranges.sh: Check testfile-dwp-5 and
> > 	testfile-dwp-4.
> > 	* tests/run-dwarf-getmacros.sh: Check testfile-dwp-5 and
> > 	testfile-dwp-4-strict.
> > 	* tests/run-get-units-split.sh: Check testfile-dwp-5,
> > 	testfile-dwp-4, and testfile-dwp-4-strict.
> 
> The code and tests look good. run-varlocs.sh seems good, which seems to
> confirm DW_SECT_LOCLISTS is handled correctly (but why doesn't it need
> a hack similar to ranges in __libdw_formptr?).

I think it's because ranges have the uniquely weird behavior in DWARF 4
GNU Debug Fission that DW_AT_GNU_ranges_base is in the skeleton file but
used to interpret the split file. This was cleaned up for DWARF 5 (as I
documented in commit c9c9ffae725009b192b40e2d89035f353ae7055f), and
there was no base attribute for location lists in DWARF 4, so it's not
applicable.

> We might want to add a test for run-next-lines.sh and run-next-
> files.sh?

Good idea, I'll send an updated version with those tests.

Thanks!
Omar