Re: [PATCH 07/14] libdw: Recognize .debug_[ct]u_index sections in dwarf_elf_begin

[Omar Sandoval <osandov@osandov.com>]

On Wed, Nov 01, 2023 at 03:03:57PM +0100, Mark Wielaard wrote:
> Hi Omar,
> 
> On Wed, 2023-09-27 at 11:20 -0700, Omar Sandoval wrote:
> > From: Omar Sandoval <osandov@fb.com>
> > 
> > DWARF package (.dwp) files have a .debug_cu_index section and,
> > optionally, a .debug_tu_index section.  Add them to the list of DWARF
> > sections.
> > 
> > Unfortunately, it's not that simple: the other debug sections in a dwp
> > file have names ending with .dwo, which confuses the checks introduced
> > by commit 5b21e70216b8 ("libdw: dwarf_elf_begin should use either plain,
> > dwo or lto DWARF sections.").  So, we also have to special case
> > .debug_cu_index and .debug_tu_index in scn_dwarf_type and check_section
> > to treat them as TYPE_DWO sections.
> 
> This seems to work, but I wonder if we should have a specific TYPE_DWP?

I tried this, and it made check_section even more confusing (because
then we need a more complicated check than result->type == section
type). I came to the same conclusion that since this is internal for
now, it didn't really matter. Although maybe the name TYPE_SPLIT would
make more sense now rather than overloading TYPE_DWO.

When this becomes public, separate TYPE_DWO and TYPE_DWP types might be
nicer so that a hypothetical dwarf_get_type function could tell you
whether you got a dwo file or a dwp file.

> It doesn't really matter now, because the enum dwarf_type is only used
> internally. But I was hoping to extend the dwarf_begin interface with a
> flag so that you can open a DWARF as a specific type. For example there
> are single file split DWARF files. Which contain both "plain" and
> ".dwo" sections. Currently you can only open them as "plain", but there
> are actually two "views" of such files.
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=28573
> 
> Do you think there are reasons to open a file as either TYPE_DWO or
> TYPE_DWP? Or doesn't that not make sense?

If we were to treat a dwp file as a dwo file, I guess we'd have to
pretend it only contained one unit and ignore the rest, which I don't
think makes much sense. So even if we had separate types for them, I
don't know if we should allow that.