DWARF 2 sections and padding ?

DWARF 2 sections and padding ?

[Brian Ford]

I am trying to add DWARF 2 support for PE/COFF on Cygwin.  PE/COFF
sections must be aligned to 512 byte boundaries.  This extra padding
confuses the DWARF 2 parsing routines because they use the sections
raw_size, thus parsing an invalid compilation unit header (all
zeros).

The actual, smaller size is available in the section header.  I am
confused about what the cooked_size is supposed to mean.  It is currently
zero, but I think it could contain the smaller virtual size.

I have read the DWARF spec several times, and although it discusses this
issue briefly for some debugging sections, it is unclear to me how this
should really work.  Unless I try to arrange for the producer (gcc) to
pad the compilation units with zeros instead of having bfd pad the end of
the sections, I don't know how to proceed.

I would like to just use an alternate size instead of the raw size (cooked
maybe?) but I don't know how this would affect other targets and I don't
want a PE specific hack in DWARF parsing code.

Any ideas?  Thanks.

-- 
Brian Ford
Senior Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
Phone: 314-551-8460
Fax:   314-551-8444

Re: DWARF 2 sections and padding ?

[Daniel Jacobowitz]

On Fri, Feb 21, 2003 at 10:16:49AM -0600, Brian Ford wrote:
> I am trying to add DWARF 2 support for PE/COFF on Cygwin.  PE/COFF
> sections must be aligned to 512 byte boundaries.  This extra padding
> confuses the DWARF 2 parsing routines because they use the sections
> raw_size, thus parsing an invalid compilation unit header (all
> zeros).
> 
> The actual, smaller size is available in the section header.  I am
> confused about what the cooked_size is supposed to mean.  It is currently
> zero, but I think it could contain the smaller virtual size.
> 
> I have read the DWARF spec several times, and although it discusses this
> issue briefly for some debugging sections, it is unclear to me how this
> should really work.  Unless I try to arrange for the producer (gcc) to
> pad the compilation units with zeros instead of having bfd pad the end of
> the sections, I don't know how to proceed.
> 
> I would like to just use an alternate size instead of the raw size (cooked
> maybe?) but I don't know how this would affect other targets and I don't
> want a PE specific hack in DWARF parsing code.
> 
> Any ideas?  Thanks.

I don't see why the raw size should include the padding; that should
just be used when _placing_ sections.  But this is a binutils/bfd
issue; redirected to that list.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

Re: DWARF 2 sections and padding ?

[Brian Ford]

On Fri, 21 Feb 2003, Daniel Jacobowitz wrote:

> On Fri, Feb 21, 2003 at 10:16:49AM -0600, Brian Ford wrote:
> > I am trying to add DWARF 2 support for PE/COFF on Cygwin.  PE/COFF
> > sections must be aligned to 512 byte boundaries.  This extra padding
> > confuses the DWARF 2 parsing routines because they use the sections
> > raw_size, thus parsing an invalid compilation unit header (all
> > zeros).
> >
> > The actual, smaller size is available in the section header.  I am
> > confused about what the cooked_size is supposed to mean.  It is currently
> > zero, but I think it could contain the smaller virtual size.
> >
> > I have read the DWARF spec several times, and although it discusses this
> > issue briefly for some debugging sections, it is unclear to me how this
> > should really work.  Unless I try to arrange for the producer (gcc) to
> > pad the compilation units with zeros instead of having bfd pad the end of
> > the sections, I don't know how to proceed.
> >
> > I would like to just use an alternate size instead of the raw size (cooked
> > maybe?) but I don't know how this would affect other targets and I don't
> > want a PE specific hack in DWARF parsing code.
> >
> > Any ideas?  Thanks.
>
> I don't see why the raw size should include the padding; that should
> just be used when _placing_ sections.  But this is a binutils/bfd
> issue; redirected to that list.
>
The raw size appears to translate directly to the size in the section
header.  The PE/COFF format states this must be a multiple of the page
size.

Other COFF targets that define ALIGN_SECTIONS_IN_FILE do the same thing,
in separate code, though.  There is one case where these other targets do
what you suggest, if COFF_PAGE_SIZE is defined:

/* In demand paged files the low order bits of the file offset
   must match the low order bits of the virtual address.  */
#ifdef COFF_PAGE_SIZE
      if ((abfd->flags & D_PAGED) != 0
          && (current->flags & SEC_ALLOC) != 0)
        sofar += (current->vma - sofar) % page_size;
#endif

I modified the PE/COFF code to do this instead and, although it violates
the format, a simple test appeared valid and runable on Windows XP.  What
do the binutils maintainers think about making this change?

-- 
Brian Ford
Senior Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
Phone: 314-551-8460
Fax:   314-551-8444