Support for DDWARF-2 debug info? (on Cygwin)

Brian Ford
Thu Jan 16 16:43:00 GMT 2003

On 16 Jan 2003, Nick Clifton wrote:

> Hi Brian,
> > My current problem is that all previous DWARF2 implementations
> > assign a VMA of zero to the .debug_* sections in the link script.
> > This violates the PE format and makes the executable unusable.
> I saw your post about this to the binutils list.
> Does the PE format require that the debugging sections be loaded into
> memory when the executable is invoked ?  The reason that the ELF
> format allows the .debug sections to have a VMA of zero is that they
> also do not have the ALLOC flag, so they are not loaded into memory.
> (A debugger wanting to access the sections for a running process must
> locate the executable on disk an load them/mmap them from there).
No, they do not need to be loaded into memory, and they do not have the
ALLOC flag set.  But, the PE format requires all sections to be adjacent
and in ascending order of VMA.  It also specifies debug sections should be
last.  I think it just tries to load everything up to the fist non-ALLOC
section, or so it seams.

> > I am still consulting the DWARF2 spec to see if gcc and gas are
> > correct in generating VMA addresses.  If so, I guess I have to fix
> > the dwarf parsing code in bfd and gdb to subtract the section base
> > VMA.
> I do not believe that the DWARF2 spec mandates the VMA addresses of
> the .debug sections.  It does say that their contents must be
> contiguous, and it does specify the meaning of their contents, but it
> does not specify the meaning of partially-complete .debug sections.
> (ie ones attached to relocations that have not yet been resolved).
No, it does not, but I did find out it mandates section relative offsets.
So, the dwarf parsing code is correct.  It is gcc that has taken a
short cut.  BTW, these problems are with fully linked executables.

>From gcc/dwarf2asm.c:122

/* Output a section-relative reference to a label.  In general this
   can only be done for debugging symbols.  E.g. on most targets with
   the GNU linker, this is accomplished with a direct reference and
   the knowledge that the debugging section will be placed at VMA 0.
   Some targets have special relocations for this that we must use.  */

dw2_asm_output_offset VPARAMS ((int size, const char *label,
                               const char *comment, ...))
  VA_OPEN (ap, comment);
  VA_FIXEDARG (ap, int, size);
  VA_FIXEDARG (ap, const char *, label);
  VA_FIXEDARG (ap, const char *, comment);

  ASM_OUTPUT_DWARF_OFFSET (asm_out_file, size, label);
  dw2_assemble_integer (size, gen_rtx_SYMBOL_REF (Pmode, label));


So, all I need to do is define ASM_OUTPUT_DWARF_OFFSET correctly in

Does anyone have I good way to define a section relative offset in
assembly for Cygwin, or do I need to define labels for the sections in the
link script as use them?  That seems messy.

> So, I think it is the case that BFD and GDB are both assuming that the
> VMA of the sections will be zero, but that this is not required.
Actually, as stated above, bfd and gdb are correct.  The VMA should not
be relevant as section relative offsets are specified.

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

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