Re: [RFC] Improve stabs debugging for mingw64 executables loaded at high addresses

[Michael Snyder <msnyder@vmware.com>]

Pierre Muller wrote:
>   On 64-bit targets, stabs suffers from the fact that addresses
> are only stored as 32-bit values.
> 
>   For mingw64, I was able to debug executables loaded
> at address 0x100000000 (the default location for executables
> compiled by Free Pascal Compiler) by using the simple
> patch below.
> 
>   The idea of the patch is quite basic:
>   add the 32 high bits of text_addr as
> to the offsets of all sections in read_dbx_symtab.
> 
>   I don't know if this is mingw64 (possibly 64-bit PE) specific or not...
> 
> All those offsets seemed to be at zero at the time I added
> the high 32-bit part of text_addr, but maybe other
> targets do something else... If this should not be applied
> for other 64-bit targets, we could of course modify
> the tests before changing the offsets, restricting it to
> targets for which it is relevant.
> 
>   I just tried to use gcc to compile an executable
> with -Wl,--image-base,0x300000000, to force high load address,
> but this lead to lots of messages:
> relocation truncated to fit R_X86_64_32 against .text
> Free Pascal compiler doesn't seem to generate any of those
> time of relocations... 
> 
>   Comments welcome,

I don't see any intrinsic harm in it, since it will only affect stabs
binaries when BFD64 is defined.

This is just a comment, though, not an approval.

> 2010-09-14  Pierre Muller  <muller@ics.u-strasbg.fr>
> 
> 	* dbxread.c (read_dbx_symtab): Add high part of text_addr value
> 	to all section offsets.
> 
> Index: src/gdb/dbxread.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/dbxread.c,v
> retrieving revision 1.116
> diff -u -p -r1.116 dbxread.c
> --- src/gdb/dbxread.c	14 May 2010 17:53:15 -0000	1.116
> +++ src/gdb/dbxread.c	1 Sep 2010 08:01:01 -0000
> @@ -1216,6 +1216,22 @@ read_dbx_symtab (struct objfile *objfile
>    text_addr = DBX_TEXT_ADDR (objfile);
>    text_size = DBX_TEXT_SIZE (objfile);
>  
> +#ifdef BFD64
> +  {
> +    /* stabs internal format only has 4 bytes for address.
> +       Use high dword of text address to fix global addresses.
> +       FIXME: this only works if the whole executable has the same
> +       high part address.  */
> +    CORE_ADDR stabs_fixup = text_addr & ~((CORE_ADDR) 0xffffffff);
> +    if (stabs_fixup)
> +      {
> +        objfile->section_offsets->offsets[SECT_OFF_TEXT (objfile)] += stabs_fixup;
> +	objfile->section_offsets->offsets[SECT_OFF_DATA (objfile)] += stabs_fixup;
> +	objfile->section_offsets->offsets[SECT_OFF_BSS (objfile)] += stabs_fixup;
> +      }
> +  }
> +#endif /* BFD64 */
> +
>    /* FIXME.  We probably want to change stringtab_global rather than add this
>       while processing every symbol entry.  FIXME.  */
>    file_string_table_offset = 0;
>