Subject: bug#20614: Segmentation fault when building on Power8 Little Endian

From: Eli Zaretskii <[email-masked]>
Date: Mon, 05 Oct 2015 11:27:17 +0300

> Cc: [email-masked]
> From: Petr Hracek <[email-masked]>
> Date: Mon, 5 Oct 2015 09:40:15 +0200
> 
> >>>> What does this GDB command display in frame #1:
> >>>>
> >>>>     (gdb) p old_section_names + NEW_SECTION_H (nn).sh_name
> >>>>
> >>>> ?
> >>>>
> >>>> It is a really strange.
> >>>> in our unexelf.c is NEW_SECTION_H defined as
> >>>>
> >>>> #define NEW_SECTION_H(n) \
> >>>> (*(ElfW (Shdr) *) ((byte *) new_section_h + new_file_h->e_shentsize * (n)))
> >>>>
> >>>> Paul Eggert did a some patch 1 or 2 year ago and modify it to
> >>>> #define NEW_SECTION_H(n) \
> >>>>     (*(ElfW (Shdr) *) entry_address (new_section_h, n, new_file_h->e_shentsize))
> >>>>
> >>>> (gdb) frame 1
> >>>> #1 0x00000000102e3724 in unexec (new_name=0x116387e8
> >>>> "/home/phracek/rpmbuild/BUILD/emacs-24.3/src/emacs", old_name=0x11638838
> >>>> "/home/phracek/rpmbuild/BUILD/emacs-24.3/src/temacs")
> >>>> at unexelf.c:1258
> >>>> 1258 if (!strcmp (old_section_names + NEW_SECTION_H (nn).sh_name, ".data")
> >>>> (gdb) p old_section_names
> >>>> $1 = 0x3fffab1dbfcc ""
> >>>> (gdb) p old_section_names + NEW_SECTION_H (nn).sh_name
> >>>> No symbol "NEW_SECTION_H" in current context.
> >>> Use the expansion of that macro instead to display the value.
> >> How to use the expansion.
> > Just type it by hand:
> >
> >   (gdb) p old_section_names + (*(ElfW (Shdr) *) ((byte *) new_section_h + new_file_h->e_shentsize * (nn)))
> >
> >> Are you connected on freenode #emacs.
> > No.
> First of all thank you for your time.
> Well,
> (gdb) p (*(ElfW (Shdr) *) ((byte *) new_section_h + 
> new_file_h->e_shentsize * (nn)))
> No symbol "ElfW" in current context.
> (gdb)
> Also ElfBitsW does not exists.
> (gdb) p ElfBitsW
> No symbol "ElfBitsW" in current context.
> (gdb)

You need to expand all these macros by hand.  They are defined at the
beginning of unexelf.c:

  #ifndef ElfW
  # define ElfBitsW(bits, type) Elf##bits##_##type
  # ifndef ELFSIZE
  #  ifdef _LP64
  #   define ELFSIZE 64
  #  else
  #   define ELFSIZE 32
  #  endif
  # endif
    /* This macro expands `bits' before invoking ElfBitsW.  */
  # define ElfExpandBitsW(bits, type) ElfBitsW (bits, type)
  # define ElfW(type) ElfExpandBitsW (ELFSIZE, type)
  #endif

I would suggest at first to rebuild Emacs with "-gdwarf-4 -g3"
compiler switches, maybe that will allow you to type expressions with
macros, and let GDB expand them using the debug info.  Maybe.