This is the minimum set of required changes to support a non-executable
GOT and PLT segment and SELinux restrictions on never allowing writable
areas to become executable nor executable areas to become writable.  It
leaves us backward compatible with old shared libraries.

o GOT needs to be made non-exec.  A non-exec GOT means we can't execute
  the blrl insn in the GOT to load the GOT pointer register for -fpic
  code.  Instead of using "bl got-4; mflr 30", use
	bcl 20,31,1f
     1: mflr 30
	addis 30,30,(got-1b)@ha
	addi 30,30,(got-1b)@l
  This example assumes r30 is being used as the GOT pointer. "got" is
  shorthand for the value of the symbol _GLOBAL_OFFSET_TABLE_.

  This will require addition of R_PPC_REL16_HA and R_PPC_REL16_LO
  relocs, support for them in ld, and of course gcc changes to generate
  the new code to load the GOT register.  -fPIC GOT pointer code can
  also benefit from the new relocs.
  The .got section will be placed before .data so that it may be made
  read-only after relocatation.  This placement is incompatible with the
  use of the GOT pointer to address .sdata using short offsets in shared
  libraries, so this feature of the SYSV ABI is no longer supported.
  With the new GOT layout, three locations are reserved.  got[0] will be
  initialised to the link-time address of .dynamic by ld, got[1] will be
  initialised to the address of dl_runtime_resolve by ld.so, and got[2]
  will be initialised to the map address by ld.so.  got[-1] will no
  longer be reserved.

o PLT should be split into two sections, one containing code which we'll
  call .glink, the other containing just data, which we'll call .plt.
  .glink will be generated by ld somewhere in the text segment, and
  contains a special symbol resolver stub, and a number of PLT call code
  stubs.  The symbol resolver stub will call the dl_runtime_resolve
  function specified by got[1] with r11 set to the plt reloc offset, and
  r12 set to the value of got[2].  These stubs need not be adjacent to
  one another or unique, and can be scattered throughout the text
  segment so as to be reachable with a branch and link instruction.
  A possible implementation is:

  # Enter with .plt entry address in r11, got pointer in r30
  PLTresolve:
	addis 11,11,(got-plt)@ha
	addi 11,11,(got-plt)@l
	sub 11,11,30			# r11 = index * 4
	add 0,11,11
	add 11,0,11			# r11 = index * 12 = reloc offset.
	lwz 0,4(30)			# got[1] address of dl_runtime_resolve
	mtctr 0
	lwz 12,8(30)			# got[2] contains the map address
	bctr

  # ith PLT code stub.
	addis 11,30,(plt+(i-1)*4-got)@ha
	lwzu 0,(plt+(i-1)*4-got)@l(11)
	mtctr 0
	bctr

  .plt will be a loaded section located after .got, and consists of an
  array of addresses.  There will also be an array of R_PPC_JMP_SLOT
  relocs in .rela.plt, with a one-one correspondence between elements of
  each array.  Each R_PPC_JMP_SLOT reloc will have r_offset pointing at
  the .plt word it relocates.  To support lazy linking, ld will set each
  .plt word to point to the symbol resolver stub in .glink.  On loading
  a shared library, ld.so should relocate the contents of .plt by adding
  the load address to each word in .plt.
  Note that this ABI does not specify a fixed GOT register, or even one
  register used throughout a binary.  If a function uses a register
  other than r30 for the GOT register, it will be specified as
  r_addend = reg*4+2 on each R_PPC_PLTREL24 reloc used by calls.  This
  of course will require the linker to generate different .glink code
  than the example implementation given here.
  To allow ld.so to support old shared libs, we need to flag a new got
  and plt layout, so we'll define a new dynamic tag, DT_PPC_GLINK which
  will be set to the link-time address of PLTresolve.

  The PLT change requires support in ld and ld.so, and gcc changes to
  ensure that the got pointer register is set properly whenever a call
  via the PLT may be necessary.  The linker will detect the difference
  between old and new object files by looking at relocs.  A new object
  file will always have R_PPC_REL16* relocs if it uses the GOT or makes
  calls that might need to go via the plt.  An old file won't have these
  (new) relocs.