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[RFC] [MIPS] Enable non-executable PT_GNU_STACK support

Increment the ABIVERSION to 5 for objects with non-executable stacks

Currently the MIPS FPU emulator uses eXecute Out of Line (XOL) on the stack to
handle instructions in the delay slots of FPU branches.  Because of this MIPS
cannot have a non-executable stack. While the solution on the kernel side is
not yet finalized, we propose changes required on the tools-side to make them
ready for a seamless transition whenever a fixed kernel becomes available.

glibc/dynamic linker:
* When non-executable stack is requested, first check AT_FLAGS in the
  auxiliary vector to decide if this kernel supports a non-executable
  stack. Persist with the non-executable mode specified on the
  PT_GNU_STACK segment only if kernel supports it, else revert to an
  executable stack.

* The 25th bit (1<<24) in AT_FLAGS is reserved for use by the kernel to
  indicate that it supports a non-executable stack on MIPS.

* glibc's ABIVERSION is incremented from 3 to 5, so that applications linked
  for this glibc can't be accidentally run against older versions. ABIVERSION
  4 has been skipped over because it was chosen for IFUNC support, which is
  still under review.

Patch under review:

* Check if assembler/dynamic linker support the new behaviour
  (ABIVERSION >= 5). If yes, enable non-executable stack by default
  for all objects.

* Increment the ABIVERSION to 5 for objects with non-executable stacks.

Increment the ABIVERSION for objects requesting non-executable stack to 5.
This change is used by the compiler to decide if it can allow
non-executable stacks as a default.

	* bfd/elfxx-mips.c
	(_bfd_mips_post_process_headers): Increment ABIVERSION for
	non-executable stack.
 bfd/elfxx-mips.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/bfd/elfxx-mips.c b/bfd/elfxx-mips.c
index 4ece819..5cd074a 100644
--- a/bfd/elfxx-mips.c
+++ b/bfd/elfxx-mips.c
@@ -16141,6 +16141,9 @@ _bfd_mips_post_process_headers (bfd *abfd, struct bfd_link_info *link_info)
   if (mips_elf_tdata (abfd)->abiflags.fp_abi == Val_GNU_MIPS_ABI_FP_64
       || mips_elf_tdata (abfd)->abiflags.fp_abi == Val_GNU_MIPS_ABI_FP_64A)
     i_ehdrp->e_ident[EI_ABIVERSION] = 3;
+  if (elf_stack_flags (abfd) && !(elf_stack_flags (abfd) & PF_X))
+    i_ehdrp->e_ident[EI_ABIVERSION] = 5;

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