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

[Faraz Shahbazker <faraz dot shahbazker at imgtec dot com>]

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.

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

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

Faraz Shahbazker (1):
  Enable non-executable PT_GNU_STACK support

 csu/init-first.c                                 |    4 ++
 elf/Makefile                                     |   11 +++--
 elf/dl-execstack-ovrd.c                          |   25 +++++++++++
 elf/dl-load.c                                    |    8 ++++
 elf/dl-support.c                                 |    4 ++
 elf/rtld.c                                       |    1 +
 elf/tst-execstack-ovrd-static.c                  |    1 +
 elf/tst-execstack-ovrd.c                         |    1 +
 sysdeps/generic/ldsodefs.h                       |    8 ++++
 sysdeps/unix/sysv/linux/mips/dl-execstack-ovrd.c |   50 ++++++++++++++++++++++
 sysdeps/unix/sysv/linux/mips/init-first.c        |   26 +++++++++++
 sysdeps/unix/sysv/linux/mips/ldsodefs.h          |    2 +-
 sysdeps/unix/sysv/linux/mips/libc-abis           |    6 +++
 13 files changed, 143 insertions(+), 4 deletions(-)
 create mode 100644 elf/dl-execstack-ovrd.c
 create mode 100644 elf/tst-execstack-ovrd-static.c
 create mode 100644 elf/tst-execstack-ovrd.c
 create mode 100644 sysdeps/unix/sysv/linux/mips/dl-execstack-ovrd.c
 create mode 100644 sysdeps/unix/sysv/linux/mips/init-first.c

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1.7.9.5