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

[Faraz Shahbazker <faraz.shahbazker@imgtec.com>]

Bump.

On 02/05/2016 10:36 AM, Faraz Shahbazker wrote:
> Enable non-executable stack mode if assembler and linker support it.
> 
> 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: https://sourceware.org/ml/libc-alpha/2016-01/msg00567.html
> 
> binutils:
> ----
> * Increment the ABIVERSION to 5 for objects with non-executable stacks.
> 
> Patch under review: https://sourceware.org/ml/binutils/2016-02/msg00087.html
> 
> gcc:
> ----
> * Check if assembler/dynamic linker support the new behaviour
>   (ABIVERSION >= 5). If yes, enable non-executable stack by default
>   for all objects.
> 
> gcc/ChangeLog
> 	* configure.ac: Check if assembler supports the new PT_GNU_STACK
> 	ABI change; if yes, enable non-executable stack mode by default.
> 	* configure: Regenerate.
> 	* config.in: Regenerate.
> 	* config/mips/mips.c: Define TARGET_ASM_FILE_END to indicate
> 	stack mode for each C file if LD_MIPS_GNUSTACK is enabled.
> 
> libgcc/ChangeLog
> 	config/mips/crti.S: Add .note.GNU-stack marker if LD_MIPS_GNUSTACK
> 	support is enabled.
> 	config/mips/crtn.S: Add .note.GNU-stack marker if LD_MIPS_GNUSTACK
> 	support is enabled.
> 	config/mips/mips16.S: Add .note.GNU-stack marker if
> 	LD_MIPS_GNUSTACK support is enabled.
> 	config/mips/vr4120-div.S: Add .note.GNU-stack marker if
> 	LD_MIPS_GNUSTACK support is enabled.
> 
> -- gcc/configure.ac gcc/config/mips/mip.c config/mips/crti.S config/mips/crtn.S config/mips/mips16.S config/mips/vr4120-div.S
> ---
>  gcc/config/mips/mips.c          |    5 +++++
>  gcc/configure.ac                |   23 +++++++++++++++++++++++
>  libgcc/config/mips/crti.S       |    6 ++++++
>  libgcc/config/mips/crtn.S       |    6 ++++++
>  libgcc/config/mips/mips16.S     |    7 +++++++
>  libgcc/config/mips/vr4120-div.S |    7 +++++++
>  6 files changed, 54 insertions(+)
> 
> diff --git a/gcc/config/mips/mips.c b/gcc/config/mips/mips.c
> index ea18ad6..c3eefc0 100644
> --- a/gcc/config/mips/mips.c
> +++ b/gcc/config/mips/mips.c
> @@ -20194,6 +20194,11 @@ mips_promote_function_mode (const_tree type ATTRIBUTE_UNUSED,
>  #undef TARGET_HARD_REGNO_SCRATCH_OK
>  #define TARGET_HARD_REGNO_SCRATCH_OK mips_hard_regno_scratch_ok
>  
> +#if HAVE_LD_MIPS_GNUSTACK
> +#undef TARGET_ASM_FILE_END
> +#define TARGET_ASM_FILE_END file_end_indicate_exec_stack
> +#endif
> +
>  struct gcc_target targetm = TARGET_INITIALIZER;
>  \f
>  #include "gt-mips.h"
> diff --git a/gcc/configure.ac b/gcc/configure.ac
> index 0a626e9..9b8190e 100644
> --- a/gcc/configure.ac
> +++ b/gcc/configure.ac
> @@ -4562,6 +4562,29 @@ pointers into PC-relative form.])
>        AC_MSG_ERROR(
>  	[Requesting --with-nan= requires assembler support for -mnan=])
>      fi
> +
> +    AC_CACHE_CHECK([linker for GNU-stack ABI support],
> +      [gcc_cv_ld_mips_gnustack],
> +      [gcc_cv_ld_mips_gnustack=no
> +       if test x$gcc_cv_as != x \
> +	       -a x$gcc_cv_ld != x \
> +	       -a x$gcc_cv_readelf != x ; then
> +	 cat > conftest.s <<EOF
> +.section .note.GNU-stack,"",%progbits
> +EOF
> +	 if $gcc_cv_as -o conftest.o conftest.s > /dev/null 2>&1 \
> +	    && $gcc_cv_ld -o conftest conftest.o > /dev/null 2>&1; then
> +	   abi_version=`$gcc_cv_readelf -h conftest 2>&1 | grep "ABI Version:" | cut -d: -f2 | tr -d '[[:space:]]'`
> +	   if test "$abi_version" -ge 5; then
> +	     gcc_cv_ld_mips_gnustack=yes
> +	   fi
> +	 fi
> +       fi
> +       rm -f conftest.s conftest.o conftest])
> +    if test x$gcc_cv_ld_mips_gnustack = xyes; then
> +	    AC_DEFINE(HAVE_LD_MIPS_GNUSTACK, 1,
> +      [Define if your linker can handle PT_GNU_STACK segments correctly.])
> +    fi
>      ;;
>      s390*-*-*)
>      gcc_GAS_CHECK_FEATURE([.gnu_attribute support],
> diff --git a/libgcc/config/mips/crti.S b/libgcc/config/mips/crti.S
> index 8521d3c..aa85d94 100644
> --- a/libgcc/config/mips/crti.S
> +++ b/libgcc/config/mips/crti.S
> @@ -21,6 +21,12 @@ a copy of the GCC Runtime Library Exception along with this program;
>  see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>  <http://www.gnu.org/licenses/>.  */
>  
> +#include "config.h"
> +#if HAVE_LD_MIPS_GNUSTACK
> +/* An executable stack is *not* required for these functions.  */
> +	.section .note.GNU-stack,"",%progbits
> +#endif
> +
>  /* 4 slots for argument spill area.  1 for cpreturn, 1 for stack.
>     Return spill offset of 40 and 20.  Aligned to 16 bytes for n32.  */
>  
> diff --git a/libgcc/config/mips/crtn.S b/libgcc/config/mips/crtn.S
> index d80c342..6a13b46 100644
> --- a/libgcc/config/mips/crtn.S
> +++ b/libgcc/config/mips/crtn.S
> @@ -21,6 +21,12 @@ a copy of the GCC Runtime Library Exception along with this program;
>  see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>  <http://www.gnu.org/licenses/>.  */
>  
> +#include "config.h"
> +#if HAVE_LD_MIPS_GNUSTACK
> +/* An executable stack is *not* required for these functions.  */
> +        .section .note.GNU-stack,"",%progbits
> +#endif
> +
>  /* 4 slots for argument spill area.  1 for cpreturn, 1 for stack.
>     Return spill offset of 40 and 20.  Aligned to 16 bytes for n32.  */
>  
> diff --git a/libgcc/config/mips/mips16.S b/libgcc/config/mips/mips16.S
> index 5080f86..b8f6df8 100644
> --- a/libgcc/config/mips/mips16.S
> +++ b/libgcc/config/mips/mips16.S
> @@ -48,6 +48,13 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>     values using the soft-float calling convention, but do the actual
>     operation using the hard floating point instructions.  */
>  
> +#include "config.h"
> +#if HAVE_LD_MIPS_GNUSTACK
> +/* An executable stack is *not* required for these functions.  */
> +        .section .note.GNU-stack,"",%progbits
> +	.previous
> +#endif
> +
>  #if defined _MIPS_SIM && (_MIPS_SIM == _ABIO32 || _MIPS_SIM == _ABIO64)
>  
>  /* This file contains 32-bit assembly code.  */
> diff --git a/libgcc/config/mips/vr4120-div.S b/libgcc/config/mips/vr4120-div.S
> index 9954bc3..262de67 100644
> --- a/libgcc/config/mips/vr4120-div.S
> +++ b/libgcc/config/mips/vr4120-div.S
> @@ -26,6 +26,13 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>     -mfix-vr4120.  div and ddiv do not give the correct result when one
>     of the operands is negative.  */
>  
> +#include "config.h"
> +#if HAVE_LD_MIPS_GNUSTACK
> +/* An executable stack is *not* required for these functions.  */
> +        .section .note.GNU-stack,"",%progbits
> +#endif
> +	.previous
> +
>  	.set	nomips16
>  
>  #define DIV								\
>