[MIPS] Add .note.GNU-stack section

On Wed, 2014-09-10 at 10:15 -0700, Eric Christopher wrote:
> 
> 
> On Wed, Sep 10, 2014 at 9:27 AM, <pinskia@gmail.com> wrote:

>         This works except you did not update the assembly files in
>         libgcc or glibc. We (Cavium) have the same patch in our tree
>         for a few released versions.

> Mind just checking yours in then Andrew?

> Thanks!
> -eric

I talked to Andrew about what files he changed in GCC and created and
tested this new patch.  Andrew also mentioned changing some assembly
files in glibc but I don't see any use of '.section .note.GNU-stack' in
any assembly files in glibc (for any platform) so I wasn't planning on
creating a glibc to add them to mips glibc assembly language files.

OK to check in this patch?

Steve Ellcey
sellcey@mips.com



2014-09-26  Steve Ellcey  <sellcey@mips.com>

	* config/mips/mips.c (TARGET_ASM_FILE_END): Define.
	* libgcc/config/mips/mips16.S: Add .note.GNU-stack section.
	* libgcc/config/mips/vr4120-div.S: Ditto.
	* libgcc/config/mips/crti.S: Ditto.
	* libgcc/config/mips/crtn.S: Ditto.

> I talked to Andrew about what files he changed in GCC and created and
> tested this new patch.  Andrew also mentioned changing some assembly
> files in glibc but I don't see any use of '.section .note.GNU-stack' in
> any assembly files in glibc (for any platform) so I wasn't planning on
> creating a glibc to add them to mips glibc assembly language files.
> 
> OK to check in this patch?

I think we need to wait for some conclusion about how the kernel will
cope with a non-executable stack before committing this. The discussion
is progressing on the glibc and kernel lists.

As far as I can tell the kernel will respond to the PT_GNU_STACK
segment already for MIPS and then lead to a crash if the FPU emulator is
invoked. If this is the case then it would be ideal if we can somehow
prevent a stack becoming non-executable unless built against a kernel
which is known to be safe for no-execstack.

Existing versions of glibc will simply follow suit with the compiler
and apply --noexecstack when building assembly files if the compiler
emits a .note.GNU-stack section. Given that behaviour then as soon
as we enable no-execstack support in the compiler then we will get
PT_GNU_STACK on the binaries and risk a crash.

I'm struggling to come up with a good way to make this safe on existing
kernels. One possibility is that MIPS would have to implement the
compiler driven no-execstack support by passing the --no-execstack
option to the assembler instead of using the .note.GNU-stack section
explicitly. This would avoid triggering the configure check in glibc
and allow us to write a MIPS specific check in glibc which looks at
both the kernel version and whether the compiler enables no-execstack.

Slightly convoluted but safe. I'm not sure how uclibc and musl deal
with this issue but we should really check them too (but they are still
secondary to glibc I think). Any other ideas?

Matthew

> 
> Steve Ellcey
> sellcey@mips.com
> 
> 
> 
> 2014-09-26  Steve Ellcey  <sellcey@mips.com>
> 
> 	* config/mips/mips.c (TARGET_ASM_FILE_END): Define.
> 	* libgcc/config/mips/mips16.S: Add .note.GNU-stack section.
> 	* libgcc/config/mips/vr4120-div.S: Ditto.
> 	* libgcc/config/mips/crti.S: Ditto.
> 	* libgcc/config/mips/crtn.S: Ditto.
> 
> 
> diff --git a/gcc/config/mips/mips.c b/gcc/config/mips/mips.c
> index f9713c1..39020d7 100644
> --- a/gcc/config/mips/mips.c
> +++ b/gcc/config/mips/mips.c
> @@ -19146,6 +19146,9 @@ mips_lra_p (void)
>  #undef TARGET_LRA_P
>  #define TARGET_LRA_P mips_lra_p
> 
> +#undef TARGET_ASM_FILE_END
> +#define TARGET_ASM_FILE_END file_end_indicate_exec_stack
> +
>  struct gcc_target targetm = TARGET_INITIALIZER;
>  

>  #include "gt-mips.h"
> diff --git a/libgcc/config/mips/crti.S b/libgcc/config/mips/crti.S
> index 6980594..93436c0 100644
> --- a/libgcc/config/mips/crti.S
> +++ b/libgcc/config/mips/crti.S
> @@ -21,6 +21,10 @@ 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/>.  */
> 
> +
> +/* An executable stack is *not* required for these functions.  */
> +	.section .note.GNU-stack,"",%progbits
> +
>  /* 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 0de2d0c..6f2c301 100644
> --- a/libgcc/config/mips/crtn.S
> +++ b/libgcc/config/mips/crtn.S
> @@ -21,6 +21,9 @@ 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/>.  */
> 
> +/* An executable stack is *not* required for these functions.  */
> +        .section .note.GNU-stack,"",%progbits
> +
>  /* 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 dde8939..58e4377 100644
> --- a/libgcc/config/mips/mips16.S
> +++ b/libgcc/config/mips/mips16.S
> @@ -35,6 +35,10 @@ 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.  */
> 
> +/* An executable stack is *not* required for these functions.  */
> +        .section .note.GNU-stack,"",%progbits
> +	.previous
> +
>  #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 76c4e7a..664d3c3 100644
> --- a/libgcc/config/mips/vr4120-div.S
> +++ b/libgcc/config/mips/vr4120-div.S
> @@ -26,6 +26,10 @@ 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.  */
> 
> +/* An executable stack is *not* required for these functions.  */
> +        .section .note.GNU-stack,"",%progbits
> +	.previous
> +
>  	.set	nomips16
> 
>  #define DIV								\
>