Hi James,
We do have PT_GNU_STACK flags set correctly, this feature is required to
workaround CPU revisions which do not have RIXI support.
RIXI support can be discovered programatically from CP0_Config3.RXI
(cpu_has_rixi in asm/cpu-features.h), so I don't follow why CPUs without
RIXI would require a kernel parameter.
The following patch introduced change in behavior with regards to
stack & heap execute-ability :
commit 1a770b85c1f1c1ee37afd7cef5237ffc4c970f04
Author: Paul Burton <paul.burton@imgtec.com>
Date: Fri Jul 8 11:06:20 2016 +0100
MIPS: non-exec stack & heap when non-exec PT_GNU_STACK is present
The stack and heap have both been executable by default on MIPS until
now. This patch changes the default to be non-executable, but only for
ELF binaries with a non-executable PT_GNU_STACK header present. This
does apply to both the heap & the stack, despite the name PT_GNU_STACK,
and this matches the behaviour of other architectures like ARM & x86.
Current MIPS toolchains do not produce the PT_GNU_STACK header, which
means that we can rely upon this patch not changing the behaviour of
existing binaries. The new default will only take effect for newly
compiled binaries once toolchains are updated to support PT_GNU_STACK,
and since those binaries are newly compiled they can be compiled
expecting the change in default behaviour. Again this matches the way in
which the ARM & x86 architectures handled their implementations of
non-executable memory.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com>
Cc: Maciej Rozycki <maciej.rozycki@imgtec.com>
Cc: Faraz Shahbazker <faraz.shahbazker@imgtec.com>
Cc: Raghu Gandham <raghu.gandham@imgtec.com>
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13765/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
....
When kernel is detecting the type of mapping it should apply :
fs/binfmt_elf.c:
...
if (elf_read_implies_exec(loc->elf_ex, executable_stack))
current->personality |= READ_IMPLIES_EXEC;
...
this effectively calls mips_elf_read_implies_exec() which performs a check:
...
if (!cpu_has_rixi) {
/* The CPU doesn't support non-executable memory */
return 1;
}
return 0;
}
This will in turn make stack & heap executable on processors without RIXI, which
are practically all processors with MIPS ISA R < 6.