[RFC] Fix si->si_code for guard page access on PowerPC

Fix si->si_code for guard page access on PowerPC

Currently, the mm code on PowerPC/POWER returns a si->si_code = 2
(SEGV_ACCERR) when the stack tries to grow beyond the stack guard
(stack ulimit). On other architectures, notably x86, the si->si_code
returned when a guard page access occurs is 1 (SEGV_MAPERR).

Although si->si_code is not historically reliable and hence no
program should trust it for any semantic behavior, the right
si->si_code for a guard page access is 1 (SEGV_MAPERR) and,
besides that, some tests still trust it in specific cases.

On PowerPC/POWER, if the mm tries to expand the stack and
hits a page mapped by the program (say, an anonymous page
with permission ---p) it generates a SIG_SEGV and a si->si_code = 2
(SEGV_ACCERR), the same way it happens on x86. But then, when this
guard page is removed (un-mapped) and the stack grows again reaching
the stack guard (stack ulimit), the mm generates a SIG_SEGV and a
si->si_code = 2 (SEGV_ACCERR) again, contrary to, for example,
what happens on x86 (si->si_code = 1 (SIG_MAPERR)). It means that
on PowerPC/POWER there is no semantic difference between a stack
growth hitting a mapped area the stack has no permission to rd/wr
and reaching the stack limit (stack ulimit), although indeed there
is a difference.

Signed-off-by: Gustavo Romero <redacted>
---
  arch/powerpc/mm/fault.c | 4 +++-
  1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index a67c6d7..6954971 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c

@@ -431,8 +431,10 @@ good_area:
            */
           fault = handle_mm_fault(mm, vma, address, flags);
           if (unlikely(fault & (VM_FAULT_RETRY|VM_FAULT_ERROR))) {
-                  if (fault & VM_FAULT_SIGSEGV)
+                  if (fault & VM_FAULT_SIGSEGV) {
+                           code = SEGV_MAPERR;
                             goto bad_area;
+                  }
                    rc = mm_fault_error(regs, address, fault);
                    if (rc >= MM_FAULT_RETURN)
                             goto bail;

Hi Gustavo,

On Fri, 2016-22-01 at 16:23:31 UTC, Gustavo Romero wrote:

Fix si->si_code for guard page access on PowerPC

...

As we discussed on IRC, I'd prefer if this case was handled in
mm_fault_error(). So please send a v2 which does that, or let us know if you
have problems.

cheers