[PATCH][arm] avoid stack corruption

["Seongbae Park (박성배, 朴成培)" <seongbae dot park at gmail dot com>]

Hi,

This patch fixes a potential stack corruption in thumb1 mode.

# cat m.c
int main() {
 return 0;
}
# install.base/bin/arm-elf-gcc -O -mthumb -fno-omit-frame-pointer -S m.c
# cat m.s
       .code   16
       .file   "m.c"
       .text
       .align  2
       .global main
       .code   16
       .thumb_func
       .type   main, %function
main:
       push    {r7, lr}
       add     r7, sp, #8
       mov     r0, #0
       mov     sp, r7
       sub     sp, sp, #8
       @ sp needed for prologue
       pop     {r7, pc}
       .size   main, .-main
       .ident  "GCC: (GNU) 4.3.0 20070618 (experimental)"
#

This is with the mainline.
As you can see, there's a window of potential stack corruption
if the interrupt arrives at the right time.
This is due to "amount" (i.e. frame size)
becoming negative in thumb1_expand_epilogue.

Not sure of how to test -mthumb -fno-omit-frame-pointer
combination effectively - how do I pass those into check-gcc tests ? -
I just tested as arm-elf on arm simulator and saw no regression.

Ok for mainline and for 4.2 and 4.1 branches ?

gcc/ChangeLog:

2007-04-19  Seongbae Park  <seongbae.park@gmail.com>

       * config/arm/arm.c (arm_get_frame_offsets): Set
       offsets->locals_base to avoid negative stack size.
       (thumb1_expand_prologue): Assert on negative stack size.

gcc/testsuite/ChangeLog:

2007-04-19  Seongbae Park  <seongbae.park@gmail.com>

        * gcc.target/arm/stack-corruption.c: New test.
--
#pragma ident "Seongbae Park, compiler, http://seongbae.blogspot.com";

Index: gcc/testsuite/gcc.target/arm/stack-corruption.c
===================================================================
--- gcc/testsuite/gcc.target/arm/stack-corruption.c     (revision 0)
+++ gcc/testsuite/gcc.target/arm/stack-corruption.c     (revision 0)
@@ -0,0 +1,8 @@
+/* { dg-do compile } */
+/* { dg-options "-O -mthumb -fno-omit-frame-pointer" } */
+
+int main() {
+  return 0;
+}
+
+/* { dg-final { scan-assembler-not "\tadd\tr7, sp, #8\n" } } */
Index: gcc/config/arm/arm.c
===================================================================
--- gcc/config/arm/arm.c        (revision 125840)
+++ gcc/config/arm/arm.c        (working copy)
@@ -11101,6 +11101,7 @@ arm_get_frame_offsets (void)
  if (leaf && frame_size == 0)
    {
      offsets->outgoing_args = offsets->soft_frame;
+      offsets->locals_base = offsets->soft_frame;
      return offsets;
    }

@@ -14662,6 +14663,7 @@ thumb1_expand_epilogue (void)
      amount = offsets->locals_base - offsets->saved_regs;
    }

+  gcc_assert (amount >= 0);
  if (amount)
    {
      if (amount < 512)