support: Add support_stack_alloc

The code to allocate a stack from xsigstack is refactored so it can
be more generic.  The new support_stack_alloc() also set PROT_EXEC
if DEFAULT_STACK_PERMS has PF_X.  This is required on some
 architectures (hppa for instance) and trying to access the rtld
global from testsuite will require more intrusive refactoring
in the ldsodefs.h header.

Checked on x86_64-linux-gnu and i686-linux-gnu.  I also ran
tst-xsigstack on both hppa and ia64.

1 files changed, 5 insertions, 38 deletions

diff --git a/support/xsigstack.c b/support/xsigstack.c
index a2f0e3269a..a471c853cb 100644
--- a/support/xsigstack.c
+++ b/support/xsigstack.c
@@ -37,8 +37,7 @@
    structures.  */
 struct sigstack_desc
 {
-  void *alloc_base;  /* Base address of the complete allocation.  */
-  size_t alloc_size; /* Size of the complete allocation.  */
+  struct support_stack stack;
   stack_t alt_stack; /* The address and size of the stack itself.  */
   stack_t old_stack; /* The previous signal stack.  */
 };
@@ -46,43 +45,11 @@ struct sigstack_desc
 void *
 xalloc_sigstack (size_t size)
 {
-  size_t pagesize = sysconf (_SC_PAGESIZE);
-  if (pagesize == -1)
-    FAIL_EXIT1 ("sysconf (_SC_PAGESIZE): %m\n");
-
-  /* Always supply at least MINSIGSTKSZ space; passing 0 as size means
-     only that much space.  No matter what the number is, round it up
-     to a whole number of pages.  */
-  size_t stacksize = roundup (size + MINSIGSTKSZ, pagesize);
-
-  /* The guard bands need to be large enough to intercept offset
-     accesses from a stack address that might otherwise hit another
-     mapping.  Make them at least twice as big as the stack itself, to
-     defend against an offset by the entire size of a large
-     stack-allocated array.  The minimum is 1MiB, which is arbitrarily
-     chosen to be larger than any "typical" wild pointer offset.
-     Again, no matter what the number is, round it up to a whole
-     number of pages.  */
-  size_t guardsize = roundup (MAX (2 * stacksize, 1024 * 1024), pagesize);
-
   struct sigstack_desc *desc = xmalloc (sizeof (struct sigstack_desc));
-  desc->alloc_size = guardsize + stacksize + guardsize;
-  /* Use MAP_NORESERVE so that RAM will not be wasted on the guard
-     bands; touch all the pages of the actual stack before returning,
-     so we know they are allocated.  */
-  desc->alloc_base = xmmap (0,
-                            desc->alloc_size,
-                            PROT_READ|PROT_WRITE,
-                            MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE|MAP_STACK,
-                            -1);
-
-  xmprotect (desc->alloc_base, guardsize, PROT_NONE);
-  xmprotect (desc->alloc_base + guardsize + stacksize, guardsize, PROT_NONE);
-  memset (desc->alloc_base + guardsize, 0xA5, stacksize);
-
-  desc->alt_stack.ss_sp    = desc->alloc_base + guardsize;
+  desc->stack = support_stack_alloc (size);
+  desc->alt_stack.ss_sp    = desc->stack.stack;
   desc->alt_stack.ss_flags = 0;
-  desc->alt_stack.ss_size  = stacksize;
+  desc->alt_stack.ss_size  = desc->stack.size;
 
   if (sigaltstack (&desc->alt_stack, &desc->old_stack))
     FAIL_EXIT1 ("sigaltstack (new stack: sp=%p, size=%zu, flags=%u): %m\n",
@@ -101,7 +68,7 @@ xfree_sigstack (void *stack)
     FAIL_EXIT1 ("sigaltstack (restore old stack: sp=%p, size=%zu, flags=%u): "
                 "%m\n", desc->old_stack.ss_sp, desc->old_stack.ss_size,
                 desc->old_stack.ss_flags);
-  xmunmap (desc->alloc_base, desc->alloc_size);
+  support_stack_free (&desc->stack);
   free (desc);
 }