[PATCH][BZ 21672] fix pthread_create crash in ia64

[Sergei Trofimovich <slyfox at gentoo dot org>]

Minimal reproducer:

    #include <pthread.h>

    static void * f (void * p) { return NULL; }

    int main (int argc, const char ** argv) {
        pthread_t t;
        pthread_create (&t, NULL, &f, NULL);

        pthread_join (t, NULL);
        return 0;
    }

    $ gcc -O0 -ggdb3 -o r bug.c -pthread && ./r

    Program terminated with signal SIGSEGV, Segmentation fault.
    #0  0x2000000000077da0 in start_thread (arg=0x0) at pthread_create.c:432
    432         __madvise (pd->stackblock, freesize - PTHREAD_STACK_MIN, MADV_DONTNEED);

Here crash happens right after attempt to free unused part of
thread's stack.

On most architectures stack grows only down or grows only up.
And there glibc decides which of unused ends of stack blocks can be freed.

ia64 maintans two stacks. Both of them grow from the opposite directions:
 - normal "sp" stack (stack for local variables) grows down
 - register stack "bsp" grows up from the opposite end of stack block

In this failure case we have prematurely freed "rsp" stack.

The change leaves a few pages from both sides of stack block.

Bug: https://sourceware.org/PR21672
Bug: https://bugs.gentoo.org/622694
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
---
 nptl/pthread_create.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index 7a970ffc5b..6e3f6db5b1 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -555,10 +555,24 @@ START_THREAD_DEFN
   size_t pagesize_m1 = __getpagesize () - 1;
 #ifdef _STACK_GROWS_DOWN
   char *sp = CURRENT_STACK_FRAME;
-  size_t freesize = (sp - (char *) pd->stackblock) & ~pagesize_m1;
+  char *freeblock = (char *) pd->stackblock;
+  size_t freesize = (sp - freeblock) & ~pagesize_m1;
   assert (freesize < pd->stackblock_size);
+# ifdef __ia64__
   if (freesize > PTHREAD_STACK_MIN)
-    __madvise (pd->stackblock, freesize - PTHREAD_STACK_MIN, MADV_DONTNEED);
+    {
+      /* On ia64 stack grows both ways!
+         - normal "sp" stack (stack for local variables) grows down
+         - register stack "bsp" grows up from the opposite end of stack block
+
+         Thus we leave PTHREAD_STACK_MIN bytes from stack block top
+         and leave same PTHREAD_STACK_MIN at stack block bottom.  */
+      freeblock += PTHREAD_STACK_MIN;
+      freesize -= PTHREAD_STACK_MIN;
+    }
+# endif
+  if (freesize > PTHREAD_STACK_MIN)
+    __madvise (freeblock, freesize - PTHREAD_STACK_MIN, MADV_DONTNEED);
 #else
   /* Page aligned start of memory to free (higher than or equal
      to current sp plus the minimum stack size).  */
-- 
2.13.1