commit a55e109709af55e6ed67d3f9536cac5d929c982e

Author: Carlos O'Donell <carlos@redhat.com>

Date:   Wed Sep 5 01:16:42 2018 -0400

    Fix tst-setcontext9 for optimized small stacks.

    If the compiler reduces the stack usage in function f1 before calling

    into function f2, then when we swapcontext back to f1 and continue

    execution we may overwrite registers that were spilled to the stack

    while f2 was executing.  Later when we return to f2 the corrupt

    registers will be reloaded from the stack and the test will crash.  This

    was most commonly observed on i686 with __x86.get_pc_thunk.dx and

    needing to save and restore $edx.  Overall i686 has few registers and

    the spilling to the stack is bound to happen, therefore the solution to

    making this test robust is to split function f1 into two parts f1a and

    f1b, and allocate f1b it's own stack such that subsequent execution does

    not overwrite the stack in use by function f2.

    Tested on i686 and x86_64.

    Signed-off-by: Carlos O'Donell <carlos@redhat.com>

    (cherry picked from commit 791b350dc725545e3f9b5db0f97ebdbc60c9735f)

diff --git a/stdlib/tst-setcontext9.c b/stdlib/tst-setcontext9.c

index 4636ce9030fa38a7..db8355766ca7b906 100644

--- a/stdlib/tst-setcontext9.c

+++ b/stdlib/tst-setcontext9.c

@@ -41,26 +41,55 @@ f2 (void)

 }

 static void

-f1 (void)

+f1b (void)

 {

-  puts ("start f1");

-  if (getcontext (&ctx[2]) != 0)

-    {

-      printf ("%s: getcontext: %m\n", __FUNCTION__);

-      exit (EXIT_FAILURE);

-    }

   if (done)

     {

-      puts ("set context in f1");

+      puts ("set context in f1b");

       if (setcontext (&ctx[3]) != 0)

 	{

 	  printf ("%s: setcontext: %m\n", __FUNCTION__);

 	  exit (EXIT_FAILURE);

 	}

     }

+  exit (EXIT_FAILURE);

+}

+

+static void

+f1a (void)

+{

+  char st2[32768];

+  puts ("start f1a");

+  if (getcontext (&ctx[2]) != 0)

+    {

+      printf ("%s: getcontext: %m\n", __FUNCTION__);

+      exit (EXIT_FAILURE);

+    }

+  ctx[2].uc_stack.ss_sp = st2;

+  ctx[2].uc_stack.ss_size = sizeof st2;

+  ctx[2].uc_link = &ctx[0];

+  makecontext (&ctx[2], (void (*) (void)) f1b, 0);

   f2 ();

 }

+/* The execution path through the test looks like this:

+   do_test (call)

+   -> "making contexts"

+   -> "swap contexts"

+   f1a (via swapcontext to ctx[1], with alternate stack)

+   -> "start f1a"

+   f2 (call)

+   -> "swap contexts in f2"

+   f1b (via swapcontext to ctx[2], with alternate stack)

+   -> "set context in f1b"

+   do_test (via setcontext to ctx[3], main stack)

+   -> "setcontext"

+   f2 (via setcontext to ctx[4], with alternate stack)

+   -> "end f2"

+

+   We must use an alternate stack for f1b, because if we don't then the

+   result of executing an earlier caller may overwrite registers

+   spilled to the stack in f2.  */

 static int

 do_test (void)

 {

@@ -79,7 +108,7 @@ do_test (void)

   ctx[1].uc_stack.ss_sp = st1;

   ctx[1].uc_stack.ss_size = sizeof st1;

   ctx[1].uc_link = &ctx[0];

-  makecontext (&ctx[1], (void (*) (void)) f1, 0);

+  makecontext (&ctx[1], (void (*) (void)) f1a, 0);

   puts ("swap contexts");

   if (swapcontext (&ctx[3], &ctx[1]) != 0)

     {