[PATCH] Fix tst-setcontext9 for optimized small stacks.

[Carlos O'Donell <carlos at redhat dot com>]

HJ,

Can I get a Reviewed-by from you for this? It looks like I'm keeping
the relative structure of the original test, but I'm not sure what
your intent was here with respect to CET testing. I'm seeing crashes
in this test on i686 testing as described in the commit message.

~~~
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>
-- 
Cheers,
Carlos.

>From 52d90e0548bee31f3f1cb62d63f40f31de5b1d97 Mon Sep 17 00:00:00 2001
From: Carlos O'Donell <carlos@redhat.com>
Date: Wed, 5 Sep 2018 01:16:42 -0400
Subject: [PATCH] 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>
---
 ChangeLog                |  6 +++++
 stdlib/tst-setcontext9.c | 47 ++++++++++++++++++++++++++++++++--------
 2 files changed, 44 insertions(+), 9 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 5f588784cb..7e03407a50 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2018-09-18  Carlos O'Donell  <carlos@redhat.com>
+
+	* stdlib/tst-setcontext9.c (f1): Rename to...
+	(f1a): ... this.
+	(f1b): New function implementing lower half of f1 in alternate stack.
+
 2018-09-18  Joseph Myers  <joseph@codesourcery.com>
 
 	* sysdeps/ieee754/ldbl-128ibm/s_ceill.c (ceil): Redirect to
diff --git a/stdlib/tst-setcontext9.c b/stdlib/tst-setcontext9.c
index 4636ce9030..db8355766c 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)
     {
-- 
2.17.1