commit 7cdd140cbccc8eb62a20d12a03bed83765cbc066
Author: Stefan Liebler <stli@linux.vnet.ibm.com>
Date:   Fri Jul 31 13:42:52 2015 +0200

    S390: Fix "backtrace() returns infinitely deep stack frames with makecontext()" [BZ #18508].
    
        On s390/s390x backtrace(buffer, size) returns the series of called functions until
        "makecontext_ret" and additional entries (up to "size") with "makecontext_ret".
        GDB-backtrace is also warning:
        "Backtrace stopped: previous frame identical to this frame (corrupt stack?)"
    
        To reproduce this scenario you have to setup a new context with makecontext()
        and activate it with setcontext(). See e.g. cf() function in testcase stdlib/tst-makecontext.c.
        Or see bug in libgo "Bug 66303 - runtime.Caller() returns infinitely deep stack frames
        on s390x " (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66303).
    
        This patch omits the cfi_startproc/cfi_endproc directives in ENTRY/END macro of
        __makecontext_ret. Thus no frame information is generated in .eh_frame and backtrace
        stops after __makecontext_ret. There is also no .eh_frame info for _start or
        thread_start functions.
    
        ChangeLog:
    
        	[BZ #18508]
        	* stdlib/Makefile ($(objpfx)tst-makecontext3):
        	Depend on $(libdl).
        	* stdlib/tst-makecontext.c (cf): Test if _Unwind_Backtrace
        	is not called infinitely times.
        	(backtrace_helper): New function.
        	(trace_arg): New struct.
        	(st1): Enlarge stack size.
        	* sysdeps/unix/sysv/linux/s390/s390-32/__makecontext_ret.S:
        	(__makecontext_ret): Omit cfi_startproc and cfi_endproc.
        	* sysdeps/unix/sysv/linux/s390/s390-64/__makecontext_ret.S:
        	Likewise.

diff --git a/stdlib/Makefile b/stdlib/Makefile
index 8f22c8d..c1e80d7 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -154,3 +154,4 @@ $(objpfx)bug-getcontext: $(link-libm)
 $(objpfx)tst-strtod-round: $(link-libm)
 $(objpfx)tst-tininess: $(link-libm)
 $(objpfx)tst-strtod-underflow: $(link-libm)
+$(objpfx)tst-makecontext: $(libdl)
diff --git a/stdlib/tst-makecontext.c b/stdlib/tst-makecontext.c
index eb6e89b..1420857 100644
--- a/stdlib/tst-makecontext.c
+++ b/stdlib/tst-makecontext.c
@@ -19,23 +19,62 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <ucontext.h>
+#include <assert.h>
+#include <unwind.h>
+#include <dlfcn.h>
+#include <gnu/lib-names.h>
 
 ucontext_t ucp;
-char st1[8192];
+char st1[16384];
 __thread int thr;
 
 int somevar = -76;
 long othervar = -78L;
 
+struct trace_arg
+{
+  int cnt, size;
+};
+
+static _Unwind_Reason_Code
+backtrace_helper (struct _Unwind_Context *ctx, void *a)
+{
+  struct trace_arg *arg = a;
+  if (++arg->cnt == arg->size)
+    return _URC_END_OF_STACK;
+  return _URC_NO_REASON;
+}
+
 void
 cf (int i)
 {
+  struct trace_arg arg = { .size = 100, .cnt = -1 };
+  void *handle;
+  _Unwind_Reason_Code (*unwind_backtrace) (_Unwind_Trace_Fn, void *);
+
   if (i != othervar || thr != 94)
     {
       printf ("i %d thr %d\n", i, thr);
       exit (1);
     }
 
+  /* Test if callback function of _Unwind_Backtrace is not called infinitely
+     times. See Bug 18508 or gcc bug "Bug 66303 - runtime.Caller() returns
+     infinitely deep stack frames on s390x.".
+     The go runtime calls backtrace_full() in
+     <gcc-src>/libbacktrace/backtrace.c, which uses _Unwind_Backtrace().  */
+  handle = dlopen (LIBGCC_S_SO, RTLD_LAZY);
+  if (handle != NULL)
+    {
+      unwind_backtrace = dlsym (handle, "_Unwind_Backtrace");
+      if (unwind_backtrace != NULL)
+	{
+	  unwind_backtrace (backtrace_helper, &arg);
+	  assert (arg.cnt != -1 && arg.cnt < 100);
+	}
+      dlclose (handle);
+    }
+
   /* Since uc_link below has been set to NULL, setcontext is supposed to
      terminate the process normally after this function returns.  */
 }
diff --git a/sysdeps/unix/sysv/linux/s390/s390-32/__makecontext_ret.S b/sysdeps/unix/sysv/linux/s390/s390-32/__makecontext_ret.S
index ab172bb..365c2b0 100644
--- a/sysdeps/unix/sysv/linux/s390/s390-32/__makecontext_ret.S
+++ b/sysdeps/unix/sysv/linux/s390/s390-32/__makecontext_ret.S
@@ -17,6 +17,14 @@
 
 #include <sysdep.h>
 
+/* We do not want .eh_frame info so that __makecontext_ret stops unwinding
+   if backtrace was called within a context created by makecontext. (There
+   is also no .eh_frame info for _start or thread_start.)  */
+#undef cfi_startproc
+#define cfi_startproc
+#undef cfi_endproc
+#define cfi_endproc
+
 ENTRY(__makecontext_ret)
 	basr  %r14,%r7
 	ltr   %r8,%r8			/* Check whether uc_link is 0.  */
diff --git a/sysdeps/unix/sysv/linux/s390/s390-64/__makecontext_ret.S b/sysdeps/unix/sysv/linux/s390/s390-64/__makecontext_ret.S
index cbd88e1..c4a43bd 100644
--- a/sysdeps/unix/sysv/linux/s390/s390-64/__makecontext_ret.S
+++ b/sysdeps/unix/sysv/linux/s390/s390-64/__makecontext_ret.S
@@ -17,6 +17,14 @@
 
 #include <sysdep.h>
 
+/* We do not want .eh_frame info so that __makecontext_ret stops unwinding
+   if backtrace was called within a context created by makecontext. (There
+   is also no .eh_frame info for _start or thread_start.)  */
+#undef cfi_startproc
+#define cfi_startproc
+#undef cfi_endproc
+#define cfi_endproc
+
 ENTRY(__makecontext_ret)
 	basr	%r14,%r7
 	ltgr	%r8,%r8			/* Check whether uc_link is 0.  */