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Re: [PATCH][BZ #18508] S390: Fix "backtrace() returns infinitely deep stack frames with makecontext()".




On 06/12/2015 11:14 AM, Florian Weimer wrote:
On 06/09/2015 05:42 PM, Stefan Liebler wrote:

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?)"

Is it possible to extract a test case from the Go program, by calling
makecontext and backtrace in the same way to Go run time does?

I think this would be a useful addition to the glibc test suite.

Yes, i can make a testcase like stdlib/tst-makecontext.c or extend it - see below.

Can i call _Unwind_Backtrace directly on all architectures? Or does it exist on all architectures? The diff below uses dlopen to load libgcc, but e.g. s390 needs libgcc_s.so.1 and others like m68k needs libgcc_s.so.2.
Is there a way to load the correct libgcc_s.so on all architectures?

As an alternative i could use the glibc backtrace() method. But at least for the x86_64 version and all others who simply includes it, this testcase would not fail, because the backtrace_helper function has a check, if the address has changed in two consecutively calls. The go runtime hasn't such a check and would fail like reported by the mentioned go-bug.

Any ideas?

Bye Stefan

diff --git a/stdlib/tst-makecontext.c b/stdlib/tst-makecontext.c
index 29a588e..33f9d9e 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>

 ucontext_t ucp;
-char st1[8192];
+char st1[16384];
 __thread int thr;

+struct trace_arg
+{
+  int cnt, size;
+};
+
 int somevar = -76;
 long othervar = -78L;

+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 = __libc_dlopen ("libgcc_s.so.1");
+  if (handle != NULL)
+    {
+      unwind_backtrace = __libc_dlsym (handle, "_Unwind_Backtrace");
+      if (unwind_backtrace != NULL)
+       {
+         unwind_backtrace (backtrace_helper, &arg);
+         assert (arg.cnt != -1 && arg.cnt < 100);
+       }
+      __libc_dlclose (handle);
+    }
+
   /* Since uc_link below has been set to NULL, setcontext is supposed to
      terminate the process normally after this function returns.  */
 }


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