RISC-V: Fix elfutils testsuite unwind failures.

The clone.S patch fixes 2 elfutils testsuite unwind failures, where the
backtrace gets stuck repeating __thread_start until we hit the backtrace
limit.  This was confirmed by building and installing a patched glibc and
then building elfutils and running its testsuite.

Unfortunately, the testcase isn't working as expected and I don't know why.
The testcase passes even when my clone.S patch is not installed.  The testcase
looks logically similarly to the elfutils testcases that are failing.  Maybe
there is a subtle difference in how the glibc unwinding works versus the
elfutils unwinding?  I don't have good gdb pthread support yet, so I haven't
found a way to debug this.  Anyways, I don't know if the testcase is useful or
not.  If the testcase isn't useful then maybe the clone.S patch is OK without
a testcase?

Jim

	[BZ #24040]
	* elf/Makefile (CFLAGS-tst-unwind-main.c): Add -DUSE_PTHREADS=0.
	* elf/tst-unwind-main.c: If USE_PTHEADS, include pthread.h and error.h
	(func): New.
	(main): If USE_PTHREADS, call pthread_create to run func.  Otherwise
	call func directly.
	* nptl/Makefile (tests): Add tst-unwind-thread.
	(CFLAGS-tst-unwind-thread.c): Define.
	* nptl/tst-unwind-thread.c: New file.
	* sysdeps/unix/sysv/linux/riscv/clone.S (__thread_start): Mark ra
	as undefined.

ChangeLog

@@ -1,3 +1,17 @@
+2019-01-13  Jim Wilson  <jimw@sifive.com>
+
+	[BZ #24040]
+	* elf/Makefile (CFLAGS-tst-unwind-main.c): Add -DUSE_PTHREADS=0.
+	* elf/tst-unwind-main.c: If USE_PTHEADS, include pthread.h and error.h
+	(func): New.
+	(main): If USE_PTHREADS, call pthread_create to run func.  Otherwise
+	call func directly.
+	* nptl/Makefile (tests): Add tst-unwind-thread.
+	(CFLAGS-tst-unwind-thread.c): Define.
+	* nptl/tst-unwind-thread.c: New file.
+	* sysdeps/unix/sysv/linux/riscv/clone.S (__thread_start): Mark ra
+	as undefined.
+
 2019-02-13  Joseph Myers  <joseph@codesourcery.com>
 
 	* iconvdata/cns11643.h (ucs4_to_cns11643): Adjust fall-through

elf/Makefile

@@ -1512,4 +1512,4 @@ $(objpfx)tst-big-note: $(objpfx)tst-big-note-lib.so
 
 $(objpfx)tst-unwind-ctor: $(objpfx)tst-unwind-ctor-lib.so
 
-CFLAGS-tst-unwind-main.c += -funwind-tables
+CFLAGS-tst-unwind-main.c += -funwind-tables -DUSE_PTHREADS=0

elf/tst-unwind-main.c

@@ -20,19 +20,41 @@
 #include <unistd.h>
 #include <support/test-driver.h>
 
+#if USE_PTHREADS
+# include <pthread.h>
+# include <error.h>
+#endif
+
 static _Unwind_Reason_Code
 callback (struct _Unwind_Context *ctx, void *arg)
 {
   return _URC_NO_REASON;
 }
 
-int
-main (void)
+static void *
+func (void *a)
 {
   /* Arrange for this test to be killed if _Unwind_Backtrace runs into an
      endless loop.  We cannot use the test driver because the complete
      call chain needs to be compiled with -funwind-tables so that
-     _Unwind_Backtrace is able to reach _start.  */
+     _Unwind_Backtrace is able to reach the start routine.  */
   alarm (DEFAULT_TIMEOUT);
   _Unwind_Backtrace (callback, 0);
+  return a;
+}
+
+int
+main (void)
+{
+#if USE_PTHREADS
+  pthread_t thr;
+  int rc = pthread_create (&thr, NULL, &func, NULL);
+  if (rc)
+    error (1, rc, "pthread_create");
+  rc = pthread_join (thr, NULL);
+  if (rc)
+    error (1, rc, "pthread_join");
+#else
+  func (NULL);
+#endif
 }

nptl/Makefile

@@ -320,7 +320,8 @@ tests = tst-attr1 tst-attr2 tst-attr3 tst-default-attr \
 	tst-cnd-timedwait tst-thrd-detach tst-mtx-basic tst-thrd-sleep \
 	tst-mtx-recursive tst-tss-basic tst-call-once tst-mtx-timedlock \
 	tst-rwlock-pwn \
-	tst-rwlock-tryrdlock-stall tst-rwlock-trywrlock-stall
+	tst-rwlock-tryrdlock-stall tst-rwlock-trywrlock-stall \
+	tst-unwind-thread
 
 tests-internal := tst-rwlock19 tst-rwlock20 \
 		  tst-sem11 tst-sem12 tst-sem13 \
@@ -710,6 +711,8 @@ $(objpfx)tst-audit-threads: $(objpfx)tst-audit-threads-mod2.so
 $(objpfx)tst-audit-threads.out: $(objpfx)tst-audit-threads-mod1.so
 tst-audit-threads-ENV = LD_AUDIT=$(objpfx)tst-audit-threads-mod1.so
 
+CFLAGS-tst-unwind-thread.c += -funwind-tables
+
 # The tests here better do not run in parallel
 ifneq ($(filter %tests,$(MAKECMDGOALS)),)
 .NOTPARALLEL:

nptl/tst-unwind-thread.c

@@ -0,0 +1,2 @@
+#define USE_PTHREADS 1
+#include "../elf/tst-unwind-main.c"

sysdeps/unix/sysv/linux/riscv/clone.S

@@ -69,6 +69,11 @@ L (error):
 
 ENTRY (__thread_start)
 L (thread_start):
+	/* Terminate call stack by noting ra is undefined.  Use a dummy
+	   .cfi_label to force starting the FDE.  */
+	.cfi_label .Ldummy
+	cfi_undefined (ra)
+
 	/* Restore the arg for user's function.  */
 	REG_L		a1,0(sp)	/* Function pointer.  */
 	REG_L		a0,SZREG(sp)	/* Argument pointer.  */