[2.28 COMMITTED] RISC-V: properly terminate call chain (bug 23125)

[2.28 COMMITTED] RISC-V: properly terminate call chain (bug 23125)

[Aurelien Jarno]

From: Andreas Schwab <schwab@suse.de>

Mark the ra register as undefined in _start, so that unwinding through
main works correctly.  Also, don't use a tail call so that ra points after
the call to __libc_start_main, not after the previous call.

(cherry picked from commit 2dd12baa045f25c52b30a34b10f72d51f2605413)
---
 ChangeLog             |  9 +++++++++
 elf/Makefile          |  5 ++++-
 elf/tst-unwind-main.c | 38 ++++++++++++++++++++++++++++++++++++++
 sysdeps/riscv/start.S |  7 ++++++-
 4 files changed, 57 insertions(+), 2 deletions(-)
 create mode 100644 elf/tst-unwind-main.c

diff --git a/ChangeLog b/ChangeLog
index d8459496bac..81555f1675a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2018-10-30  Andreas Schwab  <schwab@suse.de>
+
+	[BZ #23125]
+	* sysdeps/riscv/start.S (ENTRY_POINT): Mark ra as undefined.
+	Don't use tail call.
+	* elf/tst-unwind-main.c: New file.
+	* elf/Makefile (tests): Add tst-unwind-main.
+	(CFLAGS-tst-unwind-main.c): Define.
+
 2018-12-15  Florian Weimer  <fweimer@redhat.com>
 
 	* support/blob_repeat.c (check_mul_overflow_size_t): New function.
diff --git a/elf/Makefile b/elf/Makefile
index cd0771307f9..43f625af056 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -186,7 +186,8 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 tst-tlsalign tst-tlsalign-extern tst-nodelete-opened \
 	 tst-nodelete2 tst-audit11 tst-audit12 tst-dlsym-error tst-noload \
 	 tst-latepthread tst-tls-manydynamic tst-nodelete-dlclose \
-	 tst-debug1 tst-main1 tst-absolute-sym tst-absolute-zero tst-big-note
+	 tst-debug1 tst-main1 tst-absolute-sym tst-absolute-zero tst-big-note \
+	 tst-unwind-main
 #	 reldep9
 tests-internal += loadtest unload unload2 circleload1 \
 	 neededtest neededtest2 neededtest3 neededtest4 \
@@ -1484,3 +1485,5 @@ tst-libc_dlvsym-static-ENV = \
 $(objpfx)tst-libc_dlvsym-static.out: $(objpfx)tst-libc_dlvsym-dso.so
 
 $(objpfx)tst-big-note: $(objpfx)tst-big-note-lib.so
+
+CFLAGS-tst-unwind-main.c += -funwind-tables
diff --git a/elf/tst-unwind-main.c b/elf/tst-unwind-main.c
new file mode 100644
index 00000000000..d1236032d77
--- /dev/null
+++ b/elf/tst-unwind-main.c
@@ -0,0 +1,38 @@
+/* Test unwinding through main.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <unwind.h>
+#include <unistd.h>
+#include <support/test-driver.h>
+
+static _Unwind_Reason_Code
+callback (struct _Unwind_Context *ctx, void *arg)
+{
+  return _URC_NO_REASON;
+}
+
+int
+main (void)
+{
+  /* 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.  */
+  alarm (DEFAULT_TIMEOUT);
+  _Unwind_Backtrace (callback, 0);
+}
diff --git a/sysdeps/riscv/start.S b/sysdeps/riscv/start.S
index 4635ddb5eb8..2d6f06e6305 100644
--- a/sysdeps/riscv/start.S
+++ b/sysdeps/riscv/start.S
@@ -43,6 +43,10 @@
    __libc_start_main wants this in a5.  */
 
 ENTRY (ENTRY_POINT)
+	/* Terminate call stack by noting ra is undefined.  Use a dummy
+	   .cfi_label to force starting the FDE.  */
+	.cfi_label .Ldummy
+	cfi_undefined (ra)
 	call  .Lload_gp
 	mv    a5, a0  /* rtld_fini.  */
 	/* main may be in a shared library.  */
@@ -54,7 +58,8 @@ ENTRY (ENTRY_POINT)
 	lla   a4, __libc_csu_fini
 	mv    a6, sp  /* stack_end.  */
 
-	tail  __libc_start_main@plt
+	call  __libc_start_main@plt
+	ebreak
 END (ENTRY_POINT)
 
 /* Dynamic links need the global pointer to be initialized prior to calling
-- 
2.19.1