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Re: [PATCH] RISC-V: properly terminate call chain

On Thu, 25 Oct 2018 02:53:44 PDT (-0700), wrote:
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.

	[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.
 elf/Makefile          |  4 +++-
 elf/tst-unwind-main.c | 38 ++++++++++++++++++++++++++++++++++++++
 sysdeps/riscv/start.S |  7 ++++++-
 3 files changed, 47 insertions(+), 2 deletions(-)
 create mode 100644 elf/tst-unwind-main.c

Thanks. Your original reply to my patch has been the top of my inbox for about 6 months now but I keep getting interrupted before I can actually write up the patch.

diff --git a/elf/Makefile b/elf/Makefile
index 455ec730fc..d277d34b17 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -187,7 +187,7 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 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-unwind-ctor
+	 tst-unwind-ctor tst-unwind-main
 #	 reldep9
 tests-internal += loadtest unload unload2 circleload1 \
 	 neededtest neededtest2 neededtest3 neededtest4 \
@@ -1495,3 +1495,5 @@ $(objpfx)tst-libc_dlvsym-static.out: $(objpfx)
 $(objpfx)tst-big-note: $(objpfx)

 $(objpfx)tst-unwind-ctor: $(objpfx)
+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 0000000000..d1236032d7
--- /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
+   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
+   <>.  */
+#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;
+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.  */
+  _Unwind_Backtrace (callback, 0);
diff --git a/sysdeps/riscv/start.S b/sysdeps/riscv/start.S
index 4635ddb5eb..ba3e2cf16c 100644
--- a/sysdeps/riscv/start.S
+++ b/sysdeps/riscv/start.S
@@ -43,6 +43,10 @@
    __libc_start_main wants this in a5.  */

+	/* 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
+	unimp

 /* Dynamic links need the global pointer to be initialized prior to calling

GCC will emit an 'ebreak' when it sees unreachable code on RISC-V, for example

   $ cat test.c
   int main()
     int *ptr = 0;
     return *ptr;
   $ riscv64-sifive-elf-gcc test.c -O3 -S -o-
   	.file	"test.c"
   	.option nopic
   	.section	.text.startup,"ax",@progbits
   	.align	1
   	.globl	main
   	.type	main, @function
   	lw	a5,0(zero)
   	.size	main, .-main
   	.ident	"GCC: (GNU) 8.1.0"
Which results in a SIGBREAK instead of a SIGILL. The decision here was a bit arbitrary, but I think we should keep it consistent between the hand-written assembly and the compiler-generated assembly.

Aside from that, this is great.  Thanks!

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