RISC-V: properly terminate call chain

Message ID mvmin1qpd1z.fsf@suse.de
State New
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Series
  • RISC-V: properly terminate call chain
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Commit Message

Andreas Schwab Oct. 25, 2018, 9:53 a.m.
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

-- 
2.19.1


-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."

Comments

Palmer Dabbelt Oct. 29, 2018, 5:14 p.m. | #1
On Thu, 25 Oct 2018 02:53:44 PDT (-0700), schwab@suse.de 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)tst-libc_dlvsym-dso.so

>  $(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

> 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

> +   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 4635ddb5eb..ba3e2cf16c 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

> +	unimp

>  END (ENTRY_POINT)

>

>  /* 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
    	.text
    	.section	.text.startup,"ax",@progbits
    	.align	1
    	.globl	main
    	.type	main, @function
    main:
    	lw	a5,0(zero)
    	ebreak
    	.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!
Andreas Schwab Oct. 30, 2018, 11:09 a.m. | #2
On Okt 29 2018, Palmer Dabbelt <palmer@sifive.com> wrote:

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


Pushed with that change.

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."
Palmer Dabbelt Oct. 30, 2018, 6:03 p.m. | #3
On Tue, 30 Oct 2018 04:09:00 PDT (-0700), schwab@suse.de wrote:
> On Okt 29 2018, Palmer Dabbelt <palmer@sifive.com> wrote:

>

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

>

> Pushed with that change.


Thanks!

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)tst-libc_dlvsym-dso.so
 $(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
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
+   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 4635ddb5eb..ba3e2cf16c 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
+	unimp
 END (ENTRY_POINT)
 
 /* Dynamic links need the global pointer to be initialized prior to calling