RISC-V: properly terminate call chain
Commit Message
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
Comments
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!
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.
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!
@@ -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
new file mode 100644
@@ -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);
+}
@@ -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