V3 [PATCH] ucontext.h: Add __INDIRECT_RETURN
Commit Message
On Thu, Jul 19, 2018 at 02:21:46PM +0200, Florian Weimer wrote:
> * H. J. Lu:
>
> > +/* On x86, swapcontext returns via indirect branch when the shadow stack
> > + is enabled. Define __INDIRECT_RETURN to indicate whether swapcontext
> > + returns via indirect branch. */
> > +#if defined __CET__ && (__CET__ & 2) != 0
> > +/* Note: Functions returned via indirect branch returns once. Without
> > + indirect_return attribute, use returns_twice attribute. */
>
> I still don't understand the first sentence of the second comment.
Here is the updated patch with the first sentence removed.
OK for master?
Thanks.
H.J.
---
Add <bits/indirect-return.h> and include it in <ucontext.h>.
__INDIRECT_RETURN defined in <bits/indirect-return.h> indicates if
swapcontext requires special compiler treatment. The default
__INDIRECT_RETURN is empty.
On x86, when shadow stack is enabled, __INDIRECT_RETURN is defined
with indirect_return attribute, which has been added to GCC 9, to
indicate that swapcontext returns via indirect branch. Otherwise
__INDIRECT_RETURN is defined with returns_twice attribute.
When shadow stack is enabled, remove always_inline attribute from
prepare_test_buffer in string/tst-xbzero-opt.c to avoid:
tst-xbzero-opt.c: In function ‘prepare_test_buffer’:
tst-xbzero-opt.c:105:1: error: function ‘prepare_test_buffer’ can never be inlined because it uses setjmp
prepare_test_buffer (unsigned char *buf)
when indirect_return attribute isn't available.
* bits/indirect-return.h: New file.
* misc/sys/cdefs.h (__glibc_has_attribute): New.
* sysdeps/x86/bits/indirect-return.h: Likewise.
* stdlib/Makefile (headers): Add bits/indirect-return.h.
* stdlib/ucontext.h: Include <bits/indirect-return.h>.
(swapcontext): Add __INDIRECT_RETURN.
* string/tst-xbzero-opt.c (ALWAYS_INLINE): New.
(prepare_test_buffer): Use it.
---
bits/indirect-return.h | 25 +++++++++++++++++++++
misc/sys/cdefs.h | 6 +++++
stdlib/Makefile | 2 +-
stdlib/ucontext.h | 6 ++++-
string/tst-xbzero-opt.c | 10 ++++++++-
sysdeps/x86/bits/indirect-return.h | 35 ++++++++++++++++++++++++++++++
6 files changed, 81 insertions(+), 3 deletions(-)
create mode 100644 bits/indirect-return.h
create mode 100644 sysdeps/x86/bits/indirect-return.h
new file mode 100644
@@ -0,0 +1,25 @@
+/* Definition of __INDIRECT_RETURN. Generic version.
+ 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/>. */
+
+#ifndef _UCONTEXT_H
+# error "Never include <bits/indirect-return.h> directly; use <ucontext.h> instead."
+#endif
+
+/* __INDIRECT_RETURN is used on swapcontext to indicate if it requires
+ special compiler treatment. */
+#define __INDIRECT_RETURN
@@ -406,6 +406,12 @@
# define __glibc_likely(cond) (cond)
#endif
+#ifdef __has_attribute
+# define __glibc_has_attribute(attr) __has_attribute (attr)
+#else
+# define __glibc_has_attribute(attr) 0
+#endif
+
#if (!defined _Noreturn \
&& (defined __STDC_VERSION__ ? __STDC_VERSION__ : 0) < 201112 \
&& !__GNUC_PREREQ (4,7))
@@ -26,7 +26,7 @@ headers := stdlib.h bits/stdlib.h bits/stdlib-ldbl.h bits/stdlib-float.h \
monetary.h bits/monetary-ldbl.h \
inttypes.h stdint.h bits/wordsize.h \
errno.h sys/errno.h bits/errno.h bits/types/error_t.h \
- ucontext.h sys/ucontext.h \
+ ucontext.h sys/ucontext.h bits/indirect-return.h \
alloca.h fmtmsg.h \
bits/stdlib-bsearch.h sys/random.h bits/stdint-intn.h \
bits/stdint-uintn.h
@@ -22,6 +22,9 @@
#include <features.h>
+/* Get definition of __INDIRECT_RETURN. */
+#include <bits/indirect-return.h>
+
/* Get machine dependent definition of data structures. */
#include <sys/ucontext.h>
@@ -36,7 +39,8 @@ extern int setcontext (const ucontext_t *__ucp) __THROWNL;
/* Save current context in context variable pointed to by OUCP and set
context from variable pointed to by UCP. */
extern int swapcontext (ucontext_t *__restrict __oucp,
- const ucontext_t *__restrict __ucp) __THROWNL;
+ const ucontext_t *__restrict __ucp)
+ __THROWNL __INDIRECT_RETURN;
/* Manipulate user context UCP to continue with calling functions FUNC
and the ARGC-1 parameters following ARGC when the context is used
@@ -100,7 +100,15 @@ static ucontext_t uc_main, uc_co;
/* Always check the test buffer immediately after filling it; this
makes externally visible side effects depend on the buffer existing
and having been filled in. */
-static inline __attribute__ ((always_inline)) void
+#if defined __CET__ && !__glibc_has_attribute (__indirect_return__)
+/* Note: swapcontext returns via indirect branch when SHSTK is enabled.
+ Without indirect_return attribute, swapcontext is marked with
+ returns_twice attribute, which prevents always_inline to work. */
+# define ALWAYS_INLINE
+#else
+# define ALWAYS_INLINE __attribute__ ((always_inline))
+#endif
+static inline ALWAYS_INLINE void
prepare_test_buffer (unsigned char *buf)
{
for (unsigned int i = 0; i < PATTERN_REPS; i++)
new file mode 100644
@@ -0,0 +1,35 @@
+/* Definition of __INDIRECT_RETURN. x86 version.
+ 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/>. */
+
+#ifndef _UCONTEXT_H
+# error "Never include <bits/indirect-return.h> directly; use <ucontext.h> instead."
+#endif
+
+/* On x86, swapcontext returns via indirect branch when the shadow stack
+ is enabled. Define __INDIRECT_RETURN to indicate whether swapcontext
+ returns via indirect branch. */
+#if defined __CET__ && (__CET__ & 2) != 0
+# if __glibc_has_attribute (__indirect_return__)
+# define __INDIRECT_RETURN __attribute__ ((__indirect_return__))
+# else
+/* Without indirect_return attribute, use returns_twice attribute. */
+# define __INDIRECT_RETURN __attribute__ ((__returns_twice__))
+# endif
+#else
+# define __INDIRECT_RETURN
+#endif