[03/12] Add <bits/indirect-return.h>

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

On 07/21/2018 10:20 AM, H.J. Lu wrote:
> 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.

I saw this discussion and I think a returns_twice is the right solution
for older compilers since it removes possibly incorrect optimizations.

> 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)

OK.

> 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.

OK with suggestion.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> ---
>  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
> 
> diff --git a/bits/indirect-return.h b/bits/indirect-return.h
> new file mode 100644
> index 0000000000..47f6f15a6e
> --- /dev/null
> +++ b/bits/indirect-return.h
> @@ -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

OK. Does nothing.

> diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
> index e80a45ca68..3f6fe3cc85 100644
> --- a/misc/sys/cdefs.h
> +++ b/misc/sys/cdefs.h
> @@ -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

OK. In gcc < 5 which lacks __has_attribute this will resolve to 0,
and with that we will get returns_twice attribute being used, and
that's fine.

> +
>  #if (!defined _Noreturn \
>       && (defined __STDC_VERSION__ ? __STDC_VERSION__ : 0) < 201112 \
>       &&  !__GNUC_PREREQ (4,7))
> diff --git a/stdlib/Makefile b/stdlib/Makefile
> index 808a8ceab7..b5e55b0a55 100644
> --- a/stdlib/Makefile
> +++ b/stdlib/Makefile
> @@ -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		      \

OK.

>  	   alloca.h fmtmsg.h						      \
>  	   bits/stdlib-bsearch.h sys/random.h bits/stdint-intn.h	      \
>  	   bits/stdint-uintn.h
> diff --git a/stdlib/ucontext.h b/stdlib/ucontext.h
> index eec7611631..ec630038f6 100644
> --- a/stdlib/ucontext.h
> +++ b/stdlib/ucontext.h
> @@ -22,6 +22,9 @@
>  
>  #include <features.h>
>  
> +/* Get definition of __INDIRECT_RETURN.  */
> +#include <bits/indirect-return.h>

OK.

> +
>  /* 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;

OK.

>  
>  /* Manipulate user context UCP to continue with calling functions FUNC
>     and the ARGC-1 parameters following ARGC when the context is used
> diff --git a/string/tst-xbzero-opt.c b/string/tst-xbzero-opt.c
> index cf7041f37a..aab4a7f715 100644
> --- a/string/tst-xbzero-opt.c
> +++ b/string/tst-xbzero-opt.c
> @@ -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

OK. I don't think this will impact the semantics of the test, the compiler
should still be able to see the local call and carry out any optimizations
it needs to do.

>  prepare_test_buffer (unsigned char *buf)
>  {
>    for (unsigned int i = 0; i < PATTERN_REPS; i++)
> diff --git a/sysdeps/x86/bits/indirect-return.h b/sysdeps/x86/bits/indirect-return.h
> new file mode 100644
> index 0000000000..0587e687ac
> --- /dev/null
> +++ b/sysdeps/x86/bits/indirect-return.h
> @@ -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.  */

Suggest:

/* Newer compilers provide the indirect_return attribute, but without
   it we can use returns_twice to affect the optimizer in the same
   way and avoid unsafe optimizations.  */

> +#  define __INDIRECT_RETURN __attribute__ ((__returns_twice__))
> +# endif
> +#else
> +# define __INDIRECT_RETURN
> +#endif
>