i386: Add <startup.h> [BZ #21913]

On Linux/i386, there are 3 ways to make a system call:

1. call *%gs:SYSINFO_OFFSET.  This requires TLS initialization.
2. call *_dl_sysinfo.  This requires relocation of _dl_sysinfo.
3. int $0x80.  This is slower than #2 and #3, but works everywhere.

When an object file is compiled with PIC, #1 is prefered since it is
faster than #3 and doesn't require relocation of _dl_sysinfo.  For
dynamic executables, ld.so initializes TLS.  However, for static
executables, before TLS is initialized by __libc_setup_tls, #3 should
be used for system calls.

This patch adds <startup.h> which defines _startup_fatal and defaults
it to __libc_fatal.  It replaces __libc_fatal with _startup_fatal in
static executables where it is called before __libc_setup_tls is called.
This header file is included in all files containing functions which are
called before __libc_setup_tls is called.  On Linux/i386, when PIE is
enabled by default, _startup_fatal is turned into ABORT_INSTRUCTION and
I386_USE_SYSENTER is defined to 0 so that "int $0x80" is used for system
calls before __libc_setup_tls is called.

Tested on i686 and x86-64.  Without this patch, all statically-linked
tests will fail on i686 when the compiler defaults to -fPIE.

	[BZ #21913]
	* csu/libc-tls.c: Include <startup.h> first.
	(__libc_setup_tls): Call _startup_fatal instead of __libc_fatal.
	* elf/dl-tunables.c: Include <startup.h> first.
	* include/libc-symbols.h (BUILD_PIE_DEFAULT): New.
	* sysdeps/generic/startup.h: New file.
	* sysdeps/unix/sysv/linux/i386/startup.h: Likewise.
	* sysdeps/unix/sysv/linux/i386/brk.c [BUILD_PIE_DEFAULT != 0]
	(I386_USE_SYSENTER): New.  Defined to 0.

2 files changed, 41 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/i386/brk.c b/sysdeps/unix/sysv/linux/i386/brk.c
index 25ab1015d3..d67b279d55 100644
--- a/sysdeps/unix/sysv/linux/i386/brk.c
+++ b/sysdeps/unix/sysv/linux/i386/brk.c
@@ -16,6 +16,11 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#if BUILD_PIE_DEFAULT
+/* Can't use "call *%gs:SYSINFO_OFFSET" during statup in static PIE.  */
+# define I386_USE_SYSENTER 0
+#endif
+
 #include <errno.h>
 #include <unistd.h>
 #include <sysdep.h>
diff --git a/sysdeps/unix/sysv/linux/i386/startup.h b/sysdeps/unix/sysv/linux/i386/startup.h
new file mode 100644
index 0000000000..b73565a2a5
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/i386/startup.h
@@ -0,0 +1,36 @@
+/* Linux/i386 definitions of functions used by static libc main startup.
+   Copyright (C) 2017 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/>.  */
+
+#if BUILD_PIE_DEFAULT
+# include <abort-instr.h>
+
+/* Can't use "call *%gs:SYSINFO_OFFSET" during statup in static PIE.  */
+# define I386_USE_SYSENTER 0
+
+__attribute__ ((__noreturn__))
+static inline void
+_startup_fatal (const char *message __attribute__ ((unused)))
+{
+  /* This is only called very early during startup in static PIE.
+     FIXME: How can it be improved?  */
+  ABORT_INSTRUCTION;
+  __builtin_unreachable ();
+}
+#else
+# include_next <startup.h>
+#endif