Make getcontext(2) work on i386. It needs a small wrapper in libc

otherwise the return from the syscall stub for getcontext will pop off
the return value for the caller to the getcontext stub and it will appear
as though the setcontext() syscall returned instead of the getcontext().
The same bug exists on amd64, a fix is coming there too.

The bug can be demonstrated with this test code fragment:
main()
{
        ucontext_t top;

        if (getcontext(&top) == 0) {
                write(2, "PING!\n", 6);
                /* Cause a return value of 1 from getcontext this time */
                top.uc_mcontext.mc_eax = 1;
                setcontext(&top);
                err(1, "setcontext() returned");
        }
        write(2, "PONG!\n", 6);
        _exit(0);
}

2 files changed, 52 insertions, 2 deletions

diff --git a/lib/libc/i386/sys/Makefile.inc b/lib/libc/i386/sys/Makefile.inc
index ad76609..5adee65 100644
--- a/lib/libc/i386/sys/Makefile.inc
+++ b/lib/libc/i386/sys/Makefile.inc
@@ -4,8 +4,8 @@
 SRCS+=	i386_clr_watch.c i386_get_ioperm.c i386_get_ldt.c i386_set_ioperm.c \
 	i386_set_ldt.c i386_set_watch.c i386_vm86.c
 
-MDASM=	Ovfork.S brk.S cerror.S exect.S pipe.S ptrace.S reboot.S sbrk.S \
-	setlogin.S sigreturn.S syscall.S
+MDASM=	Ovfork.S brk.S cerror.S exect.S getcontext.S pipe.S ptrace.S \
+	reboot.S sbrk.S setlogin.S sigreturn.S syscall.S
 
 # Don't generate default code for these syscalls:
 NOASM=	break.o exit.o ftruncate.o getdomainname.o getlogin.o \
diff --git a/lib/libc/i386/sys/getcontext.S b/lib/libc/i386/sys/getcontext.S
new file mode 100644
index 0000000..b66ad30
--- /dev/null
+++ b/lib/libc/i386/sys/getcontext.S
@@ -0,0 +1,50 @@
+/*-
+ * Copyright (c) 2003 Peter Wemm <peter@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <machine/asm.h>
+__FBSDID("$FreeBSD$");
+
+#include <SYS.h>
+
+/*
+ * This has to be magic to handle the multiple returns.
+ * Otherwise, the setcontext() syscall will return here and we'll
+ * pop off the return address and go to the *setcontext* call.
+ */
+	.weak	_getcontext
+	.set	_getcontext,__sys_getcontext
+	.weak	getcontext
+	.set	getcontext,__sys_getcontext
+ENTRY(__sys_getcontext)
+	movl	(%esp),%ecx	/* save getcontext return address */
+	mov	$SYS_getcontext,%eax
+	KERNCALL
+	jb	1f
+	addl	$4,%esp		/* remove stale (setcontext) return address */
+	jmp	*%ecx		/* restore return address */
+1:
+	PIC_PROLOGUE
+	jmp	PIC_PLT(HIDENAME(cerror))