x86: fix fragile computation of vsyscall address

Previously, the address of the vsyscall page (VSYSCALL_PHYS_ADDR,
VSYSCALL_VIRT_ADDR) was computed by arithmetic on the address of the
last section.  This leads to bugs when new sections are inserted, such
as the one fixed by commit d312ceda567ab91acd756cde95ac5fbc6b40ed40.
Let's compute it from the current address instead.

Signed-off-by: Anders Kaseorg <andersk@ksplice.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/vmlinux.lds.S

@@ -135,24 +135,21 @@ SECTIONS
 #ifdef CONFIG_X86_64
 
 #define VSYSCALL_ADDR (-10*1024*1024)
-#define VSYSCALL_PHYS_ADDR ((LOADADDR(.data) + SIZEOF(.data) + \
-                            PAGE_SIZE - 1) & ~(PAGE_SIZE - 1))
-#define VSYSCALL_VIRT_ADDR ((ADDR(.data) + SIZEOF(.data) + \
-                            PAGE_SIZE - 1) & ~(PAGE_SIZE - 1))
 
-#define VLOAD_OFFSET (VSYSCALL_ADDR - VSYSCALL_PHYS_ADDR)
+#define VLOAD_OFFSET (VSYSCALL_ADDR - __vsyscall_0 + LOAD_OFFSET)
 #define VLOAD(x) (ADDR(x) - VLOAD_OFFSET)
 
-#define VVIRT_OFFSET (VSYSCALL_ADDR - VSYSCALL_VIRT_ADDR)
+#define VVIRT_OFFSET (VSYSCALL_ADDR - __vsyscall_0)
 #define VVIRT(x) (ADDR(x) - VVIRT_OFFSET)
 
+	. = ALIGN(4096);
+	__vsyscall_0 = .;
+
 	. = VSYSCALL_ADDR;
-	.vsyscall_0 : AT(VSYSCALL_PHYS_ADDR) {
+	.vsyscall_0 : AT(VLOAD(.vsyscall_0)) {
 		*(.vsyscall_0)
 	} :user
 
-	__vsyscall_0 = VSYSCALL_VIRT_ADDR;
-
 	. = ALIGN(CONFIG_X86_L1_CACHE_BYTES);
 	.vsyscall_fn : AT(VLOAD(.vsyscall_fn)) {
 		*(.vsyscall_fn)
@@ -192,11 +189,9 @@ SECTIONS
 		*(.vsyscall_3)
 	}
 
-	. = VSYSCALL_VIRT_ADDR + PAGE_SIZE;
+	. = __vsyscall_0 + PAGE_SIZE;
 
 #undef VSYSCALL_ADDR
-#undef VSYSCALL_PHYS_ADDR
-#undef VSYSCALL_VIRT_ADDR
 #undef VLOAD_OFFSET
 #undef VLOAD
 #undef VVIRT_OFFSET