kexec jump: check code size in control page

Kexec/Kexec-jump require code size in control page is less than
PAGE_SIZE/2. This patch add link-time checking for this.

ASSERT() of ld link script is used as the link-time checking mechanism.

[[e-mail]: build fix]
Signed-off-by: Huang Ying <[e-mail]>
Cc: Pavel Machek <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Acked-by: Vivek Goyal <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86/kernel/machine_kexec_32.c

···
 	}
 
 	control_page = page_address(image->control_code_page);
-	memcpy(control_page, relocate_kernel, PAGE_SIZE/2);
+	memcpy(control_page, relocate_kernel, KEXEC_CONTROL_CODE_MAX_SIZE);
 
 	relocate_kernel_ptr = control_page;
 	page_list[PA_CONTROL_PAGE] = __pa(control_page);

arch/x86/kernel/relocate_kernel_32.S

···
 #define PAGE_ATTR (_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED | _PAGE_DIRTY)
 #define PAE_PGD_ATTR (_PAGE_PRESENT)
 
-/* control_page + PAGE_SIZE/2 ~ control_page + PAGE_SIZE * 3/4 are
- * used to save some data for jumping back
+/* control_page + KEXEC_CONTROL_CODE_MAX_SIZE
+ * ~ control_page + PAGE_SIZE are used as data storage and stack for
+ * jumping back
  */
-#define DATA(offset)		(PAGE_SIZE/2+(offset))
+#define DATA(offset)		(KEXEC_CONTROL_CODE_MAX_SIZE+(offset))
 
 /* Minimal CPU state */
 #define ESP			DATA(0x0)
···
 	popl	%ebx
 	popl	%ebp
 	ret
+
+	.globl kexec_control_code_size
+.set kexec_control_code_size, . - relocate_kernel

arch/x86/kernel/vmlinux_32.lds.S

···
 
   DWARF_DEBUG
 }
+
+#ifdef CONFIG_KEXEC
+/* Link time checks */
+#include <asm/kexec.h>
+
+ASSERT(kexec_control_code_size <= KEXEC_CONTROL_CODE_MAX_SIZE,
+       "kexec control code size is too big")
+#endif

include/asm-x86/kexec.h

···
 # define PAGES_NR		17
 #endif
 
+#ifdef CONFIG_X86_32
+# define KEXEC_CONTROL_CODE_MAX_SIZE	2048
+#endif
+
 #ifndef __ASSEMBLY__
 
 #include <linux/string.h>