x32: Add a thread flag for x32 processes

An x32 process is *almost* the same thing as a 64-bit process with a
32-bit address limit, but there are a few minor differences -- in
particular core dumps are 32 bits and signal handling is different.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/include/asm/thread_info.h

@@ -96,6 +96,7 @@ struct thread_info {
 #define TIF_LAZY_MMU_UPDATES	27	/* task is updating the mmu lazily */
 #define TIF_SYSCALL_TRACEPOINT	28	/* syscall tracepoint instrumentation */
 #define TIF_ADDR32		29	/* 32-bit address space on 64 bits */
+#define TIF_X32			30	/* 32-bit native x86-64 binary */
 
 #define _TIF_SYSCALL_TRACE	(1 << TIF_SYSCALL_TRACE)
 #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
@@ -118,6 +119,7 @@ struct thread_info {
 #define _TIF_LAZY_MMU_UPDATES	(1 << TIF_LAZY_MMU_UPDATES)
 #define _TIF_SYSCALL_TRACEPOINT	(1 << TIF_SYSCALL_TRACEPOINT)
 #define _TIF_ADDR32		(1 << TIF_ADDR32)
+#define _TIF_X32		(1 << TIF_X32)
 
 /* work to do in syscall_trace_enter() */
 #define _TIF_WORK_SYSCALL_ENTRY	\

arch/x86/kernel/process_64.c

@@ -509,6 +509,7 @@ void set_personality_64bit(void)
 	/* Make sure to be in 64bit mode */
 	clear_thread_flag(TIF_IA32);
 	clear_thread_flag(TIF_ADDR32);
+	clear_thread_flag(TIF_X32);
 
 	/* Ensure the corresponding mm is not marked. */
 	if (current->mm)
@@ -528,6 +529,7 @@ void set_personality_ia32(void)
 	/* Make sure to be in 32bit mode */
 	set_thread_flag(TIF_IA32);
 	set_thread_flag(TIF_ADDR32);
+	clear_thread_flag(TIF_X32);
 	current->personality |= force_personality32;
 
 	/* Mark the associated mm as containing 32-bit tasks. */