Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Thu, Jun 23, 2016 at 12:17 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> With the goal being that I'm hoping that we can then actually get rid
> of this (at least on x86-64, even if we leave it in some other
> architectures) in 4.8.

The context here was that we could almost get rid of thread-info
entirely, at least for x86-64, by moving it into struct task_struct.

It turns out that we're not *that* far off after the obvious cleanups
I already committed, but I couldn't get things quite to work.

I'm attaching a patch that I wrote today that doesn't boot, but "looks
right". The reason I'm attaching it is because I'm hoping somebody
wants to take a look and maybe see what else I missed, but mostly
because I think the patch is interesting in a couple of cases where we
just do incredibly ugly things.

First off, some code that Andy wrote when he re-organized the entry path.

Oh Gods, Andy. That pt_regs_to_thread_info() thing made me want to do
unspeakable acts on a poor innocent wax figure that looked _exactly_
like you.

I just got rid of pt_regs_to_thread_info() entirely, and just replaced
it with current_thread_info().  I'm not at all convinced that trying
to be that clever was really a good idea.

Secondly, the x86-64 ret_from_fork calling convention was documented
wrongly. It says %rdi contains the previous task pointer. Yes it does,
but it doesn't mention that %r8 is supposed to contain the new
thread_info. That was fun to find.

And thirdly, the stack size games that asm/kprobes.h plays are just
disgusting. I stared at that code for much too long. I may in fact be
going blind as a result.

The rest was fairly straightforward, although since the end result
doesn't actually work, that "straightforward" may be broken too. But
the basic approach _looks_ sane.

Comments? Anybody want to play with this and see where I went wrong?

(Note - this patch was written on top of the two thread-info removal
patches I committed in

   da01e18a37a5 x86: avoid avoid passing around 'thread_info' in stack
dumping code
   6720a305df74 locking: avoid passing around 'thread_info' in mutex
debugging code

and depends on them, since "ti->task" no longer exists with
CONFIG_THREAD_INFO_IN_TASK. "ti" and "task" will have the same value).

                 Linus

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This is a non-working attempt at moving the thread_info into the
task_struct

 arch/x86/Kconfig                   |  1 +
 arch/x86/entry/common.c            | 21 +++++++--------------
 arch/x86/entry/entry_64.S          |  9 ++++++---
 arch/x86/include/asm/kprobes.h     | 12 ++++++------
 arch/x86/include/asm/switch_to.h   |  6 ++----
 arch/x86/include/asm/thread_info.h | 38 ++++----------------------------------
 arch/x86/kernel/dumpstack.c        |  2 +-
 arch/x86/kernel/irq_32.c           |  2 --
 arch/x86/kernel/irq_64.c           |  3 +--
 arch/x86/kernel/process.c          |  6 ++----
 arch/x86/um/ptrace_32.c            |  8 ++++----
 include/linux/init_task.h          |  9 +++++++++
 include/linux/sched.h              | 14 +++++++++++++-
 init/Kconfig                       |  3 +++
 init/init_task.c                   |  7 +++++--
 15 files changed, 64 insertions(+), 77 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d9a94da0c29f..f33bc80577c5 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -154,6 +154,7 @@ config X86
 	select SPARSE_IRQ
 	select SRCU
 	select SYSCTL_EXCEPTION_TRACE
+	select THREAD_INFO_IN_TASK
 	select USER_STACKTRACE_SUPPORT
 	select VIRT_TO_BUS
 	select X86_DEV_DMA_OPS			if X86_64
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index ec138e538c44..d5feac5f252d 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -31,13 +31,6 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/syscalls.h>
 
-static struct thread_info *pt_regs_to_thread_info(struct pt_regs *regs)
-{
-	unsigned long top_of_stack =
-		(unsigned long)(regs + 1) + TOP_OF_KERNEL_STACK_PADDING;
-	return (struct thread_info *)(top_of_stack - THREAD_SIZE);
-}
-
 #ifdef CONFIG_CONTEXT_TRACKING
 /* Called on entry from user mode with IRQs off. */
 __visible void enter_from_user_mode(void)
@@ -78,7 +71,7 @@ static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
  */
 unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
 {
-	struct thread_info *ti = pt_regs_to_thread_info(regs);
+	struct thread_info *ti = current_thread_info();
 	unsigned long ret = 0;
 	u32 work;
 
@@ -156,7 +149,7 @@ unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
 long syscall_trace_enter_phase2(struct pt_regs *regs, u32 arch,
 				unsigned long phase1_result)
 {
-	struct thread_info *ti = pt_regs_to_thread_info(regs);
+	struct thread_info *ti = current_thread_info();
 	long ret = 0;
 	u32 work = ACCESS_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY;
 
@@ -239,7 +232,7 @@ static void exit_to_usermode_loop(struct pt_regs *regs, u32 cached_flags)
 		/* Disable IRQs and retry */
 		local_irq_disable();
 
-		cached_flags = READ_ONCE(pt_regs_to_thread_info(regs)->flags);
+		cached_flags = READ_ONCE(current_thread_info()->flags);
 
 		if (!(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
 			break;
@@ -250,7 +243,7 @@ static void exit_to_usermode_loop(struct pt_regs *regs, u32 cached_flags)
 /* Called with IRQs disabled. */
 __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
 {
-	struct thread_info *ti = pt_regs_to_thread_info(regs);
+	struct thread_info *ti = current_thread_info();
 	u32 cached_flags;
 
 	if (IS_ENABLED(CONFIG_PROVE_LOCKING) && WARN_ON(!irqs_disabled()))
@@ -309,7 +302,7 @@ static void syscall_slow_exit_work(struct pt_regs *regs, u32 cached_flags)
  */
 __visible inline void syscall_return_slowpath(struct pt_regs *regs)
 {
-	struct thread_info *ti = pt_regs_to_thread_info(regs);
+	struct thread_info *ti = current_thread_info();
 	u32 cached_flags = READ_ONCE(ti->flags);
 
 	CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
@@ -332,7 +325,7 @@ __visible inline void syscall_return_slowpath(struct pt_regs *regs)
 #ifdef CONFIG_X86_64
 __visible void do_syscall_64(struct pt_regs *regs)
 {
-	struct thread_info *ti = pt_regs_to_thread_info(regs);
+	struct thread_info *ti = current_thread_info();
 	unsigned long nr = regs->orig_ax;
 
 	enter_from_user_mode();
@@ -365,7 +358,7 @@ __visible void do_syscall_64(struct pt_regs *regs)
  */
 static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
 {
-	struct thread_info *ti = pt_regs_to_thread_info(regs);
+	struct thread_info *ti = current_thread_info();
 	unsigned int nr = (unsigned int)regs->orig_ax;
 
 #ifdef CONFIG_IA32_EMULATION
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 9ee0da1807ed..f49742de2c65 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -179,7 +179,8 @@ GLOBAL(entry_SYSCALL_64_after_swapgs)
 	 * If we need to do entry work or if we guess we'll need to do
 	 * exit work, go straight to the slow path.
 	 */
-	testl	$_TIF_WORK_SYSCALL_ENTRY|_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
+	GET_THREAD_INFO(%r11)
+	testl	$_TIF_WORK_SYSCALL_ENTRY|_TIF_ALLWORK_MASK, TI_flags(%r11)
 	jnz	entry_SYSCALL64_slow_path
 
 entry_SYSCALL_64_fastpath:
@@ -217,7 +218,8 @@ entry_SYSCALL_64_fastpath:
 	 */
 	DISABLE_INTERRUPTS(CLBR_NONE)
 	TRACE_IRQS_OFF
-	testl	$_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
+	GET_THREAD_INFO(%r11)
+	testl	$_TIF_ALLWORK_MASK, TI_flags(%r11)
 	jnz	1f
 
 	LOCKDEP_SYS_EXIT
@@ -368,9 +370,10 @@ END(ptregs_\func)
  * A newly forked process directly context switches into this address.
  *
  * rdi: prev task we switched from
+ * rsi: task we're switching to
  */
 ENTRY(ret_from_fork)
-	LOCK ; btr $TIF_FORK, TI_flags(%r8)
+	LOCK ; btr $TIF_FORK, TI_flags(%rsi)	/* rsi: this newly forked task */
 
 	call	schedule_tail			/* rdi: 'prev' task parameter */
 
diff --git a/arch/x86/include/asm/kprobes.h b/arch/x86/include/asm/kprobes.h
index 4421b5da409d..1d2997e74b08 100644
--- a/arch/x86/include/asm/kprobes.h
+++ b/arch/x86/include/asm/kprobes.h
@@ -38,12 +38,12 @@ typedef u8 kprobe_opcode_t;
 #define RELATIVECALL_OPCODE 0xe8
 #define RELATIVE_ADDR_SIZE 4
 #define MAX_STACK_SIZE 64
-#define MIN_STACK_SIZE(ADDR)					       \
-	(((MAX_STACK_SIZE) < (((unsigned long)current_thread_info()) + \
-			      THREAD_SIZE - (unsigned long)(ADDR)))    \
-	 ? (MAX_STACK_SIZE)					       \
-	 : (((unsigned long)current_thread_info()) +		       \
-	    THREAD_SIZE - (unsigned long)(ADDR)))
+
+#define current_stack_top() ((unsigned long)task_stack_page(current)+THREAD_SIZE)
+#define current_stack_size(ADDR) (current_stack_top() - (unsigned long)(ADDR))
+
+#define MIN_STACK_SIZE(ADDR) \
+	(MAX_STACK_SIZE < current_stack_size(ADDR) ? MAX_STACK_SIZE : current_stack_size(ADDR))
 
 #define flush_insn_slot(p)	do { } while (0)
 
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index 8f321a1b03a1..ae0aa0612c67 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -110,18 +110,16 @@ do {									\
 	     "call __switch_to\n\t"					  \
 	     "movq "__percpu_arg([current_task])",%%rsi\n\t"		  \
 	     __switch_canary						  \
-	     "movq %P[thread_info](%%rsi),%%r8\n\t"			  \
 	     "movq %%rax,%%rdi\n\t" 					  \
-	     "testl  %[_tif_fork],%P[ti_flags](%%r8)\n\t"		  \
+	     "testl  %[_tif_fork],%P[ti_flags](%%rsi)\n\t"		  \
 	     "jnz   ret_from_fork\n\t"					  \
 	     RESTORE_CONTEXT						  \
 	     : "=a" (last)					  	  \
 	       __switch_canary_oparam					  \
 	     : [next] "S" (next), [prev] "D" (prev),			  \
 	       [threadrsp] "i" (offsetof(struct task_struct, thread.sp)), \
-	       [ti_flags] "i" (offsetof(struct thread_info, flags)),	  \
+	       [ti_flags] "i" (offsetof(struct task_struct, thread_info.flags)),	  \
 	       [_tif_fork] "i" (_TIF_FORK),			  	  \
-	       [thread_info] "i" (offsetof(struct task_struct, stack)),   \
 	       [current_task] "m" (current_task)			  \
 	       __switch_canary_iparam					  \
 	     : "memory", "cc" __EXTRA_CLOBBER)
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index 30c133ac05cd..eef687fdc90d 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -53,24 +53,22 @@ struct task_struct;
 #include <linux/atomic.h>
 
 struct thread_info {
-	struct task_struct	*task;		/* main task structure */
 	__u32			flags;		/* low level flags */
 	__u32			status;		/* thread synchronous flags */
 	__u32			cpu;		/* current CPU */
-	mm_segment_t		addr_limit;
 	unsigned int		sig_on_uaccess_error:1;
 	unsigned int		uaccess_err:1;	/* uaccess failed */
+	mm_segment_t		addr_limit;
 };
 
 #define INIT_THREAD_INFO(tsk)			\
 {						\
-	.task		= &tsk,			\
 	.flags		= 0,			\
 	.cpu		= 0,			\
 	.addr_limit	= KERNEL_DS,		\
 }
 
-#define init_thread_info	(init_thread_union.thread_info)
+#define init_thread_info	(init_task.thread_info)
 #define init_stack		(init_thread_union.stack)
 
 #else /* !__ASSEMBLY__ */
@@ -166,7 +164,7 @@ struct thread_info {
 
 static inline struct thread_info *current_thread_info(void)
 {
-	return (struct thread_info *)(current_top_of_stack() - THREAD_SIZE);
+	return (struct thread_info *)current;
 }
 
 static inline unsigned long current_stack_pointer(void)
@@ -188,35 +186,7 @@ static inline unsigned long current_stack_pointer(void)
 
 /* Load thread_info address into "reg" */
 #define GET_THREAD_INFO(reg) \
-	_ASM_MOV PER_CPU_VAR(cpu_current_top_of_stack),reg ; \
-	_ASM_SUB $(THREAD_SIZE),reg ;
-
-/*
- * ASM operand which evaluates to a 'thread_info' address of
- * the current task, if it is known that "reg" is exactly "off"
- * bytes below the top of the stack currently.
- *
- * ( The kernel stack's size is known at build time, it is usually
- *   2 or 4 pages, and the bottom  of the kernel stack contains
- *   the thread_info structure. So to access the thread_info very
- *   quickly from assembly code we can calculate down from the
- *   top of the kernel stack to the bottom, using constant,
- *   build-time calculations only. )
- *
- * For example, to fetch the current thread_info->flags value into %eax
- * on x86-64 defconfig kernels, in syscall entry code where RSP is
- * currently at exactly SIZEOF_PTREGS bytes away from the top of the
- * stack:
- *
- *      mov ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS), %eax
- *
- * will translate to:
- *
- *      8b 84 24 b8 c0 ff ff      mov    -0x3f48(%rsp), %eax
- *
- * which is below the current RSP by almost 16K.
- */
-#define ASM_THREAD_INFO(field, reg, off) ((field)+(off)-THREAD_SIZE)(reg)
+	_ASM_MOV PER_CPU_VAR(current_task),reg
 
 #endif
 
diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index d6209f3a69cb..ef8017ca5ba9 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -80,7 +80,7 @@ print_ftrace_graph_addr(unsigned long addr, void *data,
 static inline int valid_stack_ptr(struct task_struct *task,
 			void *p, unsigned int size, void *end)
 {
-	void *t = task_thread_info(task);
+	void *t = task_stack_page(task);
 	if (end) {
 		if (p < end && p >= (end-THREAD_SIZE))
 			return 1;
diff --git a/arch/x86/kernel/irq_32.c b/arch/x86/kernel/irq_32.c
index 38da8f29a9c8..c627bf8d98ad 100644
--- a/arch/x86/kernel/irq_32.c
+++ b/arch/x86/kernel/irq_32.c
@@ -130,11 +130,9 @@ void irq_ctx_init(int cpu)
 
 void do_softirq_own_stack(void)
 {
-	struct thread_info *curstk;
 	struct irq_stack *irqstk;
 	u32 *isp, *prev_esp;
 
-	curstk = current_stack();
 	irqstk = __this_cpu_read(softirq_stack);
 
 	/* build the stack frame on the softirq stack */
diff --git a/arch/x86/kernel/irq_64.c b/arch/x86/kernel/irq_64.c
index 206d0b90a3ab..38f9f5678dc8 100644
--- a/arch/x86/kernel/irq_64.c
+++ b/arch/x86/kernel/irq_64.c
@@ -41,8 +41,7 @@ static inline void stack_overflow_check(struct pt_regs *regs)
 	if (user_mode(regs))
 		return;
 
-	if (regs->sp >= curbase + sizeof(struct thread_info) +
-				  sizeof(struct pt_regs) + STACK_TOP_MARGIN &&
+	if (regs->sp >= curbase + sizeof(struct pt_regs) + STACK_TOP_MARGIN &&
 	    regs->sp <= curbase + THREAD_SIZE)
 		return;
 
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 96becbbb52e0..8f60f810a9e7 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -536,9 +536,7 @@ unsigned long get_wchan(struct task_struct *p)
 	 * PADDING
 	 * ----------- top = topmax - TOP_OF_KERNEL_STACK_PADDING
 	 * stack
-	 * ----------- bottom = start + sizeof(thread_info)
-	 * thread_info
-	 * ----------- start
+	 * ----------- bottom = start
 	 *
 	 * The tasks stack pointer points at the location where the
 	 * framepointer is stored. The data on the stack is:
@@ -549,7 +547,7 @@ unsigned long get_wchan(struct task_struct *p)
 	 */
 	top = start + THREAD_SIZE - TOP_OF_KERNEL_STACK_PADDING;
 	top -= 2 * sizeof(unsigned long);
-	bottom = start + sizeof(struct thread_info);
+	bottom = start;
 
 	sp = READ_ONCE(p->thread.sp);
 	if (sp < bottom || sp > top)
diff --git a/arch/x86/um/ptrace_32.c b/arch/x86/um/ptrace_32.c
index ebd4dd6ef73b..14e8f6a628c2 100644
--- a/arch/x86/um/ptrace_32.c
+++ b/arch/x86/um/ptrace_32.c
@@ -191,7 +191,7 @@ int peek_user(struct task_struct *child, long addr, long data)
 
 static int get_fpregs(struct user_i387_struct __user *buf, struct task_struct *child)
 {
-	int err, n, cpu = ((struct thread_info *) child->stack)->cpu;
+	int err, n, cpu = task_thread_info(child)->cpu;
 	struct user_i387_struct fpregs;
 
 	err = save_i387_registers(userspace_pid[cpu],
@@ -208,7 +208,7 @@ static int get_fpregs(struct user_i387_struct __user *buf, struct task_struct *c
 
 static int set_fpregs(struct user_i387_struct __user *buf, struct task_struct *child)
 {
-	int n, cpu = ((struct thread_info *) child->stack)->cpu;
+	int n, cpu = task_thread_info(child)->cpu;
 	struct user_i387_struct fpregs;
 
 	n = copy_from_user(&fpregs, buf, sizeof(fpregs));
@@ -221,7 +221,7 @@ static int set_fpregs(struct user_i387_struct __user *buf, struct task_struct *c
 
 static int get_fpxregs(struct user_fxsr_struct __user *buf, struct task_struct *child)
 {
-	int err, n, cpu = ((struct thread_info *) child->stack)->cpu;
+	int err, n, cpu = task_thread_info(child)->cpu;
 	struct user_fxsr_struct fpregs;
 
 	err = save_fpx_registers(userspace_pid[cpu], (unsigned long *) &fpregs);
@@ -237,7 +237,7 @@ static int get_fpxregs(struct user_fxsr_struct __user *buf, struct task_struct *
 
 static int set_fpxregs(struct user_fxsr_struct __user *buf, struct task_struct *child)
 {
-	int n, cpu = ((struct thread_info *) child->stack)->cpu;
+	int n, cpu = task_thread_info(child)->cpu;
 	struct user_fxsr_struct fpregs;
 
 	n = copy_from_user(&fpregs, buf, sizeof(fpregs));
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index f2cb8d45513d..a00f53b64c09 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -15,6 +15,8 @@
 #include <net/net_namespace.h>
 #include <linux/sched/rt.h>
 
+#include <asm/thread_info.h>
+
 #ifdef CONFIG_SMP
 # define INIT_PUSHABLE_TASKS(tsk)					\
 	.pushable_tasks = PLIST_NODE_INIT(tsk.pushable_tasks, MAX_PRIO),
@@ -183,12 +185,19 @@ extern struct task_group root_task_group;
 # define INIT_KASAN(tsk)
 #endif
 
+#ifdef CONFIG_THREAD_INFO_IN_TASK
+# define INIT_TASK_TI(tsk) .thread_info = INIT_THREAD_INFO(tsk),
+#else
+# define INIT_TASK_TI(tsk)
+#endif
+
 /*
  *  INIT_TASK is used to set up the first task table, touch at
  * your own risk!. Base=0, limit=0x1fffff (=2MB)
  */
 #define INIT_TASK(tsk)	\
 {									\
+	INIT_TASK_TI(tsk)						\
 	.state		= 0,						\
 	.stack		= &init_thread_info,				\
 	.usage		= ATOMIC_INIT(2),				\
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6e42ada26345..06236a36ba17 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1456,6 +1456,9 @@ struct tlbflush_unmap_batch {
 };
 
 struct task_struct {
+#ifdef CONFIG_THREAD_INFO_IN_TASK
+	struct thread_info thread_info;
+#endif
 	volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
 	void *stack;
 	atomic_t usage;
@@ -2539,7 +2542,9 @@ extern void set_curr_task(int cpu, struct task_struct *p);
 void yield(void);
 
 union thread_union {
+#ifndef CONFIG_THREAD_INFO_IN_TASK
 	struct thread_info thread_info;
+#endif
 	unsigned long stack[THREAD_SIZE/sizeof(long)];
 };
 
@@ -2967,7 +2972,14 @@ static inline void threadgroup_change_end(struct task_struct *tsk)
 	cgroup_threadgroup_change_end(tsk);
 }
 
-#ifndef __HAVE_THREAD_FUNCTIONS
+#ifdef CONFIG_THREAD_INFO_IN_TASK
+
+#define task_thread_info(task)		(&(task)->thread_info)
+#define task_stack_page(task)		((task)->stack)
+#define setup_thread_stack(new,old)	do { } while(0)
+#define end_of_stack(task)		((unsigned long *)task_stack_page(task))
+
+#elif !defined(__HAVE_THREAD_FUNCTIONS)
 
 #define task_thread_info(task)	((struct thread_info *)(task)->stack)
 #define task_stack_page(task)	((task)->stack)
diff --git a/init/Kconfig b/init/Kconfig
index f755a602d4a1..0c83af6d3753 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -26,6 +26,9 @@ config IRQ_WORK
 config BUILDTIME_EXTABLE_SORT
 	bool
 
+config THREAD_INFO_IN_TASK
+	bool
+
 menu "General setup"
 
 config BROKEN
diff --git a/init/init_task.c b/init/init_task.c
index ba0a7f362d9e..11f83be1fa79 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -22,5 +22,8 @@ EXPORT_SYMBOL(init_task);
  * Initial thread structure. Alignment of this is handled by a special
  * linker map entry.
  */
-union thread_union init_thread_union __init_task_data =
-	{ INIT_THREAD_INFO(init_task) };
+union thread_union init_thread_union __init_task_data = {
+#ifndef CONFIG_THREAD_INFO_IN_TASK
+	INIT_THREAD_INFO(init_task)
+#endif
+};