[PATCH v2 2/3] i387: use 'restore_fpu_checking()' directly in taskswitching code

[Linus Torvalds]

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Sun, 19 Feb 2012 11:48:44 -0800
Subject: [PATCH v2 2/3] i387: use 'restore_fpu_checking()' directly in task switching code

This inlines what is usually just a couple of instructions, but more
importantly it also fixes the theoretical error case (can that FPU
restore really ever fail? Maybe we should remove the checking).

We can't start sending signals from within the scheduler, we're much too
deep in the kernel and are holding the runqueue lock etc.  So don't
bother even trying.

Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---

I do think the "load bad FP state from user stack at a signal restore,
and then get an exception on task switch preload and try to send a
signal while holding the runqueue lock" is a real possibility on x86-32,
so this *may* actually be -stable material too.  But somebody should
really double-check my thinking. 

 arch/x86/include/asm/i387.h |   17 ++++++++++++++---
 arch/x86/kernel/traps.c     |   40 ++++++++--------------------------------
 2 files changed, 22 insertions(+), 35 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index 8df95849721d..74c607b37e87 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -29,7 +29,6 @@ extern unsigned int sig_xstate_size;
 extern void fpu_init(void);
 extern void mxcsr_feature_mask_init(void);
 extern int init_fpu(struct task_struct *child);
-extern void __math_state_restore(struct task_struct *);
 extern void math_state_restore(void);
 extern int dump_fpu(struct pt_regs *, struct user_i387_struct *);
 
@@ -269,6 +268,16 @@ static inline int fpu_restore_checking(struct fpu *fpu)
 
 static inline int restore_fpu_checking(struct task_struct *tsk)
 {
+	/* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
+	   is pending.  Clear the x87 state here by setting it to fixed
+	   values. "m" is a random variable that should be in L1 */
+	alternative_input(
+		ASM_NOP8 ASM_NOP2,
+		"emms\n\t"	  	/* clear stack tags */
+		"fildl %P[addr]",	/* set F?P to defined value */
+		X86_FEATURE_FXSAVE_LEAK,
+		[addr] "m" (tsk->thread.has_fpu));
+
 	return fpu_restore_checking(&tsk->thread.fpu);
 }
 
@@ -378,8 +387,10 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
  */
 static inline void switch_fpu_finish(struct task_struct *new, fpu_switch_t fpu)
 {
-	if (fpu.preload)
-		__math_state_restore(new);
+	if (fpu.preload) {
+		if (unlikely(restore_fpu_checking(new)))
+			__thread_fpu_end(new);
+	}
 }
 
 /*
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 77da5b475ad2..4bbe04d96744 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -571,37 +571,6 @@ asmlinkage void __attribute__((weak)) smp_threshold_interrupt(void)
 }
 
 /*
- * This gets called with the process already owning the
- * FPU state, and with CR0.TS cleared. It just needs to
- * restore the FPU register state.
- */
-void __math_state_restore(struct task_struct *tsk)
-{
-	/* We need a safe address that is cheap to find and that is already
-	   in L1. We've just brought in "tsk->thread.has_fpu", so use that */
-#define safe_address (tsk->thread.has_fpu)
-
-	/* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
-	   is pending.  Clear the x87 state here by setting it to fixed
-	   values. safe_address is a random variable that should be in L1 */
-	alternative_input(
-		ASM_NOP8 ASM_NOP2,
-		"emms\n\t"	  	/* clear stack tags */
-		"fildl %P[addr]",	/* set F?P to defined value */
-		X86_FEATURE_FXSAVE_LEAK,
-		[addr] "m" (safe_address));
-
-	/*
-	 * Paranoid restore. send a SIGSEGV if we fail to restore the state.
-	 */
-	if (unlikely(restore_fpu_checking(tsk))) {
-		__thread_fpu_end(tsk);
-		force_sig(SIGSEGV, tsk);
-		return;
-	}
-}
-
-/*
  * 'math_state_restore()' saves the current math information in the
  * old math state array, and gets the new ones from the current task
  *
@@ -631,7 +600,14 @@ void math_state_restore(void)
 	}
 
 	__thread_fpu_begin(tsk);
-	__math_state_restore(tsk);
+	/*
+	 * Paranoid restore. send a SIGSEGV if we fail to restore the state.
+	 */
+	if (unlikely(restore_fpu_checking(tsk))) {
+		__thread_fpu_end(tsk);
+		force_sig(SIGSEGV, tsk);
+		return;
+	}
 
 	tsk->fpu_counter++;
 }
-- 
1.7.9.188.g12766.dirty

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