x86: Use is_kprobe_fault to better match usage

Currently the notify_page_fault helper is used to test it the page fault was caused by a kprobe causing an early return from do_page_fault.

Change the name of the helper to is_kprobe_fault to match the usage and remove the preempt_disable/enable pair around kprobe_running() with an explicit test for preemption. The idea for this comes from a patch by Quentin Barnes to kprobes.c

Signed-off-by Harvey Harrison <[email protected]>

---
Ingo, this may not be functionally equivalent, feel free to yank it out if there is any trouble, but from what I've seen it should be OK.

Did you ever find a good kprobes test?

 arch/x86/mm/fault_32.c |   30 ++++++++++++++----------------
 arch/x86/mm/fault_64.c |   30 ++++++++++++++----------------
 2 files changed, 28 insertions(+), 32 deletions(-)

arch/x86/mm/fault_32.c +14 -16

index 051a4ec..5c48cc2 100644
@@ -12,6 +12,7 @@
 #include <linux/mman.h>
 #include <linux/mm.h>
 #include <linux/smp.h>
+#include <linux/hardirq.h>
 #include <linux/interrupt.h>
 #include <linux/init.h>
 #include <linux/tty.h>

@@ -42,23 +43,20 @@
 #define PF_RSVD	(1<<3)
 #define PF_INSTR	(1<<4)
 
-static inline int notify_page_fault(struct pt_regs *regs)
+static inline int is_kprobe_fault(struct pt_regs *regs)
 {
-#ifdef CONFIG_KPROBES
 	int ret = 0;
-
-	/* kprobe_running() needs smp_processor_id() */
-	if (!user_mode_vm(regs)) {
-		preempt_disable();
-		if (kprobe_running() && kprobe_fault_handler(regs, 14))
-			ret = 1;
-		preempt_enable();
-	}
-
-	return ret;
-#else
-	return 0;
+#ifdef CONFIG_KPROBES
+	/*
+	 * If it is a kprobe fault we can not be premptible so return before
+	 * calling kprobe_running() as it will assert on smp_processor_id if
+	 * preemption is enabled.
+	 */
+	if (!user_mode_vm(regs) && !preemptible() && kprobe_running() &&
+	    kprobe_fault_handler(regs, 14))
+		ret = 1;
 #endif
+	return ret;
 }
 
 #ifdef CONFIG_X86_32

@@ -428,7 +426,7 @@ void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code)
 				return;
 		}
 #endif
-		if (notify_page_fault(regs))
+		if (is_kprobe_fault(regs))
 			return;
 		/*
 		 * Don't take the mm semaphore here. If we fixup a prefetch

@@ -437,7 +435,7 @@ void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code)
 		goto bad_area_nosemaphore;
 	}
 
-	if (notify_page_fault(regs))
+	if (is_kprobe_fault(regs))
 		return;
 
 	/* It's safe to allow irq's after cr2 has been saved and the vmalloc

arch/x86/mm/fault_64.c +14 -16

index 97b92b6..09008e5 100644
@@ -13,6 +13,7 @@
 #include <linux/mman.h>
 #include <linux/mm.h>
 #include <linux/smp.h>
+#include <linux/hardirq.h>
 #include <linux/interrupt.h>
 #include <linux/init.h>
 #include <linux/tty.h>

@@ -45,23 +46,20 @@
 #define PF_RSVD	(1<<3)
 #define PF_INSTR	(1<<4)
 
-static inline int notify_page_fault(struct pt_regs *regs)
+static inline int is_kprobe_fault(struct pt_regs *regs)
 {
-#ifdef CONFIG_KPROBES
 	int ret = 0;
-
-	/* kprobe_running() needs smp_processor_id() */
-	if (!user_mode(regs)) {
-		preempt_disable();
-		if (kprobe_running() && kprobe_fault_handler(regs, 14))
-			ret = 1;
-		preempt_enable();
-	}
-
-	return ret;
-#else
-	return 0;
+#ifdef CONFIG_KPROBES
+	/*
+	 * If it is a kprobe fault we can not be premptible so return before
+	 * calling kprobe_running() as it will assert on smp_processor_id if
+	 * preemption is enabled.
+	 */
+	if (!user_mode_vm(regs) && !preemptible() && kprobe_running() &&
+	    kprobe_fault_handler(regs, 14))
+		ret = 1;
 #endif
+	return ret;
 }
 
 #ifdef CONFIG_X86_32

@@ -478,7 +476,7 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 				return;
 		}
 #endif
-		if (notify_page_fault(regs))
+		if (is_kprobe_fault(regs))
 			return;
 		/*
 		 * Don't take the mm semaphore here. If we fixup a prefetch

@@ -487,7 +485,7 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 		goto bad_area_nosemaphore;
 	}
 
-	if (notify_page_fault(regs))
+	if (is_kprobe_fault(regs))
 		return;
 
 	if (likely(regs->flags & X86_EFLAGS_IF))

Hi Harvey,

Quoted 7 lines · Harvey Harrison

Currently the notify_page_fault helper is used to test it the page fault was caused by a kprobe causing an early return from do_page_fault.

Change the name of the helper to is_kprobe_fault to match the usage and remove the preempt_disable/enable pair around kprobe_running() with an explicit test for preemption. The idea for this comes from a patch by Quentin Barnes to kprobes.c

Sure, that's right.
However, since other architectures also have notify_page_fault(), I think all of those code might better be changed same time for maintainability.

Quoted 10 lines · Masami Hiramatsu

Hi Harvey,

Quoted 7 lines · Harvey Harrison

Currently the notify_page_fault helper is used to test it the page fault was caused by a kprobe causing an early return from do_page_fault.

Change the name of the helper to is_kprobe_fault to match the usage and remove the preempt_disable/enable pair around kprobe_running() with an explicit test for preemption. The idea for this comes from a patch by Quentin Barnes to kprobes.c

Quoted 5 lines

Sure, that's right.
However, since other architectures also have notify_page_fault(), I think all of those code might better be changed same time for maintainability.

How about a static inline in linux/kprobes.h with a big comment above about when/why the !preemptible() check is sufficient?

Harvey

Currently the notify_page_fault helper is used to test it the page fault was caused by a kprobe causing an early return from do_page_fault.

Change the name of the helper to is_kprobe_fault to match the usage and remove the preempt_disable/enable pair around kprobe_running() with an explicit test for preemption. The idea for this comes from a patch by Quentin Barnes to kprobes.c

Signed-off-by Harvey Harrison <[email protected]>