[PATCH v3 1/7] x86: refactor kprobes_fault() like kprobe_exceptions_notify()

[Jann Horn <jannh@google.com>]

This is an extension of commit b506a9d08bae ("x86: code clarification patch
to Kprobes arch code"). As that commit explains, even though
kprobe_running() can't be called with preemption enabled, you don't have to
disable preemption - if preemption is on, you can't be in a kprobe.

Also, use X86_TRAP_PF instead of 14.

Signed-off-by: Jann Horn <jannh@google.com>
---
v3:
 - avoid unnecessary branch on return value and split up the checks
   (Borislav Petkov)

 arch/x86/mm/fault.c | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index b9123c497e0a..bcdaae1d5bf5 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -44,17 +44,19 @@ kmmio_fault(struct pt_regs *regs, unsigned long addr)
 
 static nokprobe_inline int kprobes_fault(struct pt_regs *regs)
 {
-	int ret = 0;
-
-	/* kprobe_running() needs smp_processor_id() */
-	if (kprobes_built_in() && !user_mode(regs)) {
-		preempt_disable();
-		if (kprobe_running() && kprobe_fault_handler(regs, 14))
-			ret = 1;
-		preempt_enable();
-	}
-
-	return ret;
+	if (!kprobes_built_in())
+		return 0;
+	if (user_mode(regs))
+		return 0;
+	/*
+	 * To be potentially processing a kprobe fault and to be allowed to call
+	 * kprobe_running(), we have to be non-preemptible.
+	 */
+	if (preemptible())
+		return 0;
+	if (!kprobe_running())
+		return 0;
+	return kprobe_fault_handler(regs, X86_TRAP_PF);
 }
 
 /*
-- 
2.19.0.rc0.228.g281dcd1b4d0-goog