[RFC PATCH 8/9] arm64/kprobes: Drop the XOL single-step fault PC check

[Hongyan Xia]

From: Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx>

With the armed single-step window closed (the whole path noinstr, and
every piece of instrumentable code confined to instrumentation windows
that run in KPROBE_HIT_ACTIVE/HIT_SSDONE state), no code other than the
XOL instruction itself can execute while the state is KPROBE_HIT_SS or
KPROBE_REENTER. A page fault taken in those states therefore necessarily
originates from the XOL instruction, and the faulting-PC comparison is
redundant; revert to handling all such faults as single-step faults (as
before commit 879a6754d3d1).

The simulated-kprobe early bail is kept: simulated probes have no XOL
slot and execute (inside an instrumentation window) in debug trap
context, so a fault there can still come from the simulation handlers
and must not be treated as a single-step fault.

Signed-off-by: Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx>
---
 arch/arm64/kernel/probes/kprobes.c | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index 1658b6acd803..73bbbcdbbfe5 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -309,18 +309,6 @@ int noinstr kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
 	switch (kcb->kprobe_status) {
 	case KPROBE_HIT_SS:
 	case KPROBE_REENTER:
-		/*
-		 * A page fault taken while in KPROBE_HIT_SS or
-		 * KPROBE_REENTER state is only attributable to kprobe
-		 * single-stepping if the faulting PC points to the
-		 * current kprobe's XOL instruction. If the fault occurred
-		 * elsewhere (e.g. in perf or tracing code invoked from the
-		 * debug exception path), leave it for the normal page fault
-		 * handler to process.
-		 */
-		if (instruction_pointer(regs) != (unsigned long)cur->ainsn.xol_insn)
-			break;
-
 		/*
 		 * We are here because the instruction being single
 		 * stepped caused a page fault. We reset the current
-- 
2.47.3