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Subject[PATCH v2 3/4] arm64: Make debug exception handlers visible from RCU
Date
Make debug exceptions visible from RCU so that synchronize_rcu()
correctly track the debug exception handler.

This also introduces sanity checks for user-mode exceptions as same
as x86's ist_enter()/ist_exit().

The debug exception can interrupt in idle task. For example, it warns
if we put a kprobe on a function called from idle task as below.
The warning message showed that the rcu_read_lock() caused this
problem. But actually, this means the RCU is lost the context which
is already in NMI/IRQ.

/sys/kernel/debug/tracing # echo p default_idle_call >> kprobe_events
/sys/kernel/debug/tracing # echo 1 > events/kprobes/enable
/sys/kernel/debug/tracing # [ 135.122237]
[ 135.125035] =============================
[ 135.125310] WARNING: suspicious RCU usage
[ 135.125581] 5.2.0-08445-g9187c508bdc7 #20 Not tainted
[ 135.125904] -----------------------------
[ 135.126205] include/linux/rcupdate.h:594 rcu_read_lock() used illegally while idle!
[ 135.126839]
[ 135.126839] other info that might help us debug this:
[ 135.126839]
[ 135.127410]
[ 135.127410] RCU used illegally from idle CPU!
[ 135.127410] rcu_scheduler_active = 2, debug_locks = 1
[ 135.128114] RCU used illegally from extended quiescent state!
[ 135.128555] 1 lock held by swapper/0/0:
[ 135.128944] #0: (____ptrval____) (rcu_read_lock){....}, at: call_break_hook+0x0/0x178
[ 135.130499]
[ 135.130499] stack backtrace:
[ 135.131192] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0-08445-g9187c508bdc7 #20
[ 135.131841] Hardware name: linux,dummy-virt (DT)
[ 135.132224] Call trace:
[ 135.132491] dump_backtrace+0x0/0x140
[ 135.132806] show_stack+0x24/0x30
[ 135.133133] dump_stack+0xc4/0x10c
[ 135.133726] lockdep_rcu_suspicious+0xf8/0x108
[ 135.134171] call_break_hook+0x170/0x178
[ 135.134486] brk_handler+0x28/0x68
[ 135.134792] do_debug_exception+0x90/0x150
[ 135.135051] el1_dbg+0x18/0x8c
[ 135.135260] default_idle_call+0x0/0x44
[ 135.135516] cpu_startup_entry+0x2c/0x30
[ 135.135815] rest_init+0x1b0/0x280
[ 135.136044] arch_call_rest_init+0x14/0x1c
[ 135.136305] start_kernel+0x4d4/0x500
[ 135.136597]

So make debug exception visible to RCU can fix this warning.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Cc: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
arch/arm64/mm/fault.c | 40 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 40 insertions(+)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 9568c116ac7f..a6b244240db6 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -777,6 +777,42 @@ void __init hook_debug_fault_code(int nr,
debug_fault_info[nr].name = name;
}

+/*
+ * In debug exception context, we explicitly disable preemption.
+ * This serves two purposes: it makes it much less likely that we would
+ * accidentally schedule in exception context and it will force a warning
+ * if we somehow manage to schedule by accident.
+ */
+static void debug_exception_enter(struct pt_regs *regs)
+{
+ if (user_mode(regs)) {
+ RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+ } else {
+ /*
+ * We might have interrupted pretty much anything. In
+ * fact, if we're a debug exception, we can even interrupt
+ * NMI processing. We don't want in_nmi() to return true,
+ * but we need to notify RCU.
+ */
+ rcu_nmi_enter();
+ }
+
+ preempt_disable();
+
+ /* This code is a bit fragile. Test it. */
+ RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
+}
+NOKPROBE_SYMBOL(debug_exception_enter);
+
+static void debug_exception_exit(struct pt_regs *regs)
+{
+ preempt_enable_no_resched();
+
+ if (!user_mode(regs))
+ rcu_nmi_exit();
+}
+NOKPROBE_SYMBOL(debug_exception_exit);
+
#ifdef CONFIG_ARM64_ERRATUM_1463225
DECLARE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa);

@@ -824,6 +860,8 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint,
if (interrupts_enabled(regs))
trace_hardirqs_off();

+ debug_exception_enter(regs);
+
if (user_mode(regs) && !is_ttbr0_addr(pc))
arm64_apply_bp_hardening();

@@ -832,6 +870,8 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint,
inf->sig, inf->code, (void __user *)pc, esr);
}

+ debug_exception_exit(regs);
+
if (interrupts_enabled(regs))
trace_hardirqs_on();
}
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