I was hitting the below panic continuously when attaching kprobes to
scheduler functions
[ 159.045212] Unexpected kernel BRK exception at EL1
[ 159.053753] Internal error: BRK handler: f2000006 [#1] PREEMPT SMP
[ 159.059954] Modules linked in:
[ 159.063025] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 5.11.0-rc4-00008-g1e2a199f6ccd #56
[rt-app] <notice> [1] Exiting.[ 159.071166] Hardware name: ARM Juno development board (r2) (DT)
[ 159.079689] pstate: 600003c5 (nZCv DAIF -PAN -UAO -TCO BTYPE=--)
[ 159.085723] pc : 0xffff80001624501c
[ 159.089377] lr : attach_entity_load_avg+0x2ac/0x350
[ 159.094271] sp : ffff80001622b640
[rt-app] <notice> [0] Exiting.[ 159.097591] x29: ffff80001622b640 x28: 0000000000000001
[ 159.105515] x27: 0000000000000049 x26: ffff000800b79980
[ 159.110847] x25: ffff00097ef37840 x24: 0000000000000000
[ 159.116331] x23: 00000024eacec1ec x22: ffff00097ef12b90
[ 159.121663] x21: ffff00097ef37700 x20: ffff800010119170
[rt-app] <notice> [11] Exiting.[ 159.126995] x19: ffff00097ef37840 x18: 000000000000000e
[ 159.135003] x17: 0000000000000001 x16: 0000000000000019
[ 159.140335] x15: 0000000000000000 x14: 0000000000000000
[ 159.145666] x13: 0000000000000002 x12: 0000000000000002
[ 159.150996] x11: ffff80001592f9f0 x10: 0000000000000060
[ 159.156327] x9 : ffff8000100f6f9c x8 : be618290de0999a1
[ 159.161659] x7 : ffff80096a4b1000 x6 : 0000000000000000
[ 159.166990] x5 : ffff00097ef37840 x4 : 0000000000000000
[ 159.172321] x3 : ffff000800328948 x2 : 0000000000000000
[ 159.177652] x1 : 0000002507d52fec x0 : ffff00097ef12b90
[ 159.182983] Call trace:
[ 159.185433] 0xffff80001624501c
[ 159.188581] update_load_avg+0x2d0/0x778
[ 159.192516] enqueue_task_fair+0x134/0xe20
[ 159.196625] enqueue_task+0x4c/0x2c8
[ 159.200211] ttwu_do_activate+0x70/0x138
[ 159.204147] sched_ttwu_pending+0xbc/0x160
[ 159.208253] flush_smp_call_function_queue+0x16c/0x320
[ 159.213408] generic_smp_call_function_single_interrupt+0x1c/0x28
[ 159.219521] ipi_handler+0x1e8/0x3c8
[ 159.223106] handle_percpu_devid_irq+0xd8/0x460
[ 159.227650] generic_handle_irq+0x38/0x50
[ 159.231672] __handle_domain_irq+0x6c/0xc8
[ 159.235781] gic_handle_irq+0xcc/0xf0
[ 159.239452] el1_irq+0xb4/0x180
[ 159.242600] rcu_is_watching+0x28/0x70
[ 159.246359] rcu_read_lock_held_common+0x44/0x88
[ 159.250991] rcu_read_lock_any_held+0x30/0xc0
[ 159.255360] kretprobe_dispatcher+0xc4/0xf0
[ 159.259555] __kretprobe_trampoline_handler+0xc0/0x150
[ 159.264710] trampoline_probe_handler+0x38/0x58
[ 159.269255] kretprobe_trampoline+0x70/0xc4
[ 159.273450] run_rebalance_domains+0x54/0x80
[ 159.277734] __do_softirq+0x164/0x684
[ 159.281406] irq_exit+0x198/0x1b8
[ 159.284731] __handle_domain_irq+0x70/0xc8
[ 159.288840] gic_handle_irq+0xb0/0xf0
[ 159.292510] el1_irq+0xb4/0x180
[ 159.295658] arch_cpu_idle+0x18/0x28
[ 159.299245] default_idle_call+0x9c/0x3e8
[ 159.303265] do_idle+0x25c/0x2a8
[ 159.306502] cpu_startup_entry+0x2c/0x78
[ 159.310436] secondary_start_kernel+0x160/0x198
[ 159.314984] Code: d42000c0 aa1e03e9 d42000c0 aa1e03e9 (d42000c0)
After a bit of head scratching and debugging it turned out that it is
due to kprobe handler being interrupted by a tick that causes us to go
into (I think another) kprobe handler.
The culprit was kprobe_breakpoint_ss_handler() returning DBG_HOOK_ERROR
which leads to the Unexpected kernel BRK exception.
Reverting commit ba090f9cafd5 ("arm64: kprobes: Remove redundant
kprobe_step_ctx") seemed to fix the problem for me.
Further analysis showed that kcb->kprobe_status is set to
KPROBE_REENTER when the error occurs. By teaching
kprobe_breakpoint_ss_handler() to handle this status I can no longer
reproduce the problem.
Fixes: ba090f9cafd5 ("arm64: kprobes: Remove redundant kprobe_step_ctx")
Signed-off-by: Qais Yousef <qais.yousef at arm.com>
---
Another change in behavior I noticed is that before ba090f9cafd5 ("arm64:
kprobes: Remove redundant kprobe_step_ctx") if 'cur' was NULL we wouldn't
return DBG_HOOK_ERROR, but after the change we do.
I didn't hit a problem because of that it's just something I noticed when
I realized that this commit was causing my problem.
arch/arm64/kernel/probes/kprobes.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index 89c64ada8732..66aac2881ba8 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -352,8 +352,8 @@ kprobe_breakpoint_ss_handler(struct pt_regs *regs, unsigned int esr)
unsigned long addr = instruction_pointer(regs);
struct kprobe *cur = kprobe_running();
- if (cur && (kcb->kprobe_status == KPROBE_HIT_SS)
- && ((unsigned long)&cur->ainsn.api.insn[1] == addr)) {
+ if (cur && (kcb->kprobe_status & (KPROBE_HIT_SS | KPROBE_REENTER)) &&
+ ((unsigned long)&cur->ainsn.api.insn[1] == addr)) {
kprobes_restore_local_irqflag(kcb, regs);
post_kprobe_handler(cur, kcb, regs);
--
2.25.1