[PATCH] arm64: Fix early single-stepping

[Jean-Philippe Brucker]

To use debug features such as single-step, the OS lock must be unlocked
in the debug registers. Currently this is done in postcore_initcall
which is now too late.

Commit 36dadef23fcc ("kprobes: Init kprobes in early_initcall") enabled
using kprobes from early_initcall, when OS lock is still locked. So when
kprobe attempts to single-step a patched instruction, instead of
trapping, execution continues until it throws an undef exception:

[    0.064233] Kprobe smoke test: started
[    0.151133] ------------[ cut here ]------------
[    0.151458] kernel BUG at arch/arm64/kernel/traps.c:406!
[    0.151812] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
              ...
[    0.162689] Call trace:
[    0.163014]  do_undefinstr+0x1d4/0x1f4
[    0.163336]  el1_sync_handler+0xbc/0x140
[    0.163839]  el1_sync+0x80/0x100
[    0.164154]  0xffffffc01001d004
[    0.164527]  init_kprobes+0x13c/0x154
[    0.164968]  do_one_initcall+0x54/0x2e0
[    0.165322]  kernel_init_freeable+0xf4/0x258
[    0.165783]  kernel_init+0x20/0x12c
[    0.166117]  ret_from_fork+0x10/0x30
[    0.166595] Code: 97ffff53 a9425bf5 17ffff9b f9001bf7 (d4210000)
[    0.167084] ---[ end trace 36778fdf576e9a79 ]---

To fix this, unlock the OS lock as early as possible. Do it in
traps_init() for CPU0, since KGDB wants to use single-step from that
point on according to commit b322c65f8ca3 ("arm64: Call
debug_traps_init() from trap_init() to help early kgdb").
For secondary CPUs, setup the CPU hotplug handler at early_initcall.

Fixes: 36dadef23fcc ("kprobes: Init kprobes in early_initcall")
Signed-off-by: Jean-Philippe Brucker <jean-philippe at linaro.org>
---
 arch/arm64/kernel/debug-monitors.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 75a423c3336a..80f082021234 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -135,7 +135,7 @@ static int __init debug_monitors_init(void)
 				 "arm64/debug_monitors:starting",
 				 clear_os_lock, NULL);
 }
-postcore_initcall(debug_monitors_init);
+early_initcall(debug_monitors_init);
 
 /*
  * Single step API and exception handling.
@@ -380,6 +380,7 @@ NOKPROBE_SYMBOL(aarch32_break_handler);
 
 void __init debug_traps_init(void)
 {
+	clear_os_lock(0);
 	hook_debug_fault_code(DBG_ESR_EVT_HWSS, single_step_handler, SIGTRAP,
 			      TRAP_TRACE, "single-step handler");
 	hook_debug_fault_code(DBG_ESR_EVT_BRK, brk_handler, SIGTRAP,
-- 
2.29.1