[PATCH 4/5] powerpc/jprobes: Disable preemption when triggered through ftrace

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

KPROBES_SANITY_TEST throws the below splat when CONFIG_PREEMPT is
enabled:

[    3.140410] Kprobe smoke test: started
[    3.149680] DEBUG_LOCKS_WARN_ON(val > preempt_count())
[    3.149684] ------------[ cut here ]------------
[    3.149695] WARNING: CPU: 19 PID: 1 at kernel/sched/core.c:3094 preempt_count_sub+0xcc/0x140
[    3.149699] Modules linked in:
[    3.149705] CPU: 19 PID: 1 Comm: swapper/0 Not tainted 4.13.0-rc7-nnr+ #97
[    3.149709] task: c0000000fea80000 task.stack: c0000000feb00000
[    3.149713] NIP:  c00000000011d3dc LR: c00000000011d3d8 CTR: c000000000a090d0
[    3.149718] REGS: c0000000feb03400 TRAP: 0700   Not tainted  (4.13.0-rc7-nnr+)
[    3.149722] MSR:  8000000000021033 <SF,ME,IR,DR,RI,LE>  CR: 28000282  XER: 00000000
[    3.149732] CFAR: c00000000015aa18 SOFTE: 0
<snip>
[    3.149786] NIP [c00000000011d3dc] preempt_count_sub+0xcc/0x140
[    3.149790] LR [c00000000011d3d8] preempt_count_sub+0xc8/0x140
[    3.149794] Call Trace:
[    3.149798] [c0000000feb03680] [c00000000011d3d8] preempt_count_sub+0xc8/0x140 (unreliable)
[    3.149804] [c0000000feb036e0] [c000000000046198] kprobe_handler+0x228/0x4b0
[    3.149810] [c0000000feb03750] [c0000000000269c8] program_check_exception+0x58/0x3b0
[    3.149816] [c0000000feb037c0] [c00000000000903c] program_check_common+0x16c/0x170
[    3.149822] --- interrupt: 0 at kprobe_target+0x8/0x20
                   LR = init_test_probes+0x248/0x7d0
[    3.149829] [c0000000feb03ab0] [c000000000e4f048] kp+0x0/0x80 (unreliable)
[    3.149835] [c0000000feb03b10] [c00000000004ea60] livepatch_handler+0x38/0x74
[    3.149841] [c0000000feb03ba0] [c000000000d0de54] init_kprobes+0x1d8/0x208
[    3.149846] [c0000000feb03c40] [c00000000000daa8] do_one_initcall+0x68/0x1d0
[    3.149852] [c0000000feb03d00] [c000000000ce44f0] kernel_init_freeable+0x298/0x374
[    3.149857] [c0000000feb03dc0] [c00000000000dd84] kernel_init+0x24/0x160
[    3.149863] [c0000000feb03e30] [c00000000000bfec] ret_from_kernel_thread+0x5c/0x70
[    3.149867] Instruction dump:
[    3.149871] 419effdc 3d22001b 39299240 81290000 2f890000 409effc8 3c82ffcb 3c62ffcb
[    3.149879] 3884bc68 3863bc18 4803d5fd 60000000 <0fe00000> 4bffffa8 60000000 60000000
[    3.149890] ---[ end trace 432dd46b4ce3d29f ]---
[    3.166003] Kprobe smoke test: passed successfully

The issue is that we aren't disabling preemption in
kprobe_ftrace_handler(). Disable it.

Fixes: ead514d5fb30a0 ("powerpc/kprobes: Add support for KPROBES_ON_FTRACE")
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
 arch/powerpc/kernel/kprobes-ftrace.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
index 6c089d9757c9..2d81404f818c 100644
--- a/arch/powerpc/kernel/kprobes-ftrace.c
+++ b/arch/powerpc/kernel/kprobes-ftrace.c
@@ -65,6 +65,7 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
 	/* Disable irq for emulating a breakpoint and avoiding preempt */
 	local_irq_save(flags);
 	hard_irq_disable();
+	preempt_disable();
 
 	p = get_kprobe((kprobe_opcode_t *)nip);
 	if (unlikely(!p) || kprobe_disabled(p))
@@ -86,12 +87,18 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
 		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
 		if (!p->pre_handler || !p->pre_handler(p, regs))
 			__skip_singlestep(p, regs, kcb, orig_nip);
-		/*
-		 * If pre_handler returns !0, it sets regs->nip and
-		 * resets current kprobe.
-		 */
+		else {
+			/*
+			 * If pre_handler returns !0, it sets regs->nip and
+			 * resets current kprobe. In this case, we still need
+			 * to restore irq, but not preemption.
+			 */
+			local_irq_restore(flags);
+			return;
+		}
 	}
 end:
+	preempt_enable_no_resched();
 	local_irq_restore(flags);
 }
 NOKPROBE_SYMBOL(kprobe_ftrace_handler);
-- 
2.14.1