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Subject[PATCH 4.16 065/196] powerpc/kprobes: Fix call trace due to incorrect preempt count
4.16-stable review patch.  If anyone has any objections, please let me know.


From: Naveen N. Rao <>

commit e6e133c47e6bd4d5dac05b35d06634a8e5648615 upstream.

Michael Ellerman reported the following call trace when running

BUG: using __this_cpu_write() in preemptible [00000000] code: ftracetest/6178
caller is opt_pre_handler+0xc4/0x110
CPU: 1 PID: 6178 Comm: ftracetest Not tainted 4.15.0-rc7-gcc6x-gb2cd1df #1
Call Trace:
[c0000000f9ec39c0] [c000000000ac4304] dump_stack+0xb4/0x100 (unreliable)
[c0000000f9ec3a00] [c00000000061159c] check_preemption_disabled+0x15c/0x170
[c0000000f9ec3a90] [c000000000217e84] opt_pre_handler+0xc4/0x110
[c0000000f9ec3af0] [c00000000004cf68] optimized_callback+0x148/0x170
[c0000000f9ec3b40] [c00000000004d954] optinsn_slot+0xec/0x10000
[c0000000f9ec3e30] [c00000000004bae0] kretprobe_trampoline+0x0/0x10

This is showing up since OPTPROBES is now enabled with CONFIG_PREEMPT.

trampoline_probe_handler() considers itself to be a special kprobe
handler for kretprobes. In doing so, it expects to be called from
kprobe_handler() on a trap, and re-enables preemption before returning a
non-zero return value so as to suppress any subsequent processing of the
trap by the kprobe_handler().

However, with optprobes, we don't deal with special handlers (we ignore
the return code) and just try to re-enable preemption causing the above

To address this, modify trampoline_probe_handler() to not be special.
The only additional processing done in kprobe_handler() is to emulate
the instruction (in this case, a 'nop'). We adjust the value of
regs->nip for the purpose and delegate the job of re-enabling
preemption and resetting current kprobe to the probe handlers
(kprobe_handler() or optimized_callback()).

Fixes: 8a2d71a3f273 ("powerpc/kprobes: Disable preemption before invoking probe handler for optprobes")
Cc: # v4.15+
Reported-by: Michael Ellerman <>
Signed-off-by: Naveen N. Rao <>
Acked-by: Ananth N Mavinakayanahalli <>
Signed-off-by: Michael Ellerman <>
Signed-off-by: Greg Kroah-Hartman <>

arch/powerpc/kernel/kprobes.c | 30 +++++++++++++++++-------------
1 file changed, 17 insertions(+), 13 deletions(-)

--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -455,29 +455,33 @@ static int trampoline_probe_handler(stru

kretprobe_assert(ri, orig_ret_address, trampoline_address);
- regs->nip = orig_ret_address;
- * Make LR point to the orig_ret_address.
- * When the 'nop' inside the kretprobe_trampoline
- * is optimized, we can do a 'blr' after executing the
- * detour buffer code.
+ * We get here through one of two paths:
+ * 1. by taking a trap -> kprobe_handler() -> here
+ * 2. by optprobe branch -> optimized_callback() -> opt_pre_handler() -> here
+ *
+ * When going back through (1), we need regs->nip to be setup properly
+ * as it is used to determine the return address from the trap.
+ * For (2), since nip is not honoured with optprobes, we instead setup
+ * the link register properly so that the subsequent 'blr' in
+ * kretprobe_trampoline jumps back to the right instruction.
+ *
+ * For nip, we should set the address to the previous instruction since
+ * we end up emulating it in kprobe_handler(), which increments the nip
+ * again.
+ regs->nip = orig_ret_address - 4;
regs->link = orig_ret_address;

- reset_current_kprobe();
kretprobe_hash_unlock(current, &flags);
- preempt_enable_no_resched();

hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
- /*
- * By returning a non-zero value, we are telling
- * kprobe_handler() that we don't want the post_handler
- * to run (and have re-enabled preemption)
- */
- return 1;
+ return 0;

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