[PATCH] powerpc/kprobes: Fix call trace due to incorrect preempt count

[Ananth N Mavinakayanahalli]

On Wed, Jan 17, 2018 at 05:52:24PM +0530, Naveen N. Rao wrote:
> Michael Ellerman reported the following call trace when running
> ftracetest:
> 
> 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
> trace.
> 
> 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()).
> 
> Reported-by: Michael Ellerman <mpe at ellerman.id.au>
> Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>

Acked-by: Ananth N Mavinakayanahalli <ananth at linux.vnet.ibm.com>