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

[Michael Ellerman]

Ananth N Mavinakayanahalli <ananth at linux.vnet.ibm.com> writes:

> 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>

Fixes: 51c9c0843993 ("powerpc/kprobes: Implement Optprobes")

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cheers