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

Michael Ellerman wrote:

Ananth N Mavinakayanahalli [off-list ref] writes:
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On Wed, Jan 17, 2018 at 05:52:24PM +0530, Naveen N. Rao wrote:

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Michael Ellerman reported the following call trace when running
ftracetest:
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BUG: using __this_cpu_write() in preemptible [00000000] code: ftracetes=

t/6178

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caller is opt_pre_handler+0xc4/0x110
CPU: 1 PID: 6178 Comm: ftracetest Not tainted 4.15.0-rc7-gcc6x-gb2cd1df=

 #1

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Call Trace:
[c0000000f9ec39c0] [c000000000ac4304] dump_stack+0xb4/0x100 (unreliable=

)

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[c0000000f9ec3a00] [c00000000061159c] check_preemption_disabled+0x15c/0=

x170

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

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non-zero return value so as to suppress any subsequent processing of th=

e

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trap by the kprobe_handler().
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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.
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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()).
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Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Naveen N. Rao <redacted>

Acked-by: Ananth N Mavinakayanahalli <redacted>

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

Fixes: 8a2d71a3f2737e ("powerpc/kprobes: Disable preemption before=20
invoking probe handler for optprobes")

I think this is more appropriate. I should have caught this issue with=20
kretprobes, but I am fairly certain that I ran ftracetest at that point,
but didn't see any call traces.

Regards,
Naveen

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