Re: [PATCH resend 6/8] tracing/ftrace: Add might_fault check to syscall probes

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On 2024-10-28 13:42, Thomas Gleixner wrote:
On Mon, Sep 30 2024 at 15:23, Mathieu Desnoyers wrote:
diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index a3d8ac00793e..0430890cbb42 100644
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -303,6 +303,7 @@ static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
  	 * Syscall probe called with preemption enabled, but the ring
  	 * buffer and per-cpu data require preemption to be disabled.
  	 */
+	might_fault();
  	guard(preempt_notrace)();

I find it odd that the might_fault() check is in all the implementations
and not in the tracepoint itself:

     if (syscall) {
         might_fault();
  	rcu_read_unlock_trace();
    } else ...

That's where I would have expected it to be.

You raise a good point: we should also add a might_fault() check in
__DO_TRACE() in the syscall case, so we can catch incorrect use of the
syscall tracepoint even if no probes are registered to it.

I've added the might_fault() in each tracer syscall probe to make sure
a tracer don't end up registering a faultable probe on a tracepoint
protected with preempt_disable by mistake. It validates that the tracers
are using the tracepoint registration as expected.

I'll prepare separate a patch adding this and will add it to this
series.

Thanks,

Mathieu


Thanks,

         tglx

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com





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