linux-kernel - Re: [PATCH v2 1/1] Fix int1 recursion when no perf_bp

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Date:	Fri, 11 Dec 2015 12:04:15 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v2 1/1] Fix int1 recursion when no perf_bp_event is registeredy

One thing I noticed in this section of code:

	/* Handle all the breakpoints that were triggered */
	for (i = 0; i < HBP_NUM; ++i) {
		if (likely(!(dr6 & (DR_TRAP0 << i))))
			continue;

                ... snip ...

		if (bp->hw.info.type == X86_BREAKPOINT_EXECUTE)
			args->regs->flags |= X86_EFLAGS_RF;

		rcu_read_unlock();
	}

Whoever wrote this loop did not seem to understand have observed the
code path in action on intel hardware.  There is NEVER a case I have
seen when the hardware sends multiple breakpoint statuses through dr6,
they are sent one at a time.  So the rolling check through all the
status bits is pointless since only one breakpoint will be reported by
the hardware at a time.  This is not to say that someone in the future
might may change it, but these interrupts are delivered one by one in
order and if there are duplicates (like a read/write breakpoint set at
the same address as an execute breakpoint.

Does anyone know why it was coded this way because its flat wrong.

Jeff
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