linux-kernel - [PATCH 1/1] Add hw_breakpoint

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Date:	Sat, 12 Dec 2015 15:44:59 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andy Lutomirski <luto@...capital.net>,
	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: [PATCH 1/1] Add hw_breakpoint_enable function

The current function to manually restore breakpoints
(hw_breakpoint_restore) places the contents of the thread.debugreg6
variable back into the dr6 hardware register, a piece of hardware
intended to be read, not used as local storage.  The problem with this
is that systems that register for breakpoints, including the perf
event system and kgdb/kdb, modify this variable.  I have observed
garbage stacked debug statuses being cycled through dr6 and this
variable over and over again in a loop by calls to
hw_breakpoint_restore() that cause strange behaviors in kdb and this
layer.  In short, I don't use this broken function.

This function is necessary because upon debugger entry, you must set
dr7 to 0 to disable breakpoints while inside the debugger then call
hw_breakpoint_restore() to reload the state and re-enable them.  This
function is identical to hw_breakpoint_restore() except it leaves out
writing a user defined thread.debugreg6 variable back into a hardware
register not intended for that purpose.

Signed-off-by:  Jeff V. Merkey  <jeffmerkey@...il.com>
diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 50a3fad..7ea0f78 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -425,6 +425,16 @@ void hw_breakpoint_restore(void)
 }
 EXPORT_SYMBOL_GPL(hw_breakpoint_restore);

+void hw_breakpoint_enable(void)
+{
+	set_debugreg(__this_cpu_read(cpu_debugreg[0]), 0);
+	set_debugreg(__this_cpu_read(cpu_debugreg[1]), 1);
+	set_debugreg(__this_cpu_read(cpu_debugreg[2]), 2);
+	set_debugreg(__this_cpu_read(cpu_debugreg[3]), 3);
+	set_debugreg(__this_cpu_read(cpu_dr7), 7);
+}
+EXPORT_SYMBOL_GPL(hw_breakpoint_enable);
+
 /*
  * Handle debug exception notifications.
  *
--
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