ARM: hw_breakpoint: disable preemption during debug exception handling
On ARM, debug exceptions occur in the form of data or prefetch aborts.
One difference is that debug exceptions require access to per-cpu banked
registers and data structures which are not saved in the low-level exception
code. For kernels built with CONFIG_PREEMPT, there is an unlikely scenario
that the debug handler ends up running on a different CPU from the one
that originally signalled the event, resulting in random data being read
from the wrong registers.
This patch adds a debug_entry macro to the low-level exception handling
code which checks whether the taken exception is a debug exception. If
it is, the preempt count for the faulting process is incremented. After
the debug handler has finished, the count is decremented.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index d37ed35..eeba380 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c
@@ -24,6 +24,7 @@
#define pr_fmt(fmt) "hw-breakpoint: " fmt
#include <linux/errno.h>
+#include <linux/hardirq.h>
#include <linux/perf_event.h>
#include <linux/hw_breakpoint.h>
#include <linux/smp.h>
@@ -736,14 +737,17 @@
/*
* Called from either the Data Abort Handler [watchpoint] or the
- * Prefetch Abort Handler [breakpoint].
+ * Prefetch Abort Handler [breakpoint] with preemption disabled.
*/
static int hw_breakpoint_pending(unsigned long addr, unsigned int fsr,
struct pt_regs *regs)
{
- int ret = 1; /* Unhandled fault. */
+ int ret = 0;
u32 dscr;
+ /* We must be called with preemption disabled. */
+ WARN_ON(preemptible());
+
/* We only handle watchpoints and hardware breakpoints. */
ARM_DBG_READ(c1, 0, dscr);
@@ -758,11 +762,15 @@
watchpoint_handler(addr, regs);
break;
default:
- goto out;
+ ret = 1; /* Unhandled fault. */
}
- ret = 0;
-out:
+ /*
+ * Re-enable preemption after it was disabled in the
+ * low-level exception handling code.
+ */
+ preempt_enable();
+
return ret;
}