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ARM: hw_breakpoint: disable preemption during debug exception handling

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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>
This commit is contained in:
Will Deacon 2010-11-28 14:57:24 +00:00
parent 6ee33c2712
commit 7e20269647
3 changed files with 36 additions and 5 deletions
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4
arch/arm/kernel/entry-armv.S
View File

@ -198,6 +198,7 @@ __dabt_svc:
@
@ set desired IRQ state, then call main handler
@
debug_entry r1
msr cpsr_c, r9
mov r2, sp
bl do_DataAbort
@ -324,6 +325,7 @@ __pabt_svc:
#else
bl CPU_PABORT_HANDLER
#endif
debug_entry r1
msr cpsr_c, r9 @ Maybe enable interrupts
mov r2, sp @ regs
bl do_PrefetchAbort @ call abort handler
@ -439,6 +441,7 @@ __dabt_usr:
@
@ IRQs on, then call the main handler
@
debug_entry r1
enable_irq
mov r2, sp
adr lr, BSYM(ret_from_exception)
@ -703,6 +706,7 @@ __pabt_usr:
#else
bl CPU_PABORT_HANDLER
#endif
debug_entry r1
enable_irq @ Enable interrupts
mov r2, sp @ regs
bl do_PrefetchAbort @ call abort handler

19
arch/arm/kernel/entry-header.S
View File

@ -165,6 +165,25 @@
.endm
#endif /* !CONFIG_THUMB2_KERNEL */
@
@ Debug exceptions are taken as prefetch or data aborts.
@ We must disable preemption during the handler so that
@ we can access the debug registers safely.
@
.macro debug_entry, fsr
#if defined(CONFIG_HAVE_HW_BREAKPOINT) && defined(CONFIG_PREEMPT)
ldr r4, =0x40f @ mask out fsr.fs
and r5, r4, \fsr
cmp r5, #2 @ debug exception
bne 1f
get_thread_info r10
ldr r6, [r10, #TI_PREEMPT] @ get preempt count
add r11, r6, #1 @ increment it
str r11, [r10, #TI_PREEMPT]
1:
#endif
.endm
/*
* These are the registers used in the syscall handler, and allow us to
* have in theory up to 7 arguments to a function - r0 to r6.

18
arch/arm/kernel/hw_breakpoint.c
View File

@ -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 @@ static void breakpoint_handler(unsigned long unknown, struct pt_regs *regs)
/*
* 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 @@ static int hw_breakpoint_pending(unsigned long addr, unsigned int fsr,
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;
}