On Wed, Dec 16, 2015 at 12:45:15PM -0800, Shi, Yang wrote:
> On 12/16/2015 3:13 AM, Will Deacon wrote:
> >On Tue, Dec 15, 2015 at 04:18:08PM -0800, Yang Shi wrote:
> >>The kernel just send out a SIGTRAP signal when handling ptrace breakpoint in
> >>debug exception, so it sounds safe to have interrupt enabled if it is not
> >>disabled by the parent process.
> >
> >Is this actually fixing an issue you're seeing, or did you just spot this?
> >Given that force_sig_info disable interrupts, I don't think this is really
> >worth doing.
>
> I should made more comments at the first place, sorry for the inconvenience.
>
> I did run into some problems on -rt kernel with CRIU restore, please see the
> below kernel bug log:
Thanks.
> >My worry here is that we take an interrupt and, on the return path,
> >decide to reschedule due to CONFIG_PREEMPT. If we somehow end up back
> >in the debugger, I'm concerned that it could remove the breakpoint and
> >then later see an unexpected SIGTRAP from the child.
> >
> >Having said that, I've failed to construct a non-racy scenario in which
> >that can happen, but I'm just really uncomfortable making this change
> >unless there's a real problem being solved.
>
> The patch is inspired by the similar code in other architectures, e.g. x86
> and powerpc which have hardware debug exception to handle breakpoint and
> single step like arm64. And, they have interrupt enabled in both breakpoint
> and single step. So, I'm supposed arm64 could do the same thing.
>
> For the preempt case, it might be possible, but it sounds like a exception
> handling problem to me. The preempt should not be allowed in debug exception
> (current arm64 kernel does it), and in interrupt return path the code should
> check if debug is on or not. If debug is on, preempt should be just skipped.
> Or we could disable preempt in debug exception.
Yeah, disabling preemption during the debug handler and then enabling
interrupts if it came from userspace sounds like the best option. However,
that's a fairly invasive change to entry.S at this point, so maybe we're
better off with something like the patch below in the meantime?
Will
--->8
diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 8aee3aeec3e6..7f4913f2ea3c 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -226,11 +226,29 @@ static int call_step_hook(struct pt_regs *regs, unsigned int esr)
return retval;
}
+static void send_user_sigtrap(int si_code)
+{
+ struct pt_regs *regs = current_pt_regs();
+ siginfo_t info = {
+ .si_signo = SIGTRAP,
+ .si_errno = 0,
+ .si_code = si_code,
+ .si_addr = (void __user *)instruction_pointer(regs),
+ };
+
+ if (WARN_ON(!user_mode(regs)))
+ return;
+
+ preempt_disable();
+ local_irq_enable();
+ force_sig_info(SIGTRAP, &info, current);
+ local_irq_disable();
+ preempt_enable();
+}
+
static int single_step_handler(unsigned long addr, unsigned int esr,
struct pt_regs *regs)
{
- siginfo_t info;
-
/*
* If we are stepping a pending breakpoint, call the hw_breakpoint
* handler first.
@@ -239,11 +257,7 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
return 0;
if (user_mode(regs)) {
- info.si_signo = SIGTRAP;
- info.si_errno = 0;
- info.si_code = TRAP_HWBKPT;
- info.si_addr = (void __user *)instruction_pointer(regs);
- force_sig_info(SIGTRAP, &info, current);
+ send_user_sigtrap(TRAP_HWBKPT);
/*
* ptrace will disable single step unless explicitly
@@ -307,17 +321,8 @@ static int call_break_hook(struct pt_regs *regs, unsigned int esr)
static int brk_handler(unsigned long addr, unsigned int esr,
struct pt_regs *regs)
{
- siginfo_t info;
-
if (user_mode(regs)) {
- info = (siginfo_t) {
- .si_signo = SIGTRAP,
- .si_errno = 0,
- .si_code = TRAP_BRKPT,
- .si_addr = (void __user *)instruction_pointer(regs),
- };
-
- force_sig_info(SIGTRAP, &info, current);
+ send_user_sigtrap(TRAP_BRKPT);
} else if (call_break_hook(regs, esr) != DBG_HOOK_HANDLED) {
pr_warning("Unexpected kernel BRK exception at EL1\n");
return -EFAULT;
@@ -328,7 +333,6 @@ static int brk_handler(unsigned long addr, unsigned int esr,
int aarch32_break_handler(struct pt_regs *regs)
{
- siginfo_t info;
u32 arm_instr;
u16 thumb_instr;
bool bp = false;
@@ -359,14 +363,7 @@ int aarch32_break_handler(struct pt_regs *regs)
if (!bp)
return -EFAULT;
- info = (siginfo_t) {
- .si_signo = SIGTRAP,
- .si_errno = 0,
- .si_code = TRAP_BRKPT,
- .si_addr = pc,
- };
-
- force_sig_info(SIGTRAP, &info, current);
+ send_user_sigtrap(TRAP_BRKPT);
return 0;
}
--
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