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; }