Re: BUG: debug_exception_enter() disables preemption and may call sleeping functions on aarch64 with RT

On Mon, Feb 10, 2025 at 03:06:57PM +0100, Sebastian Andrzej Siewior wrote:

On 2025-02-10 12:49:45 [+0000], Mark Rutland wrote:

quoted

Hi,

Hi,

quoted

I don't have an immediate suggestion; I'll need to go think about this
for a bit. Unfortunatealy, there are several nested cans of worms here.
:/

In theory, we can go split out the EL0 "debug exceptions" into separate
handlers, and wouldn't generally need to disable preemption for things
like BRK or single-step.

However, it's not immediately clear to me how we could handle
watchpoints or breakpoints, since for those preemption/interruption
could change the HW state under our feet, and we rely on single-step to
skip past the watchpoint/breakpoint after it is handled.

Couldn't you delay sending signals until after the preempt-disable
section? 

Looking at do_debug_exception,

void do_debug_exception(unsigned long addr_if_watchpoint, unsigned long esr,
			struct pt_regs *regs)
{
	const struct fault_info *inf = esr_to_debug_fault_info(esr);
	unsigned long pc = instruction_pointer(regs);

	debug_exception_enter(regs);

	if (user_mode(regs) && !is_ttbr0_addr(pc))
		arm64_apply_bp_hardening();

	if (inf->fn(addr_if_watchpoint, esr, regs)) {
		arm64_notify_die(inf->name, regs, inf->sig, inf->code, pc, esr);
	}

	debug_exception_exit(regs);
}
NOKPROBE_SYMBOL(do_debug_exception);


Do you mean executing the

	arm64_notify_die(inf->name, regs, inf->sig, inf->code, pc, esr);

after re-enabling the preemption or do you mean something more
sophisticated?

Luis

quoted

That, and last I looked reworking this we'd need to do a larger rework
to split out those "debug exceptions" because of that way that currently
bounces through the fault handling ligic in arch/arm64/mm/.

Mark.

Sebastian

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