linux-kernel - Re: [PATCH v3.1 09/10] x86/unwind: add ORC unwinder

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Date:   Thu, 20 Jul 2017 09:12:16 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3.1 09/10] x86/unwind: add ORC unwinder

On 07/14/2017, 07:22 PM, Josh Poimboeuf wrote:
> +void __unwind_start(struct unwind_state *state, struct task_struct *task,
> +		    struct pt_regs *regs, unsigned long *first_frame)
> +{
> +	memset(state, 0, sizeof(*state));
> +	state->task = task;
> +
> +	/*
> +	 * Refuse to unwind the stack of a task while it's executing on another
> +	 * CPU.  This check is racy, but that's ok: the unwinder has other
> +	 * checks to prevent it from going off the rails.
> +	 */
> +	if (task_on_another_cpu(task))
> +		goto done;
> +
> +	if (regs) {
> +		if (user_mode(regs))
> +			goto done;
> +
> +		state->ip = regs->ip;
> +		state->sp = kernel_stack_pointer(regs);
> +		state->bp = regs->bp;
> +		state->regs = regs;
> +		state->full_regs = true;
> +		state->signal = true;
> +
> +	} else if (task == current) {
> +		asm volatile("lea (%%rip), %0\n\t"
> +			     "mov %%rsp, %1\n\t"
> +			     "mov %%rbp, %2\n\t"
> +			     : "=r" (state->ip), "=r" (state->sp),
> +			       "=r" (state->bp));
> +
> +	} else {
> +		struct inactive_task_frame *frame = (void *)task->thread.sp;
> +
> +		state->ip = frame->ret_addr;
> +		state->sp = task->thread.sp;
> +		state->bp = frame->bp;

I wonder, if the reads from 'frame' should have READ_ONCE_NOCHECK for
the same reason as in:
commit 84936118bdf37bda513d4a361c38181a216427e0
Author: Josh Poimboeuf <jpoimboe@...hat.com>
Date:   Mon Jan 9 12:00:23 2017 -0600

    x86/unwind: Disable KASAN checks for non-current tasks
?


thanks,
-- 
js
suse labs