linux-kernel - Re: [RFC PATCH 10/10] x86/unwind: add undwarf unwinder

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Message-ID: <8854f3ea-51a0-e5c5-ef54-5515eb2c2f37@suse.cz>
Date:   Wed, 14 Jun 2017 13:45:41 +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>
Subject: Re: [RFC PATCH 10/10] x86/unwind: add undwarf unwinder

On 06/01/2017, 07:44 AM, Josh Poimboeuf wrote:
> --- /dev/null
> +++ b/arch/x86/kernel/unwind_undwarf.c
> @@ -0,0 +1,402 @@
...
> +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;
> +
> +	if (regs) {
> +		if (user_mode(regs)) {
> +			state->stack_info.type = STACK_TYPE_UNKNOWN;
> +			return;
> +		}
> +
> +		state->ip = regs->ip;
> +		state->sp = kernel_stack_pointer(regs);
> +		state->bp = regs->bp;
> +		state->regs = regs;
> +
> +	} else if (task == current) {
> +		register void *__sp asm(_ASM_SP);
> +
> +		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), "+r" (__sp));

Maybe I don't understand this completely, but what is __sp used for here?

> +		state->regs = NULL;
> +
> +	} else {

In DWARF unwinder, we also used to do here:

+#ifdef CONFIG_SMP
+       } else if (task->on_cpu) {
+               return;
+#endif
        } else {

> +		struct inactive_task_frame *frame = (void *)task->thread.sp;

Since there is no inactive_task_frame for tasks currently running (on
other CPUs). At least this always held in the past.

Though, the test is indeed racy.

> +		state->ip = frame->ret_addr;
> +		state->sp = task->thread.sp;
> +		state->bp = frame->bp;
> +		state->regs = NULL;
> +	}

thanks,
-- 
js
suse labs