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

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Message-ID: <20170601110539.helelmmngwaba7fa@hirez.programming.kicks-ass.net>
Date:   Thu, 1 Jun 2017 13:05:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>, Jiri Slaby <jslaby@...e.cz>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH 10/10] x86/unwind: add undwarf unwinder

On Thu, Jun 01, 2017 at 12:44:16AM -0500, Josh Poimboeuf wrote:

> +static struct undwarf *__undwarf_lookup(struct undwarf *undwarf,
> +					unsigned int num, unsigned long ip)
> +{
> +	struct undwarf *first = undwarf;
> +	struct undwarf *last = undwarf + num - 1;
> +	struct undwarf *mid;
> +	unsigned long u_ip;
> +
> +	while (first <= last) {
> +		mid = first + ((last - first) / 2);
> +		u_ip = undwarf_ip(mid);
> +
> +		if (ip >= u_ip) {
> +			if (ip < u_ip + mid->len)
> +				return mid;
> +			first = mid + 1;
> +		} else
> +			last = mid - 1;
> +	}
> +
> +	return NULL;
> +}

That's a bog standard binary search thing, don't we have a helper for
that someplace?

> +static struct undwarf *undwarf_lookup(unsigned long ip)
> +{
> +	struct undwarf *undwarf;
> +	struct module *mod;
> +
> +	/* Look in vmlinux undwarf section: */
> +	undwarf = __undwarf_lookup(__undwarf_start, __undwarf_end - __undwarf_start, ip);
> +	if (undwarf)
> +		return undwarf;
> +
> +	/* Look in module undwarf sections: */
> +	preempt_disable();
> +	mod = __module_address(ip);
> +	if (!mod || !mod->arch.undwarf)
> +		goto module_out;
> +	undwarf = __undwarf_lookup(mod->arch.undwarf, mod->arch.num_undwarves, ip);
> +
> +module_out:
> +	preempt_enable();
> +	return undwarf;
> +}

A few points here:

 - that first lookup is entirely pointless if !core_kernel_text(ip)

 - that preempt_{dis,en}able() muck is 'pointless', for while it shuts
   up the warnings from __modules_address(), nothing preserves the
   struct undwarf you get a pointer to after the preempt_enable().

 - what about 'interesting' things like, ftrace_trampoline, kprobe insn
   slots and bpf text?

> +static bool stack_access_ok(struct unwind_state *state, unsigned long addr,
> +			    size_t len)
> +{
> +	struct stack_info *info = &state->stack_info;
> +
> +	/*
> +	 * If the next bp isn't on the current stack, switch to the next one.
> +	 *
> +	 * We may have to traverse multiple stacks to deal with the possibility
> +	 * that info->next_sp could point to an empty stack and the next bp
> +	 * could be on a subsequent stack.
> +	 */
> +	while (!on_stack(info, (void *)addr, len)) {
> +		if (get_stack_info(info->next_sp, state->task, info,
> +				   &state->stack_mask))
> +			return false;
	}
> +
> +	return true;
> +}