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

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Date:   Thu, 1 Jun 2017 08:02:01 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        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>
Subject: Re: [RFC PATCH 10/10] x86/unwind: add undwarf unwinder

On Thu, Jun 01, 2017 at 02:47:48PM +0200, Jiri Slaby wrote:
> On 06/01/2017, 02:26 PM, Josh Poimboeuf wrote:
> > On Thu, Jun 01, 2017 at 01:05:39PM +0200, Peter Zijlstra wrote:
> >> 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?
> > 
> > I wasn't able to find one...
> 
> There is bsearch, but that doesn't support searching for a value in
> between of 2 keys. I.e. what we typically have is these keys:
>   some_function1 at 0x1000
>   some_function2 at 0x2000
> and we look for IP which can be e.g. 0x1010. The bsearch's cmp function
> currently has no option to say, yes, the last one you asked me was the
> right one, this one is after it already.

Actually, I think bsearch would work with the latest version of the data
structure.  It now has a len field, so we can do a self-contained
compare.

-- 
Josh