linux-kernel - Re: [PATCH] arm: Add unwinding annotations for 64bit division functions

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20110920145745.GG8545@e102109-lin.cambridge.arm.com>
Date:	Tue, 20 Sep 2011 15:57:45 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Nicolas Pitre <nico@...xnic.net>
Cc:	Laura Abbott <lauraa@...eaurora.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH] arm: Add unwinding annotations for 64bit division
 functions

On Tue, Sep 20, 2011 at 02:27:01PM +0100, Nicolas Pitre wrote:
> On Mon, 19 Sep 2011, Laura Abbott wrote:
> > On Mon, September 19, 2011 4:22 pm, Nicolas Pitre wrote:
> > > On Mon, 19 Sep 2011, Laura Abbott wrote:
> > >> @@ -189,7 +191,12 @@ ENTRY(__do_div64)
> > >>  	moveq	yh, xh
> > >>  	moveq	xh, #0
> > >>  	moveq	pc, lr
> > >> +UNWIND(.fnend)
> > >>
> > >> +UNWIND(.fnstart)
> > >> +UNWIND(.pad #4)
> > >> +UNWIND(.save {lr})
> > >> +Ldiv0_64:
> > >
> > > Why this phony fnend+fnstart here?
> >
> > If a division by 0 occurs, we need to be able to access the saved LR on
> > the stack which is setup right before calling the __div0 function. This
> > can't go at the top of __do_div64 because if we try to do a backtrace from
> > within __do_div64 the annotation won't be correct as the LR was never
> > saved on the stack.
> 
> I suppose the debug infrastructure always assume the same stack frame 
> for the whole function, hence you can't put that .pad and .save right 
> before the call to __div0 without the .fnstart and have it effective 
> only there?

An element in the unwinding table contains a pointer to the .fnstart
location and a set of instruction bytecodes (like .save) to the next
.fnend statement. If more 4 bytecodes (to fit in an int), it contains an
index to another table with more bytecodes. The elements in the
unwinding table are sorted by the .fnstart address.

So I think in the above case if we don't have the .fnend/.fnstart for
Ldiv0_64 we would get all the instructions in the __do_div64 unwinding
information. I don't think the unwinding bytecode is that smart to allow
other tricks like checking some conditions.

Unless anyone has a better idea to specify this:

Acked-by: Catalin Marinas <catalin.marinas@....com>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/