linux-kernel - Re: [RFC][ PATCH -tip v2 0/7] kprobes: Kprobes jump optimization support

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: <87bpoe29jo.fsf@basil.nowhere.org>
Date:	Tue, 23 Jun 2009 18:34:51 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Anders Kaseorg <andersk@...lice.com>,
	Tim Abbott <tabbott@...lice.com>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>
Subject: Re: [RFC][ PATCH -tip v2 0/7] kprobes: Kprobes jump optimization  support

Masami Hiramatsu <mhiramat@...hat.com> writes:

> Hi Andi,
>
> Andi Kleen wrote:
>> Masami Hiramatsu <mhiramat@...hat.com> writes:
>>> The gcc's crossjumping unifies equivalent code by inserting indirect
>>> jumps which jump into other function body. It is hard to know to where
>>> these jumps jump, so I decided to disable it when setting
>>> CONFIG_OPTPROBES=y.
>> 
>> That sounds quite bad. Tail call optimization is an important optimization
>> that especially on kernel style code (lots of indirect pointers
>> and sometimes deep call chains) is very useful.  It would be quite
>> sad if production kernels would lose that optimization.
>
> I think the crossjumping is not the tail call optimization,
> http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gccint/Passes.html

Statement didn't make sense then. The RTL crossjump pass you're referring
AFAIK does not jump into other functions, it only optimizes jumps 
inside a function (unless you're talking about inlines)
 
>> Also tail calls in C should always jump directly to another function,
>> so they shouldn't be particularly complex to manage.
>
> Tail call jumps directly into the head of another function,
> not the middle. Thus it is safe.

cross jumping does neither.

>
>>> I also decided not to optimize probes when it is in functions which
>>> will cause exceptions, because the exception in the kernel will jump
>>> to a fixup code and the fixup code jumps back to the middle of the
>>> same function body.
>> 
>> Note that not only exceptions do that, there are a few other cases
>> where jumps in and out of out of line sections happen. You might
>> need a more general mechanism to detect this.
>
> As far as I can see (under arch/x86), Almost all fixup entries are
> defined with ex_table entries, and others jump to the head of
> symbols(or functions). The jumps which jump into the middle of
> some functions are what I need to find, and, as far as I know,
> those fixup jumps are used with exception tables. Of course,
> I might miss some fixup codes, in that case, please let me know:-)

One case for example are out of line sections generated by gcc itself
with the right options.

-andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
--
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/