linux-kernel - Re: [PATCH] include/linux/slab.h: new KFREE() macro.

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]

Date:	Tue, 9 Jan 2007 11:19:55 -0800
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Amit Choudhary <amit2030@...oo.com>
Cc:	Valdis.Kletnieks@...edu, Pekka Enberg <penberg@...helsinki.fi>,
	Hua Zhong <hzhong@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] include/linux/slab.h: new KFREE() macro.

On Tue, 9 Jan 2007 11:02:35 -0800 (PST) Amit Choudhary wrote:

> 
> --- Valdis.Kletnieks@...edu wrote:
> 
> > On Mon, 08 Jan 2007 01:06:12 PST, Amit Choudhary said:
> > > I do not see how a double free can result in _logical_wrong_behaviour_ of the program and the
> > > program keeps on running (like an incoming packet being dropped because of double free).
> > Double
> > > free will _only_and_only_ result in system crash that can be solved by setting 'x' to NULL.
> > 
> > The problem is that very rarely is there a second free() with no intervening
> > use - what actually *happens* usually is:
> > 
> > 1) You alloc the memory
> > 2) You use the memory
> > 3) You take a reference on the memory, so you know where it is.
> > 4) You free the memory
> > 5) You use the memory via the reference you took in (3)
> > 6) You free it again - at which point you finally know for sure that
> > everything in step 5 was doing a fandango on core....
> > 
> 
> Correct. And doing kfree(x); x=NULL; is not hiding that. These issues can still be debugged by
> using the slab debugging options. One other benefit of doing this is that if someone tries to
> access the same memory again using the variable 'x', then he will get an immediate crash. And the
> problem can be solved immediately, without using the slab debugging options. I do not yet
> understand how doing this hides the bugs, obfuscates the code, etc. because I haven't seen an
> example yet, but only blanket statements.
> 
> But now I know better, since I haven't heard anything in support of this case, I have concluded
> that doing kfree(x); x=NULL; is _not_needed_ in the "linux kernel". I hope that no one does it in
> the future. And since people vehemently opposed this, I think its better to add another item on
> the kernel janitor's list to remove all the (x=NULL) statements where people are doing "kfree(x);
> x=NULL".

No thanks.  If a driver author wants to maintain driver state
that way, it's OK, but that doesn't make it a global requirement.

---
~Randy
-
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/