linux-kernel - Re: [PATCH v2 1/1] dcache: Translating dentry into pathname without taking rename

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Date:	Thu, 05 Sep 2013 22:01:34 -0400
From:	Waiman Long <waiman.long@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	George Spelvin <linux@...izon.com>,
	John Stoffel <john@...ffel.org>
Subject: Re: [PATCH v2 1/1] dcache: Translating dentry into pathname without
 taking rename_lock

On 09/05/2013 04:42 PM, Linus Torvalds wrote:
> On Thu, Sep 5, 2013 at 1:29 PM, Waiman Long<waiman.long@...com>  wrote:
>> It is not as simple as doing a strncpy().
> Yes it damn well is.
>
> Stop the f*cking stupid arguments, and instead listen to what I say.
>
> Here. Let me bold-face the most important part for you, so that you
> don't miss it in all the other crap:
>
>     MAKE prepend() JUST USE "strncpy()" INSTEAD OF "memcpy()".
>
> Nothing else. Seriously. Your "you can't do it because we copy
> backwards" arguments are pure and utter garbage, exactly BECAUSE YOU
> DON'T CHANGE ANY OF THAT. You can actually use the unreliable length
> variable BUT YOU MUST STILL STOP AT A ZERO.
>
> Get it?
>
> You're complicating the whole thing for no good reason. I'm telling
> you (and HAVE BEEN telling you multiple times) that you cannot use
> "memcpy()" because the length may not be reliable, so you need to
> check for zero in the middle and stop early. All your arguments have
> been totally pointless, because you don't seem to see that simple and
> fundamental issue. You don't change ANYTHING else. But you damn well
> not do a "memcpy", you do something that stops when it hits a NUL
> character.
>
> We call that function "strncpy()". I'd actually prefer to write it out
> by hand (because somebody could implement "strncpy()" as a
> questionable function that accesses past the NUL as long as it's
> within the 'n'), and because I think we might want to do that
> word-at-a-time version of it, but for a first approximation, just do
> that one-liner version.
>
> Don't do anything else. Don't do locking. Don't do memchr. Just make
> sure that you stop at a NUL character, and don't trust the length,
> because the length may not match the pointer. That's was always ALL
> you needed to do.
>
>                Linus
I am sorry that I misunderstand what you said. I will do what you and Al 
advise me to do.

-Longman
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