linux-kernel - Faulty has_zero()? (was: .. anybody know of any filesystems that depend on the exact VFS 'namehash' implementation?)

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Message-Id: <FA611BE7-A574-444D-9C1E-7E32CB3BD659@anderson.de>
Date:	Sat, 17 Mar 2012 13:29:08 +0100
From:	Sven Anderson <sven@...erson.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Faulty has_zero()? (was: .. anybody know of any filesystems that depend on the exact VFS 'namehash' implementation?)

Hi Linus,

Am 01.03.2012 um 23:42 schrieb Linus Torvalds:

> +/* Return the high bit set in the first byte that is a zero */
> +static inline unsigned long has_zero(unsigned long a)
> +{
> +	return ((a - ONEBYTES) & ~a) & HIGHBITS;
> +}

(I commented this on your google+ posting as well, but I'm not sure if you will notice it there.)

Out of curiosity I studied your code, and if I'm not mistaken your has_zero() function doesn't do what is expected. If there are leading 0x01 bytes in front of a NUL byte, they are also marked in the mask because of the borrow bit. You could argue, that there are no 0x01 bytes in path stings, and I agree (even with UTF-8). But you also use it for slash detection, and there you have the same effect with the '.' char, since '.' ^ '/' == 0x01. So if you have a directory name like "foobar.../" it will get handled the same as "foobar////".

Best regards

Sven

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