linux-kernel - Re: Regression introduced by bfcfaa77bdf0f775263e906015982a608df01c76 (vfs: use 'unsigned long' accesses for dcache name comparison and hashing)

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Message-ID: <20120322214157.GB12985@phenom.dumpdata.com>
Date:	Thu, 22 Mar 2012 17:41:57 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com
Subject: Re: Regression introduced by
 bfcfaa77bdf0f775263e906015982a608df01c76 (vfs: use 'unsigned long' accesses
 for dcache name comparison and hashing)

On Thu, Mar 22, 2012 at 08:36:58PM +0000, Al Viro wrote:
> On Thu, Mar 22, 2012 at 08:24:45PM +0000, Al Viro wrote:
> > 
> > OK, full_name_hash()/hash_name() definitely have a mismatch and it's on the
> > names of length 8*n: trivial experiment shows that we have
> > name hash_name full_name_hash
> > a 61 61
> > ab 6261 6261
> > abc 636261 636261
> > abcd 64636261 64636261
> > abcdabc 64c6c4c2 64c6c4c2
> > abcdabcd efcead5 c8c6c4c2
> > abcdabcd9 efceb0e efceb0e
> > 
> > Linus, which way do you prefer to shift it?  Should hash_name() change to
> > match full_name_hash() or should it be the other way round?
> > 
> > What happens is that you get multiplication by 9 and adding 0 in the former,
> > after having added the last full word.  In the latter we add the last full
> > word, see that there's nothing left and bugger off.
> 
> Guys, could you check if this fixes it?

Works for me! Thanks for coming up with a patch so quickly. Please add

Reported-and-Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>

Thanks.
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 13e6a1f..7451d6f8 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1439,10 +1439,10 @@ unsigned int full_name_hash(const unsigned char *name, unsigned int len)
>  
>  	for (;;) {
>  		a = *(unsigned long *)name;
> -		hash *= 9;
>  		if (len < sizeof(unsigned long))
>  			break;
>  		hash += a;
> +		hash *= 9;
>  		name += sizeof(unsigned long);
>  		len -= sizeof(unsigned long);
>  		if (!len)
> --
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