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

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Date:	Thu, 22 Mar 2012 20:36:58 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: Regression introduced by
 bfcfaa77bdf0f775263e906015982a608df01c76 (vfs: use 'unsigned long' accesses
 for dcache name comparison and hashing)

On Thu, Mar 22, 2012 at 08:24:45PM +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?

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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