linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference

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Date:	Thu, 19 Jul 2007 19:41:17 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Neil Brown" <neilb@...e.de>
Cc:	"David CHANIAL" <david.ml@...o-web.fr>,
	"Linux Kernel Mailinglist" <linux-kernel@...r.kernel.org>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference - nfs v3

Hi Neil,

[ okay, just searching through my lkml folder looking for
"unable to handle" :-) ]

On 7/17/07, Neil Brown <neilb@...e.de> wrote:
> On Monday July 16, david.ml@...o-web.fr wrote:
> >
> > ************
> > BUG: unable to handle kernel NULL pointer dereference at virtual address
> > 00000004

> > EIP is at encode_fsid+0x67/0x89
>
> This is presumably where the illegal access happened.
>
> > eax: e5bde8c0   ebx: f7593404   ecx: 00000000   edx: 00000006
> > esi: dc569048   edi: f75934ec   ebp: f7593404   esp: f75f1f18

Yup, ecx is to blame here ...

> > Code: e2 08 09 d1 09 c1 eb 10 8b 83 88 00 00 00 8b 40 30 89 c3 89 c1 c1 fb 1f
> > 89 d8 0f c8 89 06 89 c8 eb 1e
>
> Unfortunately "ksymoops" does seem to decode this into something quite
> useful enough.  Normally one of the numbers has <> around it.  Are you
> should you copied the number across exactly?

Yes, I think David missed posting the full "Code:" here. Unfortunate.

>  In any case, there is no place in encode_fsid where an offset of 4
>  from any register is indexed, nor an offset of -2.

But I went ahead and disassembled encode_fsid() anyway. I did
stumble across a "mov 0x4(%ecx), %edx" -- which turns out to be:

static __be32 *encode_fsid(__be32 *p, struct svc_fh *fhp)
{
	u64 f;
	switch(fsid_source(fhp)) {
	default:
	case FSIDSOURCE_DEV:
		p = xdr_encode_hyper(p, (u64)huge_encode_dev
				(fhp->fh_dentry->d_inode->i_sb->s_dev));
		break;
	case FSIDSOURCE_FSID:
		p = xdr_encode_hyper(p, (u64) fhp->fh_export->ex_fsid);
		break;
	case FSIDSOURCE_UUID:
		f = ((u64*)fhp->fh_export->ex_uuid)[0];
		f ^= ((u64*)fhp->fh_export->ex_uuid)[1]; /* ***** HERE ***** */
		p = xdr_encode_hyper(p, f);
		break;
	}
	return p;
}

Note that fhp->fh_export->ex_uuid is an unsigned char *, which is
4 bytes on an i386 (which is what David's system is). For some
reason fhp->fh_export->ex_uuid (%ecx) is NULL here, which leads
to the oops. I have _zero_ other knowledge of knfsd code, and
not really be of any other use, sorry.

Satyam
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