Re: BUG: unable to handle kernel NULL pointer dereference - nfs v3

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Ugh, not a good day for me today ... my earlier conclusion was right,
but not the reasoning behind it ... hopefully this time I'll do better :-)


On 7/19/07, Satyam Sharma <[email protected]> wrote:
Hi Neil,

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

On 7/17/07, Neil Brown <[email protected]> wrote:
> On Monday July 16, [email protected] 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:

Whoops ...

                f = ((u64*)fhp->fh_export->ex_uuid)[0]; /* *** HERE *** */
                f ^= ((u64*)fhp->fh_export->ex_uuid)[1]; /* and not here */

Anyway, %ecx is fhp->fh_export->ex_uuid above. (NULL, therefore 0).

f is u64, so on 32-bit i386, we need to store it in two local registers.
gcc does that by fetching 0x4(%ecx) into one register, and (%ecx)
into another, both those combined is the first memory load into f.

For the second memory load, again it's a (u64 *) cast, so gcc will
fetch 0xc(%ecx) and 0x8(%ecx) separately into two local registers.

And then xor them _separately_. (upper word with upper word,
lower word with lower word)

                p = xdr_encode_hyper(p, f);

These are probably the ntohl's -> bswap's.

                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).

So this wasn't quite right.

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.

But this was.

nfs3xdr.o:     file format elf32-i386
Disassembly of section .text:
00000232 <encode_fsid>:
    232:	55                   	push   %ebp
    233:	89 e5                	mov    %esp,%ebp
    235:	57                   	push   %edi
    236:	56                   	push   %esi
    237:	89 c6                	mov    %eax,%esi
    239:	53                   	push   %ebx
    23a:	89 d0                	mov    %edx,%eax
    23c:	89 d3                	mov    %edx,%ebx
    23e:	e8 fc ff ff ff       	call   23f <encode_fsid+0xd>
    243:	83 f8 01             	cmp    $0x1,%eax
    246:	74 3e                	je     286 <encode_fsid+0x54>
    248:	83 f8 02             	cmp    $0x2,%eax
    24b:	8d 7e 08             	lea    0x8(%esi),%edi

Note the lea 0x8(%esi),%edi here.

These are the other cases below.

    24e:	74 56                	je     2a6 <encode_fsid+0x74>
    250:	8b 83 84 00 00 00    	mov    0x84(%ebx),%eax
    256:	31 db                	xor    %ebx,%ebx
    258:	8b 40 24             	mov    0x24(%eax),%eax
    25b:	8b 80 08 01 00 00    	mov    0x108(%eax),%eax
    261:	8b 50 08             	mov    0x8(%eax),%edx
    264:	89 d0                	mov    %edx,%eax
    266:	0f b6 ca             	movzbl %dl,%ecx
    269:	c1 e8 14             	shr    $0x14,%eax
    26c:	81 e2 00 ff 0f 00    	and    $0xfff00,%edx
    272:	c1 e0 08             	shl    $0x8,%eax
    275:	09 c1                	or     %eax,%ecx
    277:	89 d8                	mov    %ebx,%eax
    279:	c1 e2 0c             	shl    $0xc,%edx
    27c:	09 d1                	or     %edx,%ecx
    27e:	0f c8                	bswap  %eax
    280:	89 06                	mov    %eax,(%esi)
    282:	89 c8                	mov    %ecx,%eax
    284:	eb 3e                	jmp    2c4 <encode_fsid+0x92>
    286:	8b 83 88 00 00 00    	mov    0x88(%ebx),%eax
    28c:	8b 48 30             	mov    0x30(%eax),%ecx
    28f:	89 cb                	mov    %ecx,%ebx
    291:	c1 fb 1f             	sar    $0x1f,%ebx
    294:	89 d8                	mov    %ebx,%eax
    296:	0f c8                	bswap  %eax
    298:	89 06                	mov    %eax,(%esi)
    29a:	89 c8                	mov    %ecx,%eax
    29c:	0f c8                	bswap  %eax
    29e:	89 46 04             	mov    %eax,0x4(%esi)
    2a1:	8d 46 08             	lea    0x8(%esi),%eax
    2a4:	eb 25                	jmp    2cb <encode_fsid+0x99>

Okay, this is case FSIDSOURCE_UUID:

    2a6:	8b 83 88 00 00 00    	mov    0x88(%ebx),%eax

0x88 bytes is the offset of struct fh_export * in struct svc_fh.

    2ac:	8b 48 34             	mov    0x34(%eax),%ecx

0x34 bytes is the offset of ex_uuid in struct fh_export.
%ecx == 0, which means ex_uuid was NULL.

    2af:	8b 51 04             	mov    0x4(%ecx),%edx

Upper word of: ((u64*)fhp->fh_export->ex_uuid)[0]

    2b2:	8b 59 0c             	mov    0xc(%ecx),%ebx

Upper word of: ((u64*)fhp->fh_export->ex_uuid)[1]

    2b5:	8b 01                	mov    (%ecx),%eax

Lower word of: ((u64*)fhp->fh_export->ex_uuid)[0]

    2b7:	8b 49 08             	mov    0x8(%ecx),%ecx

Lower word of: ((u64*)fhp->fh_export->ex_uuid)[1]

    2ba:	31 da                	xor    %ebx,%edx

Xor upper with upper.

    2bc:	31 c8                	xor    %ecx,%eax

Xor lower with lower.

So now we finally have the "f ^= ..." thing in %eax and %edx.
%eax is lower, %edx is the upper word of "f".

    2be:	89 d1                	mov    %edx,%ecx

gcc moves %edx to %ecx just for kicks.
So now the upper word of "u64 f" is in %ecx.

Looks like xdr_encode_hyper got inlined below.

    2c0:	0f c9                	bswap  %ecx

htonl(upperword)

    2c2:	89 0e                	mov    %ecx,(%esi)

Store that in *p.

    2c4:	0f c8                	bswap  %eax

htonl(lowerword)

    2c6:	89 46 04             	mov    %eax,0x4(%esi)

p++ and store it there in *p.

    2c9:	89 f8                	mov    %edi,%eax

As we noted the "lea 0x8(%esi),%edi" up above, %edi is precisely the
operation that does p++ as well as puts return value (__be32 *p) in %eax.

    2cb:	5b                   	pop    %ebx
    2cc:	5e                   	pop    %esi
    2cd:	5f                   	pop    %edi
    2ce:	5d                   	pop    %ebp
    2cf:	c3                   	ret

And we return.


Ahhh ... gcc generated some _beautiful_ code up there. Didn't find
a single instruction that shouldn't have been ...


Satyam
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