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Date: Thu, 19 Jul 2007 20:41:44 +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
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 <satyam.sharma@...il.com> wrote:
> 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:
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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