On Tue, Apr 20, 2010 at 03:24:59PM -0400, William A. (Andy) Adamson wrote:
> On Tue, Apr 20, 2010 at 12:51 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
> > On Tue, Apr 20, 2010 at 12:16:52PM +1000, Neil Brown wrote:
> >>
> >> Surely this can never have worked... which implies that the code has
> >> never been used?
> >>
> >> When read_buf is called to move over to the next page in the pagelist
> >> of an NFSv4 request, it sets argp->end to essentially a random
> >> number, certainly not an address within the page which argp->p now
> >> points to. So subsequent calls to READ_BUF will think there is much
> >> more than a page of spare space (the cast to u32 ensures an unsigned
> >> comparison) so we can expect to fall off the end of the second
> >> page.
> >
> > Yipes, thanks.
> >
> >> I guess we never ever receive requests with any operation starting
> >> beyond the first page!
> >
> > putfh-write-getattr, for example, is common enough. The write decoding
> > should leave arg->end set correctly. But there are two read_buf()'s in
> > decode_getattr(), and I can't see why we don't hit this bug on a write
> > that leaves that final getattr exactly straddling a page boundary.
>
> The write data is dumped into the rq_vec which has non-contiguous
> pages. So the xdr_buf head only holds the putfh result, the short
> write response header (v4 stateid, offset, how, length, etc), and then
> the getattr. so there is plenty of space.
This is the server-side write-decoding, so you could see:
rpc header | putfh | write ... data ... | getattr
^
|
page boundary here
--b.
>
> -->Andy
>
> >
> > --b.
> >
> >> [[
> >> I found this while looking at why fsstress over NFS over RDMA caused
> >> a bad memory dereference in READ32, suggesting that 'p' had a bad
> >> value. However it was ffff8801299188f0, which is not an "I've fallen
> >> off the end of the page" sort of value. So I think it must be a
> >> different bug :-( It is as if the page is being unmapped underneath
> >> us...
> >> ]]
> >> NeilBrown
> >>
> >>
> >>
> >>
> >> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> >> index e170317..34ccf81 100644
> >> --- a/fs/nfsd/nfs4xdr.c
> >> +++ b/fs/nfsd/nfs4xdr.c
> >> @@ -161,10 +161,10 @@ static __be32 *read_buf(struct nfsd4_compoundargs *argp, u32 nbytes)
> >> argp->p = page_address(argp->pagelist[0]);
> >> argp->pagelist++;
> >> if (argp->pagelen < PAGE_SIZE) {
> >> - argp->end = p + (argp->pagelen>>2);
> >> + argp->end = argp->p + (argp->pagelen>>2);
> >> argp->pagelen = 0;
> >> } else {
> >> - argp->end = p + (PAGE_SIZE>>2);
> >> + argp->end = argp->p + (PAGE_SIZE>>2);
> >> argp->pagelen -= PAGE_SIZE;
> >> }
> >> memcpy(((char*)p)+avail, argp->p, (nbytes - avail));
> >> @@ -1426,10 +1426,10 @@ nfsd4_decode_compound(struct nfsd4_compoundargs *argp)
> >> argp->p = page_address(argp->pagelist[0]);
> >> argp->pagelist++;
> >> if (argp->pagelen < PAGE_SIZE) {
> >> - argp->end = p + (argp->pagelen>>2);
> >> + argp->end = argp->p + (argp->pagelen>>2);
> >> argp->pagelen = 0;
> >> } else {
> >> - argp->end = p + (PAGE_SIZE>>2);
> >> + argp->end = argp->p + (PAGE_SIZE>>2);
> >> argp->pagelen -= PAGE_SIZE;
> >> }
> >> }
> >>
> >>
> > --
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