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SubjectRe: [PATCH] nfsd: wrong index used in inner loop
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On Mon, 2011-03-14 at 18:22 -0400, J. Bruce Fields wrote:
> On Fri, Mar 11, 2011 at 12:13:55PM +0800, Mi Jinlong wrote:
> >
> >
> > J. Bruce Fields:
> > > On Tue, Mar 08, 2011 at 10:32:26PM +0100, roel wrote:
> > >> Index i was already used in the outer loop
> > >>
> > >> Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
> > >> ---
> > >> fs/nfsd/nfs4xdr.c | 4 ++--
> > >> 1 files changed, 2 insertions(+), 2 deletions(-)
> > >>
> > >> Not 100% sure this one is needed but it looks suspicious.
> > >
> > > Looks bad to me, thanks.
> > >
> > > nfsd4_decode_create_session should probably really be broken up a little
> > > bit; if it wasn't so long this would have been more obvious.
> > >
> > > I'll see if I can slip this into 2.6.38 with a couple other last-minute
> > > patches.... Otherwise, it'll be in 2.6.39.
> > >
> > > --b.
> > >
> > >> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> > >> index 1275b86..615f0a9 100644
> > >> --- a/fs/nfsd/nfs4xdr.c
> > >> +++ b/fs/nfsd/nfs4xdr.c
> > >> @@ -1142,7 +1142,7 @@ nfsd4_decode_create_session(struct nfsd4_compoundargs *argp,
> > >>
> > >> u32 dummy;
> > >> char *machine_name;
> > >> - int i;
> > >> + int i, j;
> > >> int nr_secflavs;
> > >>
> > >> READ_BUF(16);
> > >> @@ -1215,7 +1215,7 @@ nfsd4_decode_create_session(struct nfsd4_compoundargs *argp,
> > >> READ_BUF(4);
> > >> READ32(dummy);
> > >> READ_BUF(dummy * 4);
> > >> - for (i = 0; i < dummy; ++i)
> > >> + for (j = 0; j < dummy; ++j)
> > >> READ32(dummy);
> >
> > We must not use dummy for index here.
> > After the first index, READ32(dummy) will change dummy!!!!
>
> Actually, wait, this is kind of silly. I don't see why we couldn't just
> skip the loop and do
>
> p += dummy;

This is exactly why I _hate_ the READ*() macros and their ilk, and am
really happy we got rid of them in the client.

READ_BUF() _sets_ p to whatever the value of argp->p is, and then
updates argp->p. It is just very very very hard to see that due to the
lack of transparency.

IOW: You don't need the "p += dummy" either. That happens automatically
when you next invoke READ_BUF().

Trond
--
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com



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