Re: [External] : Re: [PATCH] 9p/xen: Fix end of loop tests for list_for_each_entry

[Harshvardhan Jha]

On 27/07/21 3:00 am, Stefano Stabellini wrote:
> On Mon, 26 Jul 2021, asmadeus@xxxxxxxxxxxxx wrote:
> > Harshvardhan Jha wrote on Sun, Jul 25, 2021 at 11:21:03PM +0530:
> > > The list_for_each_entry() iterator, "priv" in this code, can never be
> > > NULL so the warning would never be printed.
> >
> > hm? priv won't be NULL but priv->client won't be client, so it will
> > return -EINVAL alright in practice?
> >
> > This does fix an invalid read after the list head, so there's a real
> > bug, but the commit message needs fixing.
>
> Agreed
>
>
> > > Signed-off-by: Harshvardhan Jha <harshvardhan.jha@xxxxxxxxxx>
> > > ---
> > >  From static analysis.  Not tested.
> >
> > +Stefano in To - I also can't test xen right now :/
> > This looks functional to me but if you have a bit of time to spare just
> > a mount test can't hurt.
>
> Yes, I did test it successfully. Aside from the commit messaged to be
> reworded:
How's this?
===========================BEGIN========================================
9p/xen: Fix end of loop tests for list_for_each_entry

This patch addresses the following problems:
 - priv can never be NULL, so this part of the check is useless
 - if the loop ran through the whole list, priv->client is invalid and
it is more appropriate and sufficient to check for the end of
list_for_each_entry loop condition.

Signed-off-by: Harshvardhan Jha <harshvardhan.jha@xxxxxxxxxx>
---
From static analysis. Not tested.
===========================END==========================================
> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Tested-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>
>
> > > ---
> > >   net/9p/trans_xen.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
> > > index f4fea28e05da..3ec1a51a6944 100644
> > > --- a/net/9p/trans_xen.c
> > > +++ b/net/9p/trans_xen.c
> > > @@ -138,7 +138,7 @@ static bool p9_xen_write_todo(struct xen_9pfs_dataring *ring, RING_IDX size)
> > >   
> > >   static int p9_xen_request(struct p9_client *client, struct p9_req_t *p9_req)
> > >   {
> > > -	struct xen_9pfs_front_priv *priv = NULL;
> > > +	struct xen_9pfs_front_priv *priv;
> > >   	RING_IDX cons, prod, masked_cons, masked_prod;
> > >   	unsigned long flags;
> > >   	u32 size = p9_req->tc.size;
> > > @@ -151,7 +151,7 @@ static int p9_xen_request(struct p9_client *client, struct p9_req_t *p9_req)
> > >   			break;
> > >   	}
> > >   	read_unlock(&xen_9pfs_lock);
> > > -	if (!priv || priv->client != client)
> > > +	if (list_entry_is_head(priv, &xen_9pfs_devs, list))
> > >   		return -EINVAL;
> > >   
> > >   	num = p9_req->tc.tag % priv->num_rings;