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

On 27/07/21 3:00 am, Stefano Stabellini wrote:

On Mon, 26 Jul 2021, asmadeus@codewreck.org wrote:

quoted

Harshvardhan Jha wrote on Sun, Jul 25, 2021 at 11:21:03PM +0530:

quoted

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

quoted

quoted

Signed-off-by: Harshvardhan Jha <redacted>
---
 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 <redacted>
---
 From static analysis. Not tested.
===========================END==========================================

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>

quoted

quoted

---
  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;