Re: Regression in 5.1.20: Reading long directory fails

Benjamin Coddington
2019-09-12T12:29:51Z

On 11 Sep 2019, at 13:54, Chuck Lever wrote:

>> On Sep 11, 2019, at 1:50 PM, Benjamin Coddington
>> <bcodding@redhat.com> wrote:
>>
>> On 11 Sep 2019, at 13:40, Benjamin Coddington wrote:
>>
>>> On 11 Sep 2019, at 13:29, Chuck Lever wrote:
>>>
>>>>> On Sep 11, 2019, at 1:26 PM, Benjamin Coddington
>>>>> <bcodding@redhat.com> wrote:
>>>>>
>>>>>
>>>>> On 11 Sep 2019, at 12:39, Chuck Lever wrote:
>>>>>
>>>>>>> On Sep 11, 2019, at 12:25 PM, Benjamin Coddington
>>>>>>> <bcodding@redhat.com> wrote:
>>>>>>>
>>>>>
>>>>>>> Instead, I think we want to make sure the mic falls squarely
>>>>>>> into the tail
>>>>>>> every time.
>>>>>>
>>>>>> I'm not clear how you could do that. The length of the page data
>>>>>> is not
>>>>>> known to the client before it parses the reply. Are you
>>>>>> suggesting that
>>>>>> gss_unwrap should do it somehow?
>>>>>
>>>>> Is it too niave to always put the mic at the end of the tail?
>>>>
>>>> The size of the page content is variable.
>>>>
>>>> The only way the MIC will fall into the tail is if the page content
>>>> is
>>>> exactly the largest expected size. When the page content is smaller
>>>> than
>>>> that, the receive logic will place part or all of the MIC in
>>>> ->pages.
>>>
>>> Ok, right. But what I meant is that xdr_buf_read_netobj() should be
>>> renamed
>>> and repurposed to be "move the mic from wherever it is to the end of
>>> xdr_buf's tail".
>>>
>>> But now I see what you mean, and I also see that it is already
>>> trying to do
>>> that.. and we don't want to overlap the copy..
>>>
>>> So, really, we need the tail to be larger than twice the mic.. less
>>> 1. That
>>> means the fix is probably just increasing rslack for krb5i.
>>
>> .. or we can keep the tighter tail space, and if we detect the mic
>> straddles
>> the page and tail, we can move the mic into the tail with 2 copies,
>> first
>> move the bit in the tail back, then move the bit in the pages.
>>
>> Which is preferred, less allocation, or in the rare case this occurs,
>> doing
>> copy twice?
>
> It sounds like the bug is that the current code does not deal
> correctly
> when the MIC crosses the boundary between ->pages and ->tail? I'd like
> to see that addressed rather than changing rslack.

Here's what I'm about to run through my testing:

diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
index 48c93b9e525e..d6ffc9011269 100644
--- a/net/sunrpc/xdr.c
+++ b/net/sunrpc/xdr.c
@@ -1238,14 +1238,21 @@ EXPORT_SYMBOL_GPL(xdr_encode_word);

/* If the netobj starting offset bytes from the start of xdr_buf is
contained
* entirely in the head or the tail, set object to point to it;
otherwise
- * try to find space for it at the end of the tail, copy it there, and
- * set obj to point to it. */
+ * try to find space for it at the end of the tail, and copy it there.
If
+ * the netobj is partly within the page data and tail, shrink the pages
to
+ * move the object into the tail */
int xdr_buf_read_netobj(struct xdr_buf *buf, struct xdr_netobj *obj,
unsigned int offset)
{
struct xdr_buf subbuf;
+ unsigned int page_range;

if (xdr_decode_word(buf, offset, &obj->len))
return -EFAULT;
+
+ page_range = buf->head->iov_len + buf->page_len - offset + 4;
+ if (page_range > 0 && page_range < obj->len)
+ xdr_shrink_pagelen(buf, page_range);
+
if (xdr_buf_subsegment(buf, &subbuf, offset + 4, obj->len))
return -EFAULT;

Is the use of xdr_shrink_pagelen() at this point in the decoding a
problem for RDMA?

Ben