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Date: Sun, 26 Aug 2018 21:45:50 -0700
From: Tushar Dave <tushar.n.dave@...cle.com>
To: John Fastabend <john.fastabend@...il.com>, ast@...nel.org,
daniel@...earbox.net, davem@...emloft.net,
sowmini.varadhan@...cle.com, santosh.shilimkar@...cle.com,
jakub.kicinski@...ronome.com, quentin.monnet@...ronome.com,
jiong.wang@...ronome.com, sandipan@...ux.vnet.ibm.com,
kafai@...com, rdna@...com, yhs@...com, netdev@...r.kernel.org
Subject: Re: [RFC v3 net-next 3/5] ebpf: fix bpf_msg_pull_data
On 08/24/2018 06:02 PM, John Fastabend wrote:
> On 08/17/2018 04:08 PM, Tushar Dave wrote:
>> Like sockmap (sk_msg), socksg also deals with struct scatterlist
>> therefore socksg programs can use existing bpf helper bpf_msg_pull_data
>> to access packet data contained in struct scatterlist. While doing some
>> prelimnary testing, there are couple of issues found with
>> bpf_msg_pull_data that are fixed in this patch.
>>
>> Also, there cannot be more than MAX_SKB_FRAGS entries in sg_data
>> therefore any checks for sg entry more than MAX_SKB_FRAGS in
>> bpf_msg_pull_data() is removed.
>
> In sockmap the scatterlist is used as a ring so the MAX_SKB_FRAGS
> check is needed to keep searching through the ring when sg_start
> is non-zero.
Okay.
>
>>
>> Besides that, I also ran into issues while put_page() is invoked.
>> e.g.
>> [ 450.568723] BUG: Bad page state in process swapper/10 pfn:2021540
>> [ 450.575632] page:ffffea0080855000 count:0 mapcount:0
>> mapping:ffff88103d006840 index:0xffff882021540000 compound_mapcount: 0
>> [ 450.588069] flags: 0x6fffff80008100(slab|head)
>> [ 450.593033] raw: 006fffff80008100 dead000000000100 dead000000000200
>> ffff88103d006840
>> [ 450.601683] raw: ffff882021540000 0000000080080007 00000000ffffffff
>> 0000000000000000
>> [ 450.610337] page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
>> [ 450.617530] bad because of flags: 0x100(slab)
>>
>> To avoid above issue, currently put_page() is disabled in this patch
>> temporarily. I am working on alternatives so that page allocated via
>> slab (in this case) can be freed without any issue.>
>> Signed-off-by: Tushar Dave <tushar.n.dave@...cle.com>
>> Acked-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
>> ---
>> net/core/filter.c | 61 +++++++++++++++++++++++++++++--------------------------
>> 1 file changed, 32 insertions(+), 29 deletions(-)
>>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index e427c8e..cc52baa 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -2316,7 +2316,7 @@ struct sock *do_msg_redirect_map(struct sk_msg_buff *msg)
>> BPF_CALL_4(bpf_msg_pull_data,
>> struct sk_msg_buff *, msg, u32, start, u32, end, u64, flags)
>> {
>> - unsigned int len = 0, offset = 0, copy = 0;
>> + unsigned int len = 0, offset = 0, copy = 0, off = 0;
>> struct scatterlist *sg = msg->sg_data;
>> int first_sg, last_sg, i, shift;
>> unsigned char *p, *to, *from;
>> @@ -2330,22 +2330,28 @@ struct sock *do_msg_redirect_map(struct sk_msg_buff *msg)
>> i = msg->sg_start;
>> do {
>> len = sg[i].length;
>> - offset += len;
>> if (start < offset + len)
>> break;
>> + offset += len;
>
> This looks like a generic fix unrelated to this series.
> Can you send that as a bugfix?
Okay.
>
>> i++;
>> - if (i == MAX_SKB_FRAGS)
>> - i = 0;
>> - } while (i != msg->sg_end);
>> + } while (i <= msg->sg_end);
>>
>
> As noted above the MAX_SKB_FRAGS check is needed because
> sg_start can be non-zero and sg_end < st_start. In these
> cases we need to search the entries at the start of the
> array (being used as a ring).
Yup!
>
>> + /* return error if start is out of range */
>> if (unlikely(start >= offset + len))
>> return -EINVAL;
>>
>> - if (!msg->sg_copy[i] && bytes <= len)
>> - goto out;
>> + /* return error if i is last entry in sglist and end is out of range */
>> + if (msg->sg_copy[i] && end > offset + len)
>> + return -EINVAL>
>> first_sg = i;
>>
>> + /* if i is not last entry in sg list and end (i.e start + bytes) is
>> + * within this sg[i] then goto out and calculate data and data_end
>> + */
>> + if (!msg->sg_copy[i] && end <= offset + len)
>> + goto out;
>> +> /* At this point we need to linearize multiple scatterlist
>> * elements or a single shared page. Either way we need to
>> * copy into a linear buffer exclusively owned by BPF. Then
>> @@ -2359,11 +2365,14 @@ struct sock *do_msg_redirect_map(struct sk_msg_buff *msg)
>> do {
>> copy += sg[i].length;
>> i++;
>> - if (i == MAX_SKB_FRAGS)
>> - i = 0;
>
> same as above, need to keep.
Yup!
>
>> - if (bytes < copy)
>> + if (end < copy)
>> break;
>> - } while (i != msg->sg_end);
>> + } while (i <= msg->sg_end);
>> +
>> + /* return error if i is last entry in sglist and end is out of range */
>> + if (i > msg->sg_end && end > offset + copy)
>> + return -EINVAL;
>> +
>> last_sg = i;
>>
>> if (unlikely(copy < end - start))
>> @@ -2373,23 +2382,25 @@ struct sock *do_msg_redirect_map(struct sk_msg_buff *msg)
>> if (unlikely(!page))
>> return -ENOMEM;
>> p = page_address(page);
>> - offset = 0;
>>
>> i = first_sg;
>> do {
>> from = sg_virt(&sg[i]);
>> len = sg[i].length;
>> - to = p + offset;
>> + to = p + off;
>
> Not really sure if the change from offset->off is needed. Looks
> like it just makes a bigger diff.
We need both offset and off because they both are used for different
calculations!
'offset' is used to calculate the 'msg->data'
i.e. msg->data = sg_virt(&sg[first_sg]) + start - offset"
'off' , on the other hand, is used for when we linearize sg.
>
>>
>> memcpy(to, from, len);
>> - offset += len;
>> + off += len;
>> sg[i].length = 0;
>> - put_page(sg_page(&sg[i]));
>> + /* if original page is allocated via slab then put_page
>> + * causes error BUG: Bad page state in process. So temporarily
>> + * disabled put_page.
>> + * Todo: fix it
>> + */
>> + //put_page(sg_page(&sg[i]));
As I said in the commit message that put_page() causes error "BUG: Bad
page state in process ..." when used for RDS.
Any clue? Have you seen something like this with sockmap?
>>
>> i++;
>> - if (i == MAX_SKB_FRAGS)
>> - i = 0;
>> - } while (i != last_sg);
>> + } while (i < last_sg);
>>
>> sg[first_sg].length = copy;
>> sg_set_page(&sg[first_sg], page, copy, 0);
>> @@ -2406,12 +2417,8 @@ struct sock *do_msg_redirect_map(struct sk_msg_buff *msg)
>> do {
>> int move_from;
>>
>> - if (i + shift >= MAX_SKB_FRAGS)
>> - move_from = i + shift - MAX_SKB_FRAGS;
>> - else
>> - move_from = i + shift;
>> -
>
> Need to keep same as above.
yup!
>
>> - if (move_from == msg->sg_end)
>> + move_from = i + shift;> + if (move_from > msg->sg_end)
>> break;
>>
>> sg[i] = sg[move_from];
>> @@ -2420,14 +2427,10 @@ struct sock *do_msg_redirect_map(struct sk_msg_buff *msg)
>> sg[move_from].offset = 0;
>>
>> i++;
>> - if (i == MAX_SKB_FRAGS)
>> - i = 0;
>> } while (1);
>> msg->sg_end -= shift;
>> - if (msg->sg_end < 0)
>> - msg->sg_end += MAX_SKB_FRAGS;
>> out:
>> - msg->data = sg_virt(&sg[i]) + start - offset;
>> + msg->data = sg_virt(&sg[first_sg]) + start - offset;
>> msg->data_end = msg->data + bytes;
>>
>> return 0;
>>
>
> Thanks,
> John
>
Thanks.
-Tushar
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