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Date: Wed, 21 Mar 2018 13:10:29 -0700
From: Yonghong Song <yhs@...com>
To: Alexander Duyck <alexander.duyck@...il.com>
CC: Eric Dumazet <edumazet@...gle.com>, <ast@...com>,
Daniel Borkmann <daniel@...earbox.net>, <diptanu@...com>,
Netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH net-next v3 1/2] net: permit skb_segment on head_frag
frag_list skb
On 3/21/18 7:59 AM, Alexander Duyck wrote:
> On Tue, Mar 20, 2018 at 10:02 PM, Yonghong Song <yhs@...com> wrote:
>>
>>
>> On 3/20/18 4:50 PM, Alexander Duyck wrote:
>>>
>>> On Tue, Mar 20, 2018 at 4:21 PM, Yonghong Song <yhs@...com> wrote:
>>>>
>>>> One of our in-house projects, bpf-based NAT, hits a kernel BUG_ON at
>>>> function skb_segment(), line 3667. The bpf program attaches to
>>>> clsact ingress, calls bpf_skb_change_proto to change protocol
>>>> from ipv4 to ipv6 or from ipv6 to ipv4, and then calls bpf_redirect
>>>> to send the changed packet out.
>>>>
>>>> 3472 struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>>> 3473 netdev_features_t features)
>>>> 3474 {
>>>> 3475 struct sk_buff *segs = NULL;
>>>> 3476 struct sk_buff *tail = NULL;
>>>> ...
>>>> 3665 while (pos < offset + len) {
>>>> 3666 if (i >= nfrags) {
>>>> 3667 BUG_ON(skb_headlen(list_skb));
>>>> 3668
>>>> 3669 i = 0;
>>>> 3670 nfrags =
>>>> skb_shinfo(list_skb)->nr_frags;
>>>> 3671 frag = skb_shinfo(list_skb)->frags;
>>>> 3672 frag_skb = list_skb;
>>>> ...
>>>>
>>>> call stack:
>>>> ...
>>>> #1 [ffff883ffef03558] __crash_kexec at ffffffff8110c525
>>>> #2 [ffff883ffef03620] crash_kexec at ffffffff8110d5cc
>>>> #3 [ffff883ffef03640] oops_end at ffffffff8101d7e7
>>>> #4 [ffff883ffef03668] die at ffffffff8101deb2
>>>> #5 [ffff883ffef03698] do_trap at ffffffff8101a700
>>>> #6 [ffff883ffef036e8] do_error_trap at ffffffff8101abfe
>>>> #7 [ffff883ffef037a0] do_invalid_op at ffffffff8101acd0
>>>> #8 [ffff883ffef037b0] invalid_op at ffffffff81a00bab
>>>> [exception RIP: skb_segment+3044]
>>>> RIP: ffffffff817e4dd4 RSP: ffff883ffef03860 RFLAGS: 00010216
>>>> RAX: 0000000000002bf6 RBX: ffff883feb7aaa00 RCX: 0000000000000011
>>>> RDX: ffff883fb87910c0 RSI: 0000000000000011 RDI: ffff883feb7ab500
>>>> RBP: ffff883ffef03928 R8: 0000000000002ce2 R9: 00000000000027da
>>>> R10: 000001ea00000000 R11: 0000000000002d82 R12: ffff883f90a1ee80
>>>> R13: ffff883fb8791120 R14: ffff883feb7abc00 R15: 0000000000002ce2
>>>> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
>>>> #9 [ffff883ffef03930] tcp_gso_segment at ffffffff818713e7
>>>> --- <IRQ stack> ---
>>>> ...
>>>>
>>>> The triggering input skb has the following properties:
>>>> list_skb = skb->frag_list;
>>>> skb->nfrags != NULL && skb_headlen(list_skb) != 0
>>>> and skb_segment() is not able to handle a frag_list skb
>>>> if its headlen (list_skb->len - list_skb->data_len) is not 0.
>>>>
>>>> This patch addressed the issue by handling skb_headlen(list_skb) != 0
>>>> case properly if list_skb->head_frag is true, which is expected in
>>>> most cases. The head frag is processed before list_skb->frags
>>>> are processed.
>>>>
>>>> Reported-by: Diptanu Gon Choudhury <diptanu@...com>
>>>> Signed-off-by: Yonghong Song <yhs@...com>
>>>> ---
>>>> net/core/skbuff.c | 51
>>>> +++++++++++++++++++++++++++++++++++++--------------
>>>> 1 file changed, 37 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>>> index 715c134..59bbc06 100644
>>>> --- a/net/core/skbuff.c
>>>> +++ b/net/core/skbuff.c
>>>> @@ -3475,7 +3475,7 @@ struct sk_buff *skb_segment(struct sk_buff
>>>> *head_skb,
>>>> struct sk_buff *segs = NULL;
>>>> struct sk_buff *tail = NULL;
>>>> struct sk_buff *list_skb = skb_shinfo(head_skb)->frag_list;
>>>> - skb_frag_t *frag = skb_shinfo(head_skb)->frags;
>>>> + skb_frag_t *frag = skb_shinfo(head_skb)->frags, *head_frag =
>>>> NULL;
>>>
>>>
>>> I think you misunderstood me. I wasn't saying you allocate head_frag.
>>> I was saying you could move the declaration down.
>>
>>
>> Sorry for my misunderstanding. I did understand your intention of moving
>> the declaration down in order to save stack space. I thought that we cannot
>> really move declaration down (although it works in C, but semantically it is
>> not quite right, more later), so I moved on to
>> use runtime allocation. But indeed skb_frag_t is not big (16 bytes), it
>> could live on the stack.
>>
>>>
>>>> unsigned int mss = skb_shinfo(head_skb)->gso_size;
>>>> unsigned int doffset = head_skb->data -
>>>> skb_mac_header(head_skb);
>>>> struct sk_buff *frag_skb = head_skb;
>>>> @@ -3664,19 +3664,39 @@ struct sk_buff *skb_segment(struct sk_buff
>>>> *head_skb,
>>>>
>>>> while (pos < offset + len) {
>>>
>>>
>>> So right here in the loop you could add a "skb_frag_t head_frag;" just
>>> so we declare it here and save ourselves the stack space.
>>
>>
>> I actually tried to move "skb_frag_t head_frag". The stack size remains the
>> same, 0xc0. This is related to how C compiler allocates stack space.
>> The declaration place won't decide the stack size as long as the declaration
>> dictates the usage. The stack size is really determined by liveness
>> analysis.
>>
>> Further, we have code like:
>> do {
>> ....
>> while (pos < offset + len) {
>> if (i >= nfrags) {
>> ...
>> head_frag = ...
>> }
>> ... = head_frag; // head_frag access guaranteed after
>> // above definition, but it may not
>> // be in the same outer do-while loop.
>> }
>> ...
>> } while (((offset += len) < head_skb->len);
>>
>> So the use of head_frag maybe in different outer loop iterations.
>> So I feel the definition of head_frag should be outside the
>> outer do-while loop, which is the main function scope. I will add some
>> comments here.
>
> So the point I had is that head_frag doesn't need to live that long.
> All you are doing is arranging the data so that you can essentially
> just dump it into *nskb_frag.
>
> One alternative you could look at would be to rearrange the code so
> that the MAX_SKB_FRAGS check occurs first, then perform the check for
> (i >= nfrags). Doing it that way we end up bypassing the need for
> head_frag entirely as you could just populate *nskb_frag directly. You
> could probably just add an inline structure that would convert the
> head frag into a skb_frag_t structure and return that. Something along
> the lines of:
> *nskb_frag = (i < 0) ? skb_head_frag_to_page_desc(frag_skb) : *frag;
This mechanism (inline function skb_head_frag_to_page_desc) works great!
I removed the local variable and the total stack size remains unchanged
with the patch.
>
> Doing it that way you could pull out the bits below that were
> populating head frag and just have it all handled as an inline
> function. An added advantage is that it should make the line wrapping
> easier to deal with. :-)
>
>>>
>>>> if (i >= nfrags) {
>>>> - BUG_ON(skb_headlen(list_skb));
>>>> -
>>>> i = 0;
>>>> + if (skb_headlen(list_skb)) {
>>>> + struct page *page;
>>>> +
>>>> + BUG_ON(!list_skb->head_frag);
>>>> +
>>>> + page =
>>>> virt_to_head_page(list_skb->head);
>>>> + if (!head_frag) {
>>>> + head_frag =
>>>> kmalloc(sizeof(skb_frag_t),
>>>> +
>>>> GFP_KERNEL);
>>>> + if (!head_frag)
>>>> + goto err;
>>>> + }
>>>
>>>
>>> Please no memory allocation. I just meant you could allocate it on the
>>> stack later.
>>>
>>>> + head_frag->page.p = page;
>>>> + head_frag->page_offset =
>>>> list_skb->data -
>>>> + (unsigned char
>>>> *)page_address(page);
>>>> + head_frag->size =
>>>> skb_headlen(list_skb);
>>>> + /* set i = -1 so we will pick
>>>> head_frag
>>>> + * instead of
>>>> skb_shinfo(list_skb)->frags
>>>> + * when i == -1.
>>>> + */
>>>> + i = -1;
>>>> + }
>>>
>>>
>>> So it took me a bit to pick up on the fact that line below wasn't
>>> removed. So we are basically trying to do this all in one pass now. Do
>>> I have that right?
>>>
>>> One thing you could look at doing to save yourself the extra "if"
>>> later would be to pull frag pointer before you go through skb_headlen
>>> check above. Then if you are going to use a head_frag you could just
>>> do a "i--; frag--;" combination just to rewind and make the room for
>>> the increment to come later. That way you don't have an invalid frag
>>> pointer floating around. That way you only have to do this once
>>> instead of having to do a conditional check per fragment.
>>
>>
>> Right. This indeed make code more cleaner.
>>
>>>
>>>> nfrags = skb_shinfo(list_skb)->nr_frags;
>>>> - frag = skb_shinfo(list_skb)->frags;
>>>
>>>
>>> This patch might be more readable if you were to just insert the
>>> skb_headlen() bits down here and left the i=0 through frag = .. in one
>>> piece.
>>
>>
>> Right. Will implement as suggested.
>>
>>>
>>>> - frag_skb = list_skb;
>>>> -
>>>> - BUG_ON(!nfrags);
>>>> -
>>>> - if (skb_orphan_frags(frag_skb,
>>>> GFP_ATOMIC) ||
>>>> - skb_zerocopy_clone(nskb, frag_skb,
>>>> - GFP_ATOMIC))
>>>> - goto err;
>>>> + if (nfrags) {
>>>> + frag =
>>>> skb_shinfo(list_skb)->frags;
>>>> + frag_skb = list_skb;
>>>> +
>>>> + if (skb_orphan_frags(frag_skb,
>>>> GFP_ATOMIC) ||
>>>> + skb_zerocopy_clone(nskb,
>>>> frag_skb,
>>>> +
>>>> GFP_ATOMIC))
>>>> + goto err;
>>>> + }
>>>>
>>>> list_skb = list_skb->next;
>>>> }
>>>> @@ -3689,7 +3709,7 @@ struct sk_buff *skb_segment(struct sk_buff
>>>> *head_skb,
>>>> goto err;
>>>> }
>>>>
>>>> - *nskb_frag = *frag;
>>>> + *nskb_frag = (i == -1) ? *head_frag : *frag;
>>>
>>>
>>> So this would be better as "*nskb_frag = (i < 0) ? head_frag : *frag;".
>>
>>
>> Good suggestion. Will implement as suggested.
>>
>>
>>>
>>>> __skb_frag_ref(nskb_frag);
>>>> size = skb_frag_size(nskb_frag);
>>>>
>>>> @@ -3702,7 +3722,8 @@ struct sk_buff *skb_segment(struct sk_buff
>>>> *head_skb,
>>>>
>>>> if (pos + size <= offset + len) {
>>>> i++;
>>>> - frag++;
>>>> + if (i != 0)
>>>> + frag++;
>>>> pos += size;
>>>> } else {
>>>> skb_frag_size_sub(nskb_frag, pos + size
>>>> - (offset + len));
>>>> @@ -3774,10 +3795,12 @@ struct sk_buff *skb_segment(struct sk_buff
>>>> *head_skb,
>>>> swap(tail->destructor, head_skb->destructor);
>>>> swap(tail->sk, head_skb->sk);
>>>> }
>>>> + kfree(head_frag);
>>>> return segs;
>>>>
>>>> err:
>>>> kfree_skb_list(segs);
>>>> + kfree(head_frag);
>>>> return ERR_PTR(err);
>>>> }
>>>> EXPORT_SYMBOL_GPL(skb_segment);
>>>> --
>>>> 2.9.5
>>>>
>>
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