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Date:   Mon, 7 Dec 2020 16:30:07 +0100
From:   Marco Elver <elver@google.com>
To:     Eric Dumazet <eric.dumazet@gmail.com>
Cc:     Eric Dumazet <edumazet@google.com>,
        netdev <netdev@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Miller <davem@davemloft.net>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Jann Horn <jannh@google.com>, Jakub Kicinski <kuba@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Willem de Bruijn <willemb@google.com>,
        syzbot <syzbot+7b99aafdcc2eedea6178@syzkaller.appspotmail.com>
Subject: Re: WARNING in sk_stream_kill_queues (5)
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On Thu, Dec 03, 2020 at 07:01PM +0100, Eric Dumazet wrote:
> On 12/3/20 6:41 PM, Marco Elver wrote:
> 
> > One more experiment -- simply adding
> > 
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -207,7 +207,21 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> >  	 */
> >  	size = SKB_DATA_ALIGN(size);
> >  	size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> > +	size = 1 << kmalloc_index(size); /* HACK */
> >  	data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc);
> > 
> > 
> > also got rid of the warnings. Something must be off with some value that
> > is computed in terms of ksize(). If not, I don't have any explanation
> > for why the above hides the problem.
> 
> Maybe the implementations of various macros (SKB_DATA_ALIGN and friends)
> hae some kind of assumptions, I will double check this.

I looked at some of these macros and am wondering why SKB_TRUESIZE()
uses SKB_DATA_ALIGN(sizeof(struct sk_buff)). Because I don't understand
how the memcaches that allocate sk_buff are aligned or somehow always
return SKB_DATA_ALIGN(sizeof(struct sk_buff)) sized objects -- a simple
BUG_ON(ksize(skb) != SKB_DATA_ALIGN(sizeof(struct sk_buff))) triggers.

Alas, doing something like:

--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -235,7 +235,7 @@
 
 /* return minimum truesize of one skb containing X bytes of data */
 #define SKB_TRUESIZE(X) ((X) +						\
-			 SKB_DATA_ALIGN(sizeof(struct sk_buff)) +	\
+			 sizeof(struct sk_buff) +			\
 			 SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))

does not fix the problem.

Still trying to debug, because I don't want this to block the SLUB
enablement of KFENCE, even if it turns out it's not KFENCE. :-/

Thanks,
-- Marco