netdev - Re: [PATCH RFC] solos-pci: Fix BUG() with shared skb

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Date:	Wed, 04 Sep 2013 14:30:56 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	dwmw2@...radead.org
Cc:	netdev@...r.kernel.org, simon@...e.lp0.eu,
	linux-atm-general@...ts.sourceforge.net, nathan@...verse.com.au
Subject: Re: [PATCH RFC] solos-pci: Fix BUG() with shared skb

From: David Woodhouse <dwmw2@...radead.org>
Date: Tue, 03 Sep 2013 16:45:25 +0100

> Simon reported this BUG():
> 
>  kernel BUG at net/core/skbuff.c:1065!
>  Call Trace:
>   [<f9b7c12c>] ? pppoatm_send+0x3f/0x1a0 [pppoatm]
>   [<f8751797>] psend+0xa9/0x14a [solos_pci]
>   [<f9b7c248>] pppoatm_send+0x15b/0x1a0 [pppoatm]
>   [<f8a2f77d>] ppp_push+0x76/0x533 [ppp_generic]
> 
> (Rest of backtrace at http://s85.org/mn0aOxMN ― the skb appears to be
> IPv6, forwarded from another interface over PPPoATM.)
> 
> I wasn't expecting to see shared skbs in the ATM driver's ->send()
> function. Is this the right fix?

skb_realloc_headroom() should do everything you need.

This is what ethernet drivers do to prepend custom headers
when skb_headroom() is not large enough.

For example, see drivers/net/ethernet/sun/niu.c:niu_start_xmit().
There, the driver is attempting to prepend a TX descriptor to the
SKB.

If the SKB is shared, skb_realloc_headroom() will do the clone
for you if necessary.
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