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Date:   Thu, 25 Jun 2020 17:10:51 -0700
From:   Thomas Pedersen <thomas@adapt-ip.com>
To:     Nick Owens <mischief@offblast.org>
Cc:     linux-wireless@vger.kernel.org,
        Yan-Hsuan Chuang <yhchuang@realtek.com>
Subject: Re: [PATCH] rtw88: fix skb_under_panic in tx path
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On 2020-06-25 13:18, Nick Owens wrote:
> hello :)

Hi Nick :)

> this change fixes a reliable crash on my thinkpad A485.
> 
> please note i have no prior experience doing kernel development or
> sending patches, and i'm not sure if this is a correct approach.

You probably want to submit patches with git-send-email. See 
https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches

> From aa589182d30a0f99e1b3201ed4f3830e8af71dac Mon Sep 17 00:00:00 2001
> From: Nick Owens <mischief@offblast.org>
> Date: Thu, 25 Jun 2020 12:55:41 -0700
> Subject: [PATCH] rtw88: fix skb_under_panic in tx path
> 
> fixes the following panic on my thinkpad A485
> 
> Oops#1 Part3
> <0>[ 3743.881656] skbuff: skb_under_panic: text:000000005f69fd98
> len:208 put:48 head:000000009e2719e8 data:00000000bd3795e0 tail:0xc2
> end:0x2c0 dev:wlp2s0

skb->head and skb->data here are really far (0.5GB) apart. Maybe 
skb->data actually got corrupted earlier?

> diff --git a/drivers/net/wireless/realtek/rtw88/pci.c
> b/drivers/net/wireless/realtek/rtw88/pci.c
> index d735f3127fe8..21b3b268cb25 100644
> --- a/drivers/net/wireless/realtek/rtw88/pci.c
> +++ b/drivers/net/wireless/realtek/rtw88/pci.c
> @@ -741,6 +741,12 @@ static int rtw_pci_tx_write_data(struct rtw_dev 
> *rtwdev,
>  	else if (!avail_desc(ring->r.wp, ring->r.rp, ring->r.len))
>  		return -ENOSPC;
> 
> +	if (skb_headroom(skb) < chip->tx_pkt_desc_sz &&
> +	    pskb_expand_head(skb, chip->tx_pkt_desc_sz - skb_headroom(skb),
> 0, GFP_ATOMIC)) {
> +		dev_err(rtwdev->dev, "no headroom available");
> +		return -ENOMEM;
> +	}
> +

If it is a headroom issue, you can actually express the needed headroom 
needed by the driver in hw->extra_tx_headroom during init and avoid the 
pskb_expand_head() here.

-- 
thomas