Subject: Re: [PATCH 6/6] ath10k: sdio: replace skb_trim with explicit set of
 skb->len
To:     Kalle Valo <kvalo@codeaurora.org>
Cc:     linux-wireless@vger.kernel.org, ath10k@lists.infradead.org
References: <20190409190851.4557-1-erik.stromdahl@gmail.com>
 <20190409190851.4557-7-erik.stromdahl@gmail.com>
 <87zhovcqhl.fsf@kamboji.qca.qualcomm.com>
From:   Erik Stromdahl <erik.stromdahl@gmail.com>
Message-ID: <deca77d1-f171-e0cd-b571-89c2f8bafc87@gmail.com>
Date:   Mon, 15 Apr 2019 17:11:27 +0200
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On 4/12/19 3:17 PM, Kalle Valo wrote:
> Erik Stromdahl <erik.stromdahl@gmail.com> writes:
> 
>> This patch fixes a bug with padding of the skb data buffer.
>> Since skb_trim can only be used to reduce the skb len, it is useless when
>> we pad (increase the length of) the skb. Instead we must set skb->len
>> directly.
>>
>> Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
>> ---
>>   drivers/net/wireless/ath/ath10k/sdio.c | 7 ++++++-
>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/wireless/ath/ath10k/sdio.c b/drivers/net/wireless/ath/ath10k/sdio.c
>> index 3eb241cb8a25..989e3f563f3d 100644
>> --- a/drivers/net/wireless/ath/ath10k/sdio.c
>> +++ b/drivers/net/wireless/ath/ath10k/sdio.c
>> @@ -1496,7 +1496,12 @@ static int ath10k_sdio_hif_tx_sg(struct ath10k *ar, u8 pipe_id,
>>   		skb = items[i].transfer_context;
>>   		padded_len = ath10k_sdio_calc_txrx_padded_len(ar_sdio,
>>   							      skb->len);
>> -		skb_trim(skb, padded_len);
>> +		/* FIXME: unsure if just extending the skb len is the right
>> +		 * thing to do since we might read outside the skb->data
>> +		 * buffer. But we really don't want to realloc the skb just to
>> +		 * pad the length.
>> +		 */
>> +		skb->len = padded_len;
> 
> Good catch! But I don't think you can modify skb->len directly like
> that. There is skb_pad() but that doesn't change skb->len, so that most
> likely needs more changes. So maybe skb_put() is the safest here?
> 
I have tried a few different solutions for this, but none seems to be
bullet proof.

skb_pad() raises a BUG() if there is not enough space in skb->data.

The best candidate so far has been skb_put_padto(). It pads and reallocates
the skb if needed.

The problem is that it also cause a panic if there is more than one reference
to the skb (skb_shared() returns true).

Some of the management frames via nl80211 have a refcount of 2.
In this case it is not possible to free and allocate the skb since there are
other users/references.

I think I will have to make some kind of solution where I copy the content of
the skb to an internal buffer instead.