netdev - Re: [Intel-wired-lan] [PATCH net] igc: Fix passing 0 to ERR_PTR in igc_xdp_run

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Message-ID: <8e4ef7f6-1d7d-45dc-b26e-4d9bc37269de@intel.com>
Date: Wed, 16 Oct 2024 16:06:34 -0700
From: Jacob Keller <jacob.e.keller@...el.com>
To: Simon Horman <horms@...nel.org>, Yue Haibing <yuehaibing@...wei.com>
CC: <anthony.l.nguyen@...el.com>, <przemyslaw.kitszel@...el.com>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<pabeni@...hat.com>, <ast@...nel.org>, <daniel@...earbox.net>,
	<hawk@...nel.org>, <john.fastabend@...il.com>, <vedang.patel@...el.com>,
	<andre.guedes@...el.com>, <maciej.fijalkowski@...el.com>,
	<jithu.joseph@...el.com>, <intel-wired-lan@...ts.osuosl.org>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<bpf@...r.kernel.org>
Subject: Re: [Intel-wired-lan] [PATCH net] igc: Fix passing 0 to ERR_PTR in
 igc_xdp_run_prog()



On 10/16/2024 11:53 AM, Simon Horman wrote:
> On Wed, Oct 16, 2024 at 06:53:10PM +0800, Yue Haibing wrote:
>> Return NULL instead of passing to ERR_PTR while res is IGC_XDP_PASS,
>> which is zero, this fix smatch warnings:
>> drivers/net/ethernet/intel/igc/igc_main.c:2533
>>  igc_xdp_run_prog() warn: passing zero to 'ERR_PTR'
>>
>> Fixes: 26575105d6ed ("igc: Add initial XDP support")
>> Signed-off-by: Yue Haibing <yuehaibing@...wei.com>
>> ---
>>  drivers/net/ethernet/intel/igc/igc_main.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
>> index 6e70bca15db1..c3d6e20c0be0 100644
>> --- a/drivers/net/ethernet/intel/igc/igc_main.c
>> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
>> @@ -2530,7 +2530,7 @@ static struct sk_buff *igc_xdp_run_prog(struct igc_adapter *adapter,
>>  	res = __igc_xdp_run_prog(adapter, prog, xdp);
>>  
>>  out:
>> -	return ERR_PTR(-res);
>> +	return res ? ERR_PTR(-res) : NULL;
> 
> I think this is what PTR_ERR_OR_ZERO() is for.

Not quite. PTR_ERR_OR_ZERO is intended for the case where you are
extracting an error from a pointer. This is converting an error into a
pointer.

I am not sure what is really expected here. If res is zero, shouldn't we
be returning an skb pointer and not NULL?

Why does igc_xdp_run_prog even return a sk_buff pointer at all? It never
actually returns an skb...

This feels like the wrong fix entirely.

__igc_xdp_run_prog returns a custom value for the action, between
IGC_XDP_PASS, IGC_XDP_TX, IGC_XDP_REDIRECT, or IGC_XDP_CONSUMED.

This function is called by igc_xdp_run_prog which converts this to a
negative error code with the sk_buff pointer type.

All so that we can assign a value to the skb pointer in
ice_clean_rx_irq, and check it with IS_ERR

I don't like this fix, I think we could drop the igc_xdp_run_prog
wrapper, call __igc_xdp_run_prog directly and check its return value
instead of this method of using an error pointer.