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

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Message-ID: <ZxDvCoAo2puufMiH@boxer>
Date: Thu, 17 Oct 2024 13:03:38 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Yue Haibing <yuehaibing@...wei.com>
CC: Jacob Keller <jacob.e.keller@...el.com>, Simon Horman <horms@...nel.org>,
	<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>, <jithu.joseph@...el.com>,
	<intel-wired-lan@...ts.osuosl.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <bpf@...r.kernel.org>, <kurt@...utronix.de>
Subject: Re: [Intel-wired-lan] [PATCH net] igc: Fix passing 0 to ERR_PTR in
 igc_xdp_run_prog()

On Thu, Oct 17, 2024 at 11:55:05AM +0800, Yue Haibing wrote:
> On 2024/10/17 7:12, Jacob Keller wrote:
> > 
> > 
> > On 10/16/2024 4:06 PM, Jacob Keller wrote:
> >>
> >>
> >> 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.
> > 
> > Indeed, this SKB error stuff was added by 26575105d6ed ("igc: Add
> > initial XDP support") which claims to be aligning with other Intel drivers.
> > 
> 
> Thanks for review，maybe can fix this as commit 12738ac4754e ("i40e: Fix sparse errors in i40e_txrx.c")?

Yes please get rid of this logic. Historically speaking, i40e started this
and other drivers followed, but I chose in ice implementation to avoid
that :)

Kurt, if you'll be sending next revision for igb xsk support, then avoid
the logic we talk about here as well, please.

> 
> > But the other Intel drivers just have a function that returns the xdp
> > result and checks it directly.
> > 
> > Perhaps this is due to the way that the igc driver shares rings between
> > XDP and the regular path?
> > 
> > Its not clear to me, but I think this fix is not what I would do.
> > 
> > .