netdev - Re: [PATCH 1/1] xdp: avoid calling kfree twice

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Date:   Mon, 7 Dec 2020 13:51:36 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Zhu Yanjun <yanjun.zhu@...el.com>, zyjzyj2000@...il.com,
        magnus.karlsson@...el.com, netdev@...r.kernel.org,
        jonathan.lemon@...il.com
Subject: Re: [PATCH 1/1] xdp: avoid calling kfree twice

On 2020-12-08 07:50, Zhu Yanjun wrote:
> From: Zhu Yanjun <zyjzyj2000@...il.com>
> 
> In the function xdp_umem_pin_pages, if npgs != umem->npgs and
> npgs >= 0, the function xdp_umem_unpin_pages is called. In this
> function, kfree is called to handle umem->pgs, and then in the
> function xdp_umem_pin_pages, kfree is called again to handle
> umem->pgs. Eventually, umem->pgs is freed twice.
>

Hi Zhu,

Thanks for the cleanup! kfree(NULL) is valid, so this is not a 
double-free, but still a nice cleanup!


> Signed-off-by: Zhu Yanjun <zyjzyj2000@...il.com>
> ---
>   net/xdp/xdp_umem.c | 17 +++++------------
>   1 file changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> index 56a28a686988..ff5173f72920 100644
> --- a/net/xdp/xdp_umem.c
> +++ b/net/xdp/xdp_umem.c
> @@ -97,7 +97,6 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
>   {
>   	unsigned int gup_flags = FOLL_WRITE;
>   	long npgs;
> -	int err;
>   
>   	umem->pgs = kcalloc(umem->npgs, sizeof(*umem->pgs),
>   			    GFP_KERNEL | __GFP_NOWARN);
> @@ -112,20 +111,14 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
>   	if (npgs != umem->npgs) {
>   		if (npgs >= 0) {
>   			umem->npgs = npgs;
> -			err = -ENOMEM;
> -			goto out_pin;
> +			xdp_umem_unpin_pages(umem);
> +			return -ENOMEM;
>   		}
> -		err = npgs;
> -		goto out_pgs;
> +		kfree(umem->pgs);
> +		umem->pgs = NULL;
> +		return npgs;

I'd like an explicit cast "(int)" here (-Wconversion). Please spin a v2
with the cast, with my:

Acked-by: Björn Töpel <bjorn.topel@...el.com>

added.


Cheers!
Björn


>   	}
>   	return 0;
> -
> -out_pin:
> -	xdp_umem_unpin_pages(umem);
> -out_pgs:
> -	kfree(umem->pgs);
> -	umem->pgs = NULL;
> -	return err;
>   }
>   
>   static int xdp_umem_account_pages(struct xdp_umem *umem)
>