linux-kernel - Re: staging: emxx_udc question on i_write

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Date:   Thu, 3 Nov 2022 13:42:56 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     Dan Carpenter <error27@...il.com>
Cc:     outreachy@...ts.linux.dev,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: staging: emxx_udc question on i_write_length datatype

On Thu, Nov 03, 2022 at 10:55:27AM +0300, Dan Carpenter wrote:
> On Thu, Nov 03, 2022 at 12:57:09PM +0530, Deepak R Varma wrote:
> > Hello,
> > While reviewing this [1] coccicheck warning, I observed something that concerned
> > me. The variable i_write_length is declared to be of u32 type. Later it is
> > assigned a value DMA_MAX_COUNT * mpkt; which is 256 * u32;
> >
> > I am unable to estimate if mpkt (or max packet size) can attain value greater
> > than 16777215 in which case the result will overflow the 32 bits of
> > i_write_length. Is it safe to make i_write_length to be a u64?
> >
> > [1] drivers/staging/emxx_udc/emxx_udc.c:1007:28-29: WARNING opportunity for min()
> >
>
> drivers/staging/emxx_udc/emxx_udc.c
>    983  static int _nbu2ss_in_dma(struct nbu2ss_udc *udc, struct nbu2ss_ep *ep,
>    984                            struct nbu2ss_req *req, u32 num, u32 length)
>    985  {
>    986          dma_addr_t      p_buffer;
>    987          u32             mpkt;           /* MaxPacketSize */
>    988          u32             lmpkt;          /* Last Packet Data Size */
>    989          u32             dmacnt;         /* IN Data Size */
>    990          u32             i_write_length;
>    991          u32             data;
>    992          int             result = -EINVAL;
>    993          struct fc_regs __iomem *preg = udc->p_regs;
>    994
>    995          if (req->dma_flag)
>    996                  return 1;               /* DMA is forwarded */
>    997
>    998  #ifdef USE_DMA
>    999          if (req->req.actual == 0)
>   1000                  _nbu2ss_dma_map_single(udc, ep, req, USB_DIR_IN);
>   1001  #endif
>   1002          req->dma_flag = true;
>   1003
>   1004          /* MAX Packet Size */
>   1005          mpkt = _nbu2ss_readl(&preg->EP_REGS[num].EP_PCKT_ADRS) & EPN_MPKT;
>                                                                          ^^^^^^^^
> mpkt is 0-0x7ff so 256 * 0x7ff will not be greater than UINT_MAX.

Thank you Dan. I understand now. "& EPN_MPKT" keeps the size under control.
Appreciate very much.

./drv

>
>   1006
>   1007          if ((DMA_MAX_COUNT * mpkt) < length)
>   1008                  i_write_length = DMA_MAX_COUNT * mpkt;
>   1009          else
>   1010                  i_write_length = length;
>   1011
>   1012          /*------------------------------------------------------------*/
>   1013          /* Number of transmission packets */
>   1014          if (mpkt < i_write_length) {
>
> regards,
> dan carpenter
>