linux-kernel - Re: [PATCH 08/15] usb: ehci: avoid gcc-10 zero-length-bounds warning

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Message-ID: <CAK8P3a19DSnyvzSRCFr6fQGAMudQwhZY7Wy2yS6HL8y3TsfoLw@mail.gmail.com>
Date:   Fri, 1 May 2020 22:06:49 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        USB list <linux-usb@...r.kernel.org>
Subject: Re: [PATCH 08/15] usb: ehci: avoid gcc-10 zero-length-bounds warning

On Fri, May 1, 2020 at 4:42 AM Alan Stern <stern@...land.harvard.edu> wrote:
> On Thu, 30 Apr 2020, Arnd Bergmann wrote:

>
> No, they don't.

> >       /* PORTSC: offset 0x44 */
> > -     u32             port_status[0]; /* up to N_PORTS */
> > +     union {
> > +             u32             port_status[9]; /* up to N_PORTS */
>
> This array can have up to 15 elements, meaning that it can extend out
> to offset 0x80.

Ok, thanks for the clarification!

> >  /* EHCI 1.1 addendum */
> >  #define PORTSC_SUSPEND_STS_ACK 0
> >  #define PORTSC_SUSPEND_STS_NYET 1
> > @@ -165,7 +166,8 @@ struct ehci_regs {
> >  #define PORT_CONNECT (1<<0)          /* device connected */
> >  #define PORT_RWC_BITS   (PORT_CSC | PORT_PEC | PORT_OCC)
> >
> > -     u32             reserved3[9];
> > +             u32             reserved3[9];
> > +     };
> >
> >       /* USBMODE: offset 0x68 */
> >       u32             usbmode;        /* USB Device mode */
>
> As you see, this next field actually lies inside the preceding array.
> It's not a real conflict; any hardware which supports the usbmode field
> uses only the first element of the port_status array.
>
> I don't know how you want to handle this.  Doing
>
> #define usbmode port_status[9]
>
> doesn't seem like a very good approach, but I can't think of anything
> better at the moment.  Maybe just set the array size to 9, as you did,
> but with a comment explaining what's really going on.

The easiest change would be to use an anonymous struct like this

        /* PORTSC: offset 0x44 */
        union {
                u32             port_status[15]; /* up to N_PORTS */
/* EHCI 1.1 addendum */
#define PORTSC_SUSPEND_STS_ACK 0
...
#define PORT_RWC_BITS   (PORT_CSC | PORT_PEC | PORT_OCC)
                struct {
                        u32             reserved3[9];

        /* USBMODE: offset 0x68 */
                        u32             usbmode;        /* USB Device mode */
#define USBMODE_SDIS    (1<<3)          /* Stream disable */
#define USBMODE_BE      (1<<2)          /* BE/LE endianness select */
#define USBMODE_CM_HC   (3<<0)          /* host controller mode */
#define USBMODE_CM_IDLE (0<<0)          /* idle state */

                        u32             reserved4[5];
                };
        };
        u32             reserved5;

It doesn't really improve readability, but it should correctly
reflect the layout as you described it.

       Arnd