Hi Gustavo,
On Tue, Feb 11, 2020 at 10:49 PM Gustavo A. R. Silva
<[email protected]> wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> unadvertenly introduced[3] to the codebase from now on.
>
> All these instances of code were found with the help of the following
> Coccinelle script:
>
> @@
> identifier S, member, array;
> type T1, T2;
> @@
>
> struct S {
> ...
> T1 member;
> T2 array[
> - 0
> ];
> };
I've stumbled across one more in include/uapi/linux/usb/ch9.h:
struct usb_key_descriptor {
__u8 bLength;
__u8 bDescriptorType;
__u8 tTKID[3];
__u8 bReserved;
__u8 bKeyData[0];
} __attribute__((packed));
And it seems people are (ab)using one-sized arrays for flexible arrays, too:
struct usb_string_descriptor {
__u8 bLength;
__u8 bDescriptorType;
__le16 wData[1]; /* UTF-16LE encoded */
} __attribute__ ((packed));
As this is UAPI, we have to be careful for regressions, though.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
On Thu, 13 Feb 2020 at 12:09, Geert Uytterhoeven <[email protected]> wrote:
>
> Hi Gustavo,
>
> On Tue, Feb 11, 2020 at 10:49 PM Gustavo A. R. Silva
> <[email protected]> wrote:
> > The current codebase makes use of the zero-length array language
> > extension to the C90 standard, but the preferred mechanism to declare
> > variable-length types such as these ones is a flexible array member[1][2],
> > introduced in C99:
> >
> > struct foo {
> > int stuff;
> > struct boo array[];
> > };
> >
> > By making use of the mechanism above, we will get a compiler warning
> > in case the flexible array does not occur last in the structure, which
> > will help us prevent some kind of undefined behavior bugs from being
> > unadvertenly introduced[3] to the codebase from now on.
> >
> > All these instances of code were found with the help of the following
> > Coccinelle script:
> >
> > @@
> > identifier S, member, array;
> > type T1, T2;
> > @@
> >
> > struct S {
> > ...
> > T1 member;
> > T2 array[
> > - 0
> > ];
> > };
>
> I've stumbled across one more in include/uapi/linux/usb/ch9.h:
>
> struct usb_key_descriptor {
> __u8 bLength;
> __u8 bDescriptorType;
>
> __u8 tTKID[3];
> __u8 bReserved;
> __u8 bKeyData[0];
> } __attribute__((packed));
>
> And it seems people are (ab)using one-sized arrays for flexible arrays, too:
>
> struct usb_string_descriptor {
> __u8 bLength;
> __u8 bDescriptorType;
>
> __le16 wData[1]; /* UTF-16LE encoded */
> } __attribute__ ((packed));
>
> As this is UAPI, we have to be careful for regressions, though.
>
These were probably taken straight from the specification. The [1]
trick is used a lot in the UEFI specification as well, for instance.