[systemd-devel] [PATCH] fix strict aliasing violations in src/udev/udev-builtin-usb_id.c

[Lennart Poettering]

On Thu, 05.03.15 04:58, Zbigniew Jędrzejewski-Szmek (zbyszek at in.waw.pl) wrote:

> > +                uint8_t        bLength;
> > +                uint8_t        bDescriptorType;
> > +                uint8_t        bInterfaceNumber;
> > +                uint8_t        bAlternateSetting;
> > +                uint8_t        bNumEndpoints;
> > +                uint8_t        bInterfaceClass;
> > +                uint8_t        bInterfaceSubClass;
> > +                uint8_t        bInterfaceProtocol;
> > +                uint8_t        iInterface;
> >          } __attribute__((packed));
> >  
> >          if (asprintf(&filename, "%s/descriptors", udev_device_get_syspath(dev)) < 0)
> > @@ -179,21 +180,21 @@ static int dev_if_packed_info(struct udev_device *dev, char *ifs_str, size_t len
> >  
> >          ifs_str[0] = '\0';
> >          while (pos < size && strpos+7 < len-2) {
> > -                struct usb_interface_descriptor *desc;
> > +                struct usb_interface_descriptor desc;
> >                  char if_str[8];
> >  
> > -                desc = (struct usb_interface_descriptor *) &buf[pos];
> > -                if (desc->bLength < 3)
> > +                memcpy(&desc, &buf[pos], sizeof(desc));
> Copying it seems suboptimal. But is this actually an aliasing
> violation? buf is a char array, and [1] says: "a character type
> may alias any other type".
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc-4.6.1/gcc/Optimize-Options.html#index-fstrict_002daliasing-825

Also, I greatly prefer using unions for these things, to make the
aliasing explicit, rather than copying things.

Lennart

-- 
Lennart Poettering, Red Hat