On Mon, 2015-03-09 at 16:28 +0100, Ingo Molnar wrote: > * Denys Vlasenko <vda.li...@googlemail.com> wrote: > > > On Mon, Mar 9, 2015 at 3:58 PM, Ingo Molnar <mi...@kernel.org> wrote: > > >> -/** > > >> - * offsetofend(TYPE, MEMBER) > > >> - * > > >> - * @TYPE: The type of the structure > > >> - * @MEMBER: The member within the structure to get the end offset of > > >> - * > > >> - * Simple helper macro for dealing with variable sized structures passed > > >> - * from user space. This allows us to easily determine if the provided > > >> - * structure is sized to include various fields. > > >> - */ > > >> -#define offsetofend(TYPE, MEMBER) \ > > >> - (offsetof(TYPE, MEMBER) + sizeof(((TYPE *)0)->MEMBER)) > > > > > > So I like it, and because it is not particularly trivial when to use > > > this primitive it was explained nicely in a description in the vfio.h > > > version. > > > > > > But you lost that nice description during the code move!! > > > > That description was clearly specific to how that macro is used in > > drivers/vfio/*.c, along the lines of > > > > minsz = offsetofend(struct vfio_eeh_pe_op, op); > > Hm, but here 'minsz' == sizeof(struct vfio_eeh_pe_op), so the vfio > usage does not seem to be justified. > > > if (copy_from_user(&op, (void __user *)arg, minsz)) > > return -EFAULT; > > if (op.argsz < minsz || op.flags) > > return -EINVAL; > > > > But the macro is generic, it has many other uses besides this one. > > So I might be missing something, but what generic uses does it have, > beyond structures that have some rare size related weirdness, such as > alignment attributes? In 99% of the cases: > > sizeof(struct) == offsetofend(struct, last_member) > > right?
The idea in the vfio code is to allow the structure to evolve over time while maintaining compatibility. We effectively create a header in the structure with the base functionality and flags and structure size can tell us what optional fields are present. A better examples is drivers/vfio/pci/vfio_pci.c:vfio_pci_ioctl(), particularly VFIO_DEVICE_SET_IRQS where we read what's being provided in the "header" and then read beyond the minimum structure size if directed. Thanks, Alex > > Nevertheless, I can resend a version where comment survives if you > > want... > > So maybe extend it to a description that you think describes its uses > correctly? People will keep wondering about when to use this. > > Thanks, > > Ingo > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
