Re: [PATCH 5.14 018/334] nbd: add the check to prevent overflow in __nbd_ioctl()

[Nick Desaulniers]

On Mon, Sep 13, 2021 at 1:42 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Sep 13, 2021 at 1:16 PM Nick Desaulniers
> <ndesaulniers@xxxxxxxxxx> wrote:
> >
> > Do we have access to _Generic in GCC 4.9?
>
> We've ended up using it unconditionally since last year, so yes.

Sorry, grepping would have taken < 1s. I'm very lazy.
http://threevirtues.com/

>
> In fact, the compiler version tests got removed when we raised the gcc
> version requirement to 4.9 in commit 6ec4476ac825 ("Raise gcc version
> requirement to 4.9"):
>
>    "In particular, raising the minimum to 4.9 means that we can now just
>     assume _Generic() exists, which is likely the much better replacement
>     for a lot of very convoluted built-time magic with conditionals on
>     sizeof and/or __builtin_choose_expr() with same_type() etc"
>
> but we haven't used it much since.
>
> The "seqprop" code for picking the right lock for seqlock is perhaps
> the main example, and staring at that code will make you go blind, so
> look away.

Looking at my patch:
https://lore.kernel.org/stable/20210913203201.1844253-1-ndesaulniers@xxxxxxxxxx/
I don't think _Generic helps us in the case of dispatching based on
the result of is_signed_type() (the operands could undergo type
promotion, so we'd need lots of cases that are more concisely covered
by is_signed_type()). It could replace the nested checks in div_64
with nested _Generics, I think. Not sure it's a huge win for
readability.  Maybe cut the number of expansions of the parameters in
half though. Let me give it a try just to see what it looks like.
-- 
Thanks,
~Nick Desaulniers