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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 13 Sep 2021 13:50:17 -0700
Message-ID: <CAKwvOd=ZG8sf1ZOkuidX_49VGkQE+BJDa19_vR4gh2FNQ2F_9Q@mail.gmail.com>
Subject: Re: [PATCH 5.14 018/334] nbd: add the check to prevent overflow in __nbd_ioctl()
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Arnd Bergmann <arnd@kernel.org>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Baokun Li <libaokun1@huawei.com>,
        open list <linux-kernel@vger.kernel.org>,
        linux-stable <stable@vger.kernel.org>,
        Hulk Robot <hulkci@huawei.com>,
        Josef Bacik <josef@toxicpanda.com>,
        Jens Axboe <axboe@kernel.dk>, Sasha Levin <sashal@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        lkft-triage@lists.linaro.org, llvm@lists.linux.dev,
        Kees Cook <keescook@chromium.org>
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On Mon, Sep 13, 2021 at 1:42 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, Sep 13, 2021 at 1:16 PM Nick Desaulniers
> <ndesaulniers@google.com> 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@google.com/
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