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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 6 Dec 2021 16:13:00 -0800
Message-ID: <CAHk-=whLU+dk7EmPu5UC6DDSd76_dO4bVd4BkvxmR4W5-mmAgg@mail.gmail.com>
Subject: Re: [PATCH] block: switch to atomic_t for request references
To:     Kees Cook <keescook@chromium.org>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Peter Zijlstra <peterz@infradead.org>,
        Christoph Hellwig <hch@infradead.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, Dec 6, 2021 at 3:28 PM Kees Cook <keescook@chromium.org> wrote:
>
> I'm not arguing for refcount_t -- I'm arguing for an API that isn't a
> regression of features that have been protecting the kernel from bugs.

Maybe somebody could actually just fix refcount_t instead. Somebody
who cares about that currently horrendously bad interface.

Fix it to not do the fundamentally broken saturation that actively
destroys state: fix it to have a safe "try to increment", instead of
an unsafe "increment and do bad things".

Fix it to not unnecessarily use expensive compare-and-exchange loops,
when you can safely just race a bit, safe in the knowledge that you're
not going to race 2**31 times.

IOW, I think that "try_get_page()" function is basically the *much*
superior version of what is currently a broken "refcount_inc()".

And yes, it does warn about that overflow case that you claim only
refcount_t does. And does so without the broken semantics that
refcount h as.

                Linus