linux-kernel - Re: [PATCH v26 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Message-ID: <CABb0KFEfmRz+Z_-7GygTL12E5Y254dvoUfWe4uSv9-wOx+Cs8w@mail.gmail.com>
Date:   Fri, 4 Aug 2023 19:17:24 +0200
From:   Michał Mirosław <emmir@...gle.com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>
Subject: Re: [PATCH v26 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On Fri, 4 Aug 2023 at 17:59, Andrei Vagin <avagin@...il.com> wrote:
>
> On Thu, Aug 3, 2023 at 8:25 AM Michał Mirosław <emmir@...gle.com> wrote:
> >
> > On Thu, 3 Aug 2023 at 17:09, Andrei Vagin <avagin@...il.com> wrote:
> > > On Thu, Jul 27, 2023 at 02:36:34PM +0500, Muhammad Usama Anjum wrote:
> > [...]
> > > > +     n_pages = (*end - addr) / PAGE_SIZE;
> > > > +     if (check_add_overflow(p->found_pages, n_pages, &total_pages) ||
> > > > +         total_pages > p->arg.max_pages) {
> > >
> > > why do we need to use check_add_overflow here?
> > >
> > > > +             size_t n_too_much = total_pages - p->arg.max_pages;
> > >
> > > it is unsafe to use total_pages if check_add_overflow returns non-zero.
> >
> > Since we're adding unsigned integers, this is well defined even after overflow.
>
> The description of check_add_overflow declares that is unsafe:
> https://elixir.bootlin.com/linux/latest/source/include/linux/overflow.h#L62
>
> It actually doesn't matter, because it should be impossible to
> overflow total_pages
> and we can consider not to use check_add_overflow here.

It seems the doc warning is quite new (d219d2a9a92e / Mon Aug 29
13:37:17 2022 -0700). The underlying __builtin_add_overflow() is
well-defined for any integer type, though. Even staying with C99,
arithmetic on unsigned integers is always defined as being done modulo
2^n.

Best Regards
Michał Mirosław