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Date:   Fri, 22 Sep 2023 11:32:02 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@suse.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, Yosry Ahmed <yosryahmed@google.com>,
        Yu Zhao <yuzhao@google.com>, Miaohe Lin <linmiaohe@huawei.com>,
        Yafang Shao <laoar.shao@gmail.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] mm/memcg: Annotate struct mem_cgroup_threshold_ary with
 __counted_by
Message-ID: <202309221128.6AC35E3@keescook>
References: <20230922175327.work.985-kees@kernel.org>
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On Fri, Sep 22, 2023 at 11:25:56AM -0700, Shakeel Butt wrote:
> On Fri, Sep 22, 2023 at 10:53 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > Prepare for the coming implementation by GCC and Clang of the __counted_by
> > attribute. Flexible array members annotated with __counted_by can have
> > their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
> > (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
> > functions).
> >
> > As found with Coccinelle[1], add __counted_by for struct mem_cgroup_threshold_ary.
> >
> > [1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci
> >
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Shakeel Butt <shakeelb@google.com>
> > Cc: Roman Gushchin <roman.gushchin@linux.dev>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >  include/linux/memcontrol.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 45d0c10e86cc..e0cfab58ab71 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -143,7 +143,7 @@ struct mem_cgroup_threshold_ary {
> >         /* Size of entries[] */
> >         unsigned int size;
> >         /* Array of thresholds */
> > -       struct mem_cgroup_threshold entries[];
> > +       struct mem_cgroup_threshold entries[] __counted_by(size);
> 
> Does 'size' here have to be a member of the same struct as entries? We
> have nodeinfo[] in struct mem_cgroup whose size is nr_node_ids which
> is global. Will __counted_by() work for that?

Not presently, no. This may come in future expansions of the feature.
We're also hoping to gain expressions for places where a size isn't a
native count, like for big endian, or a byte count that includes the
entire struct, etc. For now, though, the feature is narrowly scoped just
to get the common case landed.

-Kees

-- 
Kees Cook