linux-kernel - Re: [PATCH] mm/vmalloc: fix wrong behavior in vread

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Message-Id: <20210714162910.4cb5434f7745dcf7ce9d85c2@linux-foundation.org>
Date:   Wed, 14 Jul 2021 16:29:10 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Chen Wandun <chenwandun@...wei.com>
Cc:     <serapheim.dimitro@...phix.com>, <urezki@...il.com>,
        <wangkefeng.wang@...wei.com>, <weiyongjun1@...wei.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Hulk Robot <hulkci@...wei.com>
Subject: Re: [PATCH] mm/vmalloc: fix wrong behavior in vread

On Wed, 14 Jul 2021 09:59:59 +0800 Chen Wandun <chenwandun@...wei.com> wrote:

> commit f608788cd2d6 ("mm/vmalloc: use rb_tree instead of list for vread()
> lookups") use rb_tree instread of list to speed up lookup, but function
> __find_vmap_area is try to find a vmap_area that include target address,
> if target address is smaller than the leftmost node in vmap_area_root,
> it will return NULL, then vread will read nothing. This behavior is
> different from the primitive semantics.

What is "the primitive semantics"?  Does this refer to behaviour prior
to f608788cd2d6?

> The correct way is find the first vmap_are that bigger than target addr,
> that is what function find_vmap_area_exceed_addr does.

Is this problem observable from userspace?  If so, what are the effects
and what must an application do to trigger it?