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

[Chen Wandun]

在 2021/7/15 7:29, Andrew Morton 写道:
> On Wed, 14 Jul 2021 09:59:59 +0800 Chen Wandun <chenwandun@xxxxxxxxxx> 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?

If address is smaller than the leftmost node in vmap_area_root and

the read size is big enough when reading [addr, addr + count),

"the primitive semantics" will fill holes by zero and copy valid vmap_area.

In such scenario,  f608788cd2d6 will read noting.

> > 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?

Reading /proc/kcore will be affected.

Thanks,

Wandun

> .