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Date: Tue, 19 Sep 2017 19:15:11 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, kernel-team <kernel-team@lge.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [PATCH] zram: fix null dereference of handle
Message-ID: <20170919100941.GA401@jagdpanzerIV.localdomain>
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 <20170919065924.GA29971@bbox>
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Hi Minchan,

On (09/19/17 15:59), Minchan Kim wrote:
[..]
> > another question, "!handle  ==  value & ZRAM_SAME"? if so, then why not
> > just check for `flags & ZRAM_SAME'? if not then:
> > 
> > -  for `value & ZRAM_SAME' you fill the page with zram_get_element(zram, index)
> >    and return 0. ok.
> > 
> > -  for !handle.... you also fill the page with zram_get_element(zram, index)
> >    and return 0. is this ok? shouldn't !handle return error in this case?
> 
> We discussed it before that we shouldn't return error.
> Userspace can ask reading unallocated buffer freely.

ok, so this is intentional behaviour.

> And in this case, it fills the buffer zero because handle and element is unified.
> However, if your concern is readability, I will make it more explict.

correct.
... but I thought that we would also return an error.

> > I really suspect that there are some paths that can lead to !handle
> > entry, that will not be ZRAM_SAME. e.g. error return from compression
> > path.
> 
> Could you be more specific?

I just meant that there are error paths in zram write, which will leave
us both with !handle entries and !ZRAM_SAME. but it seems that this is
the intentional behaviour.

	-ss