linux-kernel - Re: [PATCH] zsmalloc: calling zs_map

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Date:   Tue, 26 Sep 2017 17:26:32 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] zsmalloc: calling zs_map_object() from irq is a bug

On Wed, 20 Sep 2017 15:47:39 +0900 Minchan Kim <minchan@...nel.org> wrote:

> On Wed, Sep 20, 2017 at 03:39:41PM +0900, Sergey Senozhatsky wrote:
> > Use BUG_ON(in_interrupt()) in zs_map_object(). Calling this
> > function from IRQ is a bug, because we use per-CPU mappings
> > and interrupt may corrupt those buffers.
> > 
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> 
> IMHO, corruption the buffer would be not enough to be a BUG_ON
> which stop the system fully so user loses any chances to shut
> down smooth/hunt it down.
> 
> More serious thing of our case is that it can leak other user's
> data by overwriting, which is more concern I am thinking now.
> 

In an off-list email Linus has basically stated that all new uses of
BUG_ON should include words in the changelog or code comments
explaining why it is justifiable to kill the machine when the condition
triggers.

So can we please have a v2 with the appropriate comment?