linux-kernel - Re: [PATCH] bcache: safeguard a dangerous addressing in closure

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Message-ID: <0b4be514-0ce8-efd9-4e17-8772729ac336@lyle.org>
Date:   Thu, 12 Oct 2017 10:44:47 -0700
From:   Michael Lyle <mlyle@...e.org>
To:     Liang Chen <liangchen.linux@...il.com>,
        linux-bcache@...r.kernel.org
Cc:     i@...y.li, kent.overstreet@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bcache: safeguard a dangerous addressing in closure_queue

On 10/12/2017 07:37 AM, Liang Chen wrote:
> The use of the union reduces the size of closure struct by taking advantage
> of the current size of its members. The offset of func in work_struct equals
> the size of the first three members, so that work.work_func will just
> reference the forth member - fn.
> 
> This is smart but dangerous. It can be broken if work_struct or the other
> structs get changed, and can be a bit difficult to debug.
> 
> Signed-off-by: Liang Chen <liangchen.linux@...il.com>

So the objective here is to make sure that struct work_struct and the
anonymous struct remain identical?  I agree that's a potential problem
for future maintenance.

Could we use BUILD_BUG_ON with offsets and sizes to do the same, to get
compile-time checking and avoid doing anything at runtime (I know the
compiler can usually omit the BUG but better to be safe)?  Otherwise a
kernel that triggered this problem would compile, and it'd only be if
someone actually used bcache that it would trigger.

Mike