linux-kernel - Re: [PATCH 28/31] Constify struct super

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Date:	Thu, 10 Dec 2009 19:24:21 +0100
From:	Emese Revfy <re.emese@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org, ralf@...ux-mips.org
Subject: Re: [PATCH 28/31] Constify struct super_operations for 2.6.32 v1

Al Viro wrote:
> On Wed, Dec 09, 2009 at 01:24:34AM +0100, Emese Revfy wrote:
> 
>> If constifying the function pointer fields reduces readability,
>> what would you say for turning then into typedefs, something like this:
>>
>> typedef int (* super_ops_statfs) (struct dentry *, struct kstatfs *);
>> struct super_operations {
>> ...
>> 	const super_ops_statfs statfs;
>> ...
>> };
> 
> Even worse, since one has to go back to typedef to figure out WTF is
> going on.
>  
>>> Moreover, you *still* are not
>>> covering the real policy - these suckers should be statically allocated,
>>> not just never modified.
>> If the super ops are allocated on the stack then they will be overwritten
>> during later syscalls and will eventually crash the system on a future
>> dereference, that is, this kind of problem manifests during development.
>>
>> If the super ops are allocated by kmalloc/etc, then they will have to be
>> explicitly initialised by writing to specific fields, my patch would prevent
>> that.
>>
>> So in the end the programmer is forced to allocate and initialise super ops
>> statically.
> 
> ... unless they go ahead and use memcpy(), etc.
> 
> What you really want is
> 	* no conversions to any other pointer types for pointers to it
> and to any aggregate types containing it
> 	* no conversions from any other pointer types for the same set of
> types
> 	* all objects of that type have static storage duration
> 	* no lvalues of that type are modifiable
> 
> Which is not a job for C compiler.  Yes, (4) means that memcpy() et.al.
> give undefined behaviour.  And you get fsck-all satisfaction from knowing
> that, since C compiler is not going to warn you about it.  sparse might,
> if we teach it to do so.  Preferably - with minimal intrusiveness of
> syntax being used.

I think, all these instruments (constification, sparse, etc.) are not 
for preventing a programmer from circumventing the policy (that's impossible),
but to make it easy for the reviewer to notice it when he does so.
My patch achieves this in a very simple way for the currently uncovered case of dynamically
allocated ops structures.
--
Emese
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