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

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Date:	Wed, 9 Dec 2009 00:47:34 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Emese Revfy <re.emese@...il.com>
Cc:	Alexey Dobriyan <adobriyan@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 28/31] Constify struct super_operations for 2.6.32 v1

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.
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