linux-kernel - Re: [PATCH v1] firmware loader: introduce module parameter to customize fw search path

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Date:	Fri, 26 Oct 2012 16:57:13 +1100
From:	Kevin Easton <kevin@...rana.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ming Lei <ming.lei@...onical.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] firmware loader: introduce module parameter to
 customize fw search path

Quoting Al Viro <viro@...IV.linux.org.uk>:

> On Thu, Oct 25, 2012 at 08:38:25PM -0700, Linus Torvalds wrote:
>
>> It's valid to cast a non-const pointer to a const one. It's the
>> *other* way around that is invalid.
>>
>> So marking fw_path[] as having 'const char *' elements just means that
>> we won't be changing those elements through the fw_path[] array
>> (correct: we only read them). The fact that one of those same pointers
>> is then also available through a non-const pointer variable means that
>> they can change through *that* pointer, but that doesn't change the
>> fact that fw_path[] itself contains const pointers.
>>
>> Remember: in C, a "const pointer" does *not* mean that the thing it
>> points to cannot change. It only means that it cannot change through
>> *that* pointer.
>
> It's a bit trickier, unfortunately - pointer to pointer to const char
> and pointer to pointer to char do not mix.  Just for fun, try to constify
> envp and argv arguments of call_usermodehelper()...

That's because if it _was_ allowed, you could use it to silently  
launder the const away:

const char *c = "rodata";
char *x;
const char **y;

y = &x;
*y = c;

/* We now have (const char) values accessible through a (char *) pointer x */



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