Quoting Al Viro <v...@zeniv.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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