Date:   Wed, 2 Mar 2022 10:29:31 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Laight <David.Laight@...lab.com>
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Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop body
 as a ptr

On 02/03/2022 00.55, Linus Torvalds wrote:
> On Tue, Mar 1, 2022 at 3:19 PM David Laight <David.Laight@...lab.com> wrote:
>>

> With the "don't use iterator outside the loop" approach, the exact
> same code works in both the old world order and the new world order,
> and you don't have the semantic confusion. And *if* you try to use the
> iterator outside the loop, you'll _mostly_ (*) get a compiler warning
> about it not being initialized.
> 
>              Linus
> 
> (*) Unless somebody initializes the iterator pointer pointlessly.
> Which clearly does happen. Thus the "mostly". It's not perfect, and
> that's most definitely not nice - but it should at least hopefully
> make it that much harder to mess up.

This won't help the current issue (because it doesn't exist and might
never), but just in case some compiler people are listening, I'd like to
have some sort of way to tell the compiler "treat this variable as
uninitialized from here on". So one could do

#define kfree(p) do { __kfree(p); __magic_uninit(p); } while (0)

with __magic_uninit being a magic no-op that doesn't affect the
semantics of the code, but could be used by the compiler's "[is/may be]
used uninitialized" machinery to flag e.g. double frees on some odd
error path etc. It would probably only work for local automatic
variables, but it should be possible to just ignore the hint if p is
some expression like foo->bar or has side effects. If we had that, the
end-of-loop test could include that to "uninitialize" the iterator.

Maybe sparse/smatch or some other static analyzer could implement such a
magic thing? Maybe it's better as a function attribute
[__attribute__((uninitializes(1)))] to avoid having to macrofy all
functions that release resources.

Rasmus

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