linux-kernel - Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

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Date:   Mon, 28 Feb 2022 11:56:07 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christian König <christian.koenig@....com>
Cc:     Jakob Koschel <jakobkoschel@...il.com>,
        alsa-devel@...a-project.org, linux-aspeed@...ts.ozlabs.org,
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Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop body
 as a ptr

On Mon, Feb 28, 2022 at 4:19 AM Christian König
<christian.koenig@....com> wrote:
>
> I don't think that using the extra variable makes the code in any way
> more reliable or easier to read.

So I think the next step is to do the attached patch (which requires
that "-std=gnu11" that was discussed in the original thread).

That will guarantee that the 'pos' parameter of list_for_each_entry()
is only updated INSIDE the for_each_list_entry() loop, and can never
point to the (wrongly typed) head entry.

And I would actually hope that it should actually cause compiler
warnings about possibly uninitialized variables if people then use the
'pos' pointer outside the loop. Except

 (a) that code in sgx/encl.c currently initializes 'tmp' to NULL for
inexplicable reasons - possibly because it already expected this
behavior

 (b) when I remove that NULL initializer, I still don't get a warning,
because we've disabled -Wno-maybe-uninitialized since it results in so
many false positives.

Oh well.

Anyway, give this patch a look, and at least if it's expanded to do
"(pos) = NULL" in the entry statement for the for-loop, it will avoid
the HEAD type confusion that Jakob is working on. And I think in a
cleaner way than the horrid games he plays.

(But it won't avoid possible CPU speculation of such type confusion.
That, in my opinion, is a completely different issue)

I do wish we could actually poison the 'pos' value after the loop
somehow - but clearly the "might be uninitialized" I was hoping for
isn't the way to do it.

Anybody have any ideas?

                Linus

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