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Date: Fri, 1 Jun 2018 15:50:57 +0200
From: Peter Rosin <peda@...ntia.se>
To: Kees Cook <keescook@...omium.org>
Cc: Matthew Wilcox <mawilcox@...rosoft.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Rasmus Villemoes <linux@...musvillemoes.dk>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v3 00/16] Provide saturating helpers for allocation

Kees Cook wrote:
> This is a stab at providing three new helpers for allocation size
> calculation:
> 
> struct_size(), array_size(), and array3_size().
> 
> These are implemented on top of Rasmus's overflow checking functions. The
> existing allocators are adjusted to use the more efficient overflow
> checks as well.
> 
> While the tree-wide conversions continue to be largely unchanged,
> I've updated their commit logs a bit with some more details on
> rationale and options. Notably, while there are NO plans to replace
> kmalloc_array() and kcalloc() with kmalloc(array_size(...),...) and
> kzalloc(array_size(...),...), the treewide conversions only add the
> new helpers, as making the ..._array() and ...calloc() conversions
> balloons the Coccinelle script terribly (I haven't found a way to
> make the replacement function name depend on the matched regular expression).
> So, while nothing does:
>     kmalloc_array(a, b, ...) -> kmalloc(array_size(a, b), ...)
> the treewide changes DO perform changes like this:
>     kmalloc(a * b, ...) -> kmalloc(array_size(a, b), ...)
> 
> It should also be noted that the treewide changes overlap with a few
> recently reported "real" overflows, so these aren't theoretical fixes.
> 
> At the very least, I'd like to get the helpers and self-test landed in
> the v4.18 merge window (coming right up!) since those are relatively
> self-contained. If the treewide changes need adjustment we've got,
> in theory, through the end of -rc2 to land those.

In some places you make an effort to have the count as the first
argument, e.g. in "treewide: Use array_size() for kmalloc()-family"

-	kbuf = kmalloc(sizeof(*kbuf) * maxevents, GFP_KERNEL);
+	kbuf = kmalloc(array_size(maxevents, sizeof(*kbuf)), GFP_KERNEL);

which is reordered, and from the same patch

-	mapping->bitmaps = kzalloc(extensions * sizeof(unsigned long *),
-				GFP_KERNEL);
+	mapping->bitmaps = kzalloc(array_size(extensions, sizeof(unsigned long *)),
+				   GFP_KERNEL);

which is not reordered. That is all fine by me.

But then, in "treewide: Use array_size() for devm_*alloc()-like, leftovers"
this reordering thing is not happening, e.g.

 	values = devm_kzalloc(&pdev->dev,
-			      sizeof(*mux->data.values) * mux->data.n_values,
+			      array_size(sizeof(*mux->data.values), mux->data.n_values),
 			      GFP_KERNEL);

Also, the above shows two of numerous examples of the tools breaking the
80 column "rule", even though the surrounding code makes decent effort to
uphold it.

I can see why these things happen, but they are annoying.

Cheers,
Peter

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