[Cocci] [Fwd: Re: [PATCH] rtc: sun6i: Use struct_size() in kzalloc()]

Julia Lawall julia.lawall at lip6.fr
Fri Aug 24 00:13:32 CEST 2018



On Thu, 23 Aug 2018, Kees Cook wrote:

> On Thu, Aug 23, 2018 at 3:00 PM, Julia Lawall <julia.lawall at lip6.fr> wrote:
> >
> >
> > On Thu, 23 Aug 2018, Kees Cook wrote:
> >
> >> On Thu, Aug 23, 2018 at 2:48 PM, Joe Perches <joe at perches.com> wrote:
> >> > Forwarding a question about coccinelle and isomorphisms from Kees Cook
> >> >
> >> > ---------- Forwarded message ----------
> >> > From: Kees Cook <keescook at chromium.org>
> >> > To: "Gustavo A. R. Silva" <gustavo at embeddedor.com>
> >> > Cc: Alessandro Zummo <a.zummo at towertech.it>, Alexandre Belloni <alexandre.belloni at bootlin.com>, Maxime Ripard <maxime.ripard at bootlin.com>, Chen-Yu Tsai <wens at csie.org>, linux-rtc at vger.kernel.org, linux-arm-kernel <linux-arm-kernel at lists.infradead.org>, LKML <linux-kernel at vger.kernel.org>
> >> > Bcc:
> >> > Date: Thu, 23 Aug 2018 13:56:35 -0700
> >> > Subject: Re: [PATCH] rtc: sun6i: Use struct_size() in kzalloc()
> >> > On Thu, Aug 23, 2018 at 11:51 AM, Gustavo A. R. Silva
> >> > <gustavo at embeddedor.com> wrote:
> >> >> One of the more common cases of allocation size calculations is finding
> >> >> the size of a structure that has a zero-sized array at the end, along
> >> >> with memory for some number of elements for that array. For example:
> >> >>
> >> >> struct foo {
> >> >>         int stuff;
> >> >>         void *entry[];
> >> >> };
> >> >>
> >> >> instance = kzalloc(sizeof(struct foo) + sizeof(void *) * count, GFP_KERNEL);
> >> >>
> >> >> Instead of leaving these open-coded and prone to type mistakes, we can
> >> >> now use the new struct_size() helper:
> >> >>
> >> >> instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL);
> >> >>
> >> >> Signed-off-by: Gustavo A. R. Silva <gustavo at embeddedor.com>
> >> >> ---
> >> >>  drivers/rtc/rtc-sun6i.c | 3 +--
> >> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >> >>
> >> >> diff --git a/drivers/rtc/rtc-sun6i.c b/drivers/rtc/rtc-sun6i.c
> >> >> index 2cd5a7b..fe07310 100644
> >> >> --- a/drivers/rtc/rtc-sun6i.c
> >> >> +++ b/drivers/rtc/rtc-sun6i.c
> >> >> @@ -199,8 +199,7 @@ static void __init sun6i_rtc_clk_init(struct device_node *node)
> >> >>         if (!rtc)
> >> >>                 return;
> >> >>
> >> >> -       clk_data = kzalloc(sizeof(*clk_data) + (sizeof(*clk_data->hws) * 2),
> >> >> -                          GFP_KERNEL);
> >> >> +       clk_data = kzalloc(struct_size(clk_data, hws, 2), GFP_KERNEL);
> >> >>         if (!clk_data) {
> >> >>                 kfree(rtc);
> >> >>                 return;
> >> >
> >> > This looks like entirely correct to me, but I'm surprised the
> >> > Coccinelle script didn't discover this. I guess the isomorphisms don't
> >> > cover the parenthesis?
> >>
> >> I had this:
> >>
> >> @@
> >> identifier alloc =~
> >> "kmalloc|kzalloc|kmalloc_node|kzalloc_node|vmalloc|vzalloc|kvmalloc|kvzalloc";
> >> identifier VAR, ELEMENT;
> >> expression COUNT;
> >> @@
> >>
> >> - alloc(sizeof(*VAR) + COUNT * sizeof(*VAR->ELEMENT)
> >> + alloc(struct_size(VAR, ELEMENT, COUNT)
> >>   , ...)
> >>
> >> But I needed to explicitly change the rule to:
> >>
> >> (
> >> - alloc(sizeof(*VAR) + COUNT * sizeof(*VAR->ELEMENT)
> >> + alloc(struct_size(VAR, ELEMENT, COUNT)
> >>   , ...)
> >> |
> >> - alloc(sizeof(*VAR) + (COUNT * sizeof(*VAR->ELEMENT))
> >> + alloc(struct_size(VAR, ELEMENT, COUNT)
> >>   , ...)
> >> )
> >>
> >> to add the ()s. I would expect arithmetic commutative expressions to
> >> match? But I had to add parens?
> >
> > Isomorphisms don't add parens.  They only remove them.  If they added
> > them, you would end up with the possibility of having them everywhere, in
> > all permutations, which would be slow and useless.
>
> Would a rule for:
>
> a + (b * c)
>
> match:
>
> a + b * c

I would say yes.  Basically it removes the parentheses but doesn't reparse
the code, so it doesn't redo the associativity.  Since * has higher
precedence than +, then both will be matched.  On the other hand, if you
put:

(a + b) * c

It will consider a pattern with the parentheses removed, but that pattern
won't match anything, because real trees that consist of a + b * c are
parsed in a different way.

julia


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