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Message-ID: <20210511130352.GI2546@brightrain.aerifal.cx>
Date: Tue, 11 May 2021 09:03:53 -0400
From: Rich Felker <dalias@...c.org>
To: David CARLIER <devnexen@...il.com>, g@...t70.net,
musl@...ts.openwall.com
Subject: Re: [PATCH 1/1] reallocarray casting overflow check
explicitally
On Tue, May 11, 2021 at 09:33:06AM +0200, Szabolcs Nagy wrote:
> * David CARLIER <devnexen@...il.com> [2021-05-10 20:48:26 +0100]:
> > From 04e3caa580cfe501ad00d85d63040de329962823 Mon Sep 17 00:00:00 2001
> > From: David Carlier <devnexen@...il.com>
> > Date: Mon, 10 May 2021 20:45:12 +0100
> > Subject: [PATCH] reallocarray overflow check cast to size_t
> >
> > ---
> > src/malloc/reallocarray.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/src/malloc/reallocarray.c b/src/malloc/reallocarray.c
> > index 4a6ebe46..2556c917 100644
> > --- a/src/malloc/reallocarray.c
> > +++ b/src/malloc/reallocarray.c
> > @@ -4,7 +4,7 @@
> >
> > void *reallocarray(void *ptr, size_t m, size_t n)
> > {
> > - if (n && m > -1 / n) {
> > + if (n && m > (size_t)-1 / n) {
>
> you need to explain this change more i think.
>
> the usual arithmetic conversion rules of c already convert both
> operands of / to size_t, unless size_t has lower conversion rank
> than signed int, but such a system would have problems because
> size_t was always promoted to int so you would need a lot more
> casts in musl (and other software) to avoid signed int overflow
> and negative arithmetics.
I think the idea was just to make it explicit, but generally casting
is discouraged in musl. The corresponding code in calloc has a cast
but it's a historical artifact and contrary to the coding style used
throughout most of the source.
Rich
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