[musl] [PATCH 1/1] reallocarray casting overflow check explicitally

[musl] [PATCH 1/1] reallocarray casting overflow check explicitally

[David CARLIER]

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From 04e3caa580cfe501ad00d85d63040de329962823 Mon Sep 17 00:00:00 2001
From: David Carlier <devnexen@gmail.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) {
 		errno = ENOMEM;
 		return 0;
 	}
-- 
2.20.1

Re: [musl] [PATCH 1/1] reallocarray casting overflow check explicitally

[Szabolcs Nagy]

* David CARLIER <devnexen@gmail.com> [2021-05-10 20:48:26 +0100]:
> From 04e3caa580cfe501ad00d85d63040de329962823 Mon Sep 17 00:00:00 2001
> From: David Carlier <devnexen@gmail.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.

>  		errno = ENOMEM;
>  		return 0;
>  	}
> -- 
> 2.20.1
> 

Re: [musl] [PATCH 1/1] reallocarray casting overflow check explicitally

[Rich Felker]

On Tue, May 11, 2021 at 09:33:06AM +0200, Szabolcs Nagy wrote:
> * David CARLIER <devnexen@gmail.com> [2021-05-10 20:48:26 +0100]:
> > From 04e3caa580cfe501ad00d85d63040de329962823 Mon Sep 17 00:00:00 2001
> > From: David Carlier <devnexen@gmail.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