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Date: Sun, 2 Aug 2020 11:07:16 +0300
From: Dmitry Samersoff <dms@...ersoff.net>
To: musl@...ts.openwall.com, Ariadne Conill <ariadne@...eferenced.org>
Subject: Re: [PATCH v3] implement recallocarray(3)

Ariadne,

BSD (jemalloc) realloc always perform a new allocation and may return 
completely new pointer ever if new_size <= old_size.

Also contract for realloc says that if allocation fails original content 
should remain untouched.

I don't know how musl malloc perform in this case, but it might be 
better to move all memset after realloc and use newptr as a memeset 
base, with an appropriate error checking.

-Dmitry


On 02.08.2020 0:42, Ariadne Conill wrote:
> This OpenBSD extension is similar to reallocarray(3), but
> zero-initializes the new memory area.
> 
> This extension is placed in _BSD_SOURCE, like
> reallocarray(3).
> 
> Changes from v2:
> - drop overflow checking for old size
> 
> Changes from v1:
> - use realloc() instead of reallocarray()
> ---
>   include/stdlib.h           |  1 +
>   src/malloc/recallocarray.c | 27 +++++++++++++++++++++++++++
>   2 files changed, 28 insertions(+)
>   create mode 100644 src/malloc/recallocarray.c
> 
> diff --git a/include/stdlib.h b/include/stdlib.h
> index b54a051f..a0412ad4 100644
> --- a/include/stdlib.h
> +++ b/include/stdlib.h
> @@ -146,6 +146,7 @@ int clearenv(void);
>   #define WCOREDUMP(s) ((s) & 0x80)
>   #define WIFCONTINUED(s) ((s) == 0xffff)
>   void *reallocarray (void *, size_t, size_t);
> +void *recallocarray (void *, size_t, size_t, size_t);
>   #endif
>   
>   #ifdef _GNU_SOURCE
> diff --git a/src/malloc/recallocarray.c b/src/malloc/recallocarray.c
> new file mode 100644
> index 00000000..a7827604
> --- /dev/null
> +++ b/src/malloc/recallocarray.c
> @@ -0,0 +1,27 @@
> +#define _BSD_SOURCE
> +#include <errno.h>
> +#include <stdlib.h>
> +#include <string.h>
> +
> +void *recallocarray(void *ptr, size_t om, size_t m, size_t n)
> +{
> +	void *newptr;
> +	size_t old_size = om * n, new_size;
> +
> +	if (n && m > -1 / n) {
> +		errno = ENOMEM;
> +		return 0;
> +	}
> +	new_size = m * n;
> +
> +	if (new_size <= old_size) {
> +		memset((char *) ptr + new_size, 0, old_size - new_size);
> +	}
> +
> +	newptr = realloc(ptr, m * n);
> +	if (new_size > old_size) {
> +		memset((char *) ptr + old_size, 0, new_size - old_size);
> +	}
> +
> +	return newptr;
> +}
> 

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