Re: [PATCH] fix strdupa evaulating expression twice

[Rich Felker <dalias@libc.org>]

On Sun, Aug 28, 2016 at 10:59:26AM +0000, Noam Meltzer wrote:
> 

> From bf23b7b8fd39eaca6a05173eaf543e1bce3319ab Mon Sep 17 00:00:00 2001
> From: Noam Meltzer <tsnoam@gmail.com>
> Date: Sun, 28 Aug 2016 13:53:24 +0300
> Subject: [PATCH] fix strdupa evaulating expression twice
> 
> calling strdupa with va_arg as its expression caused unexpected
> behaviour. now the expression is evaulated only once.
> ---
>  include/string.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/string.h b/include/string.h
> index ff9badb..976faaf 100644
> --- a/include/string.h
> +++ b/include/string.h
> @@ -85,7 +85,10 @@ size_t strlcpy (char *, const char *, size_t);
>  #endif
>  
>  #ifdef _GNU_SOURCE
> -#define	strdupa(x)	strcpy(alloca(strlen(x)+1),x)
> +#define	strdupa(x) (__extension__ ({ \
> +		const char *__xval = x; \
> +		strcpy(alloca(strlen(__xval)+1),__xval); \
> +	}))
>  int strverscmp (const char *, const char *);
>  int strcasecmp_l (const char *, const char *, locale_t);
>  int strncasecmp_l (const char *, const char *, size_t, locale_t);

The intent of the form as written is to be actual C (modulo use of
alloca) rather than "GNU C". Aside from that, strdupa is essentially
always-unsafe and should probably be removed or at least made into a
warning...

Rich