Re: musl & strndupa?

[Rich Felker <dalias@aerifal.cx>]

On Wed, Jan 01, 2014 at 07:42:47PM +0000, Raphael Cohn wrote:
> Hi,
> 
> I'm trying to compile 'audit' (aka libaudit, auditd, etc - from
> http://people.redhat.com/sgrubb/audit/index.html version 2.3.2). Using musl
> 0.9.14.
> 
> The file 'src/ausearch-lol.c' uses a reference to 'strndupa', which I
> presume is an alloca version of strndup, and presumably a _GNU_SOURCE
> feature. I can't seem to see a definition for it in musl, although strdupa
> exists in string.h (Indeed, http://linux.die.net/man/3/strdup suggests as
> much).
> 
> Is this intentional? If so, what would anyone suggest as a work around? My
> guess would be  #define strndupa(x, t) strncpy(alloca(strlen(x)+1),x,t)
> but I'd like a second opinion...

That's roughly the way to do it, but you need strnlen, not strlen, and
there are various other details like properly parenthesizing macro
arguments. In addition, there's no way to avoid multiple-evaluations
of arguments unless you use the GNU C statement-expressions extension.

It should be noted that almost any use of alloca is either a bug
(potentially exploitable stack overflow) or useless (because the size
is bounded and thus could/should just be replaced by a fixed-size
array). This is the main reason I've been hesitant to go to the
trouble of providing this and dealing with the multiple-evaluation or
#ifdef __GNUC__ issue -- really, any software using alloca (and by
extension, strdupa or strndupa) should be fixed.

Rich