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Message-ID: <20160724192107.GO15995@brightrain.aerifal.cx>
Date: Sun, 24 Jul 2016 15:21:07 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: dirname() / basename() - musl vs FreeBSD and OpenBSD
On Sun, Jul 24, 2016 at 09:12:49PM +0200, Daniel Cegiełka wrote:
> Hi,
>
> I came across a very strange problem when I ports code from OpenBSD to
> musl-libc, and it seems, that a lot of problems can be caused by
> dirname().
>
> http://pubs.opengroup.org/onlinepubs/009695399/functions/dirname.html
>
> "The dirname() function >>> may <<< modify the string pointed to by
> path, and may return a pointer to static storage that may then be
> overwritten by subsequent calls to dirname()."
There is actually no other option for implementing this function. The
contract does not require that the argument string be a valid pathname
or have a bounded length like PATH_MAX; it operates on general
strings. And "No errors are defined", so failure is not an option. The
only way to implement this function is for it to modify its argument.
> OpenBSD and FreeBSD:
>
> http://man.openbsd.org/OpenBSD-5.8/dirname.3
> https://www.freebsd.org/cgi/man.cgi?query=dirname&sektion=3
>
> "dirname() returns a pointer to internal static storage space that
> will be overwritten by subsequent calls (each function has its own
> separate storage).
>
> Other vendor implementations of dirname() may modify the contents of
> the string passed to dirname(); this should be taken into account when
> writing code which calls this function if portability is desired."
>
> NetBSD:
>
> http://netbsd.gw.com/cgi-bin/man-cgi?dirname+3+NetBSD-7.0
>
> "BUGS
> (...)
> The dirname() function returns a pointer to static storage that
> may be overwritten by subse-
> quent calls to dirname(). This is not strictly a bug; it is
> explicitly allowed by IEEE Std
> 1003.1-2001 (``POSIX.1'')."
It is a bug because it necessarily returns wrong results for extremely
long strings.
> so:
>
> #include <libgen.h> /* musl libc dirname() */
> #include <stdio.h>
>
> int main()
> {
> char s1[] = "/usr/lib/";
> char s2[] = "/usr/lib/";
> char *p1, *p2;
>
> p1 = dirname(s1);
> p2 = openbsd_dirname(s2);
>
> printf("musl: s1: %s, p1: %s\n", s1, p1);
> printf("openbsd_dirname: s2: %s, p2: %s\n", s2, p2);
> return 0;
> }
>
> # ./a.out
> musl: s1: /usr, p1: /usr
> openbsd_dirname: s2: /usr/lib/, p2: /usr
>
> So if you use the code from OpenBSD or FreeBSD, then you should be
> very careful... grep, sed, patch, diff... etc. everything is
> potentially error prone.
>
> musl has very good support for code from *BSD, so is the ability that
> dirname() in musl does not overwrite argument of the function? It will
> not change anything in relation to the IEEE Std 1003.1-2001, but it
> will be much safer for the code from FreeBSD and OpenBSD.
Not an option, for the above reason.
> btw. the same problem applies to basename():
>
> http://netbsd.gw.com/cgi-bin/man-cgi?basename+3+NetBSD-7.0
>
> http://man.openbsd.org/OpenBSD-5.8/man3/basename.3
>
> https://www.freebsd.org/cgi/man.cgi?query=basename&apropos=0&sektion=3&manpath=FreeBSD+10.3-RELEASE+and+Ports&arch=default&format=html
Same applies there.
Rich
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