segfault bb_make_directory + dirname with musl

[Denys Vlasenko]

On Wed, Nov 30, 2016 at 3:46 AM, Daniel Sabogal <dsabogalcc at gmail.com> wrote:
> The following commands cause busybox to segfault on musl-based systems.
>
> $ install -D a /
> $ install -D a /b
> $ install -D a /b/
>
> This happens because the code in
>
> https://git.busybox.net/busybox/tree/coreutils/install.c?h=1_25_1#n196
>
> passes the result of dirname() to bb_make_directory() which modifies its
> contents. For paths of the above forms, musl's dirname returns a string
> literal "/" which shouldn't be modified.
>
> See http://git.musl-libc.org/cgit/musl/tree/src/misc/dirname.c
>
> There are a few other occurrences of the code shown above, but I've not
> checked to see if they could be made to segfault.

Does this fix the problem?

                        /* Bypass leading non-'/'s and then subsequent '/'s */
                        while (*s) {
                                if (*s == '/') {
                                        do {
                                                ++s;
                                        } while (*s == '/');
                                        c = *s; /* Save the current char */
====added line==>                       if (c)
                                                *s = '\0'; /* and
replace it with nul */
                                        break;