sort: another mkstemp use case

[Tobias Stoeckmann]

When creating a new temporary file name, use mkstemp instead of just
taking a rather predictable path, which could even be a symlink by
a malicious user (granted, that is very unlikely).

Index: file.c
===================================================================
RCS file: /cvs/src/usr.bin/sort/file.c,v
retrieving revision 1.6
diff -u -p -r1.6 file.c
--- file.c      1 Apr 2015 19:06:18 -0000       1.6
+++ file.c      1 Apr 2015 19:48:25 -0000
@@ -167,12 +167,13 @@ file_is_tmp(const char *fn)
 char *
 new_tmp_file_name(void)
 {
-       static size_t tfcounter = 0;
-       static const char *fn = ".bsdsort.";
        char *ret;
+       int fd;
 
-       sort_asprintf(&ret, "%s/%s%d.%lu", tmpdir, fn, (int)getpid(),
-           (unsigned long)(tfcounter++));
+       sort_asprintf(&ret, "%s/.bsdsort.XXXXXXXXXX", tmpdir);
+       if ((fd = mkstemp(ret)) == -1)
+               err(2, "%s", ret);
+       close(fd);
        tmp_file_atexit(ret);
        return ret;
 }