uml: improved error handling while locating temp dir

* arch/um/os-Linux/mem.c (make_tempfile): Don't deref NULL upon failed malloc.

* arch/um/os-Linux/mem.c (make_tempfile): Handle NULL tempdir.
Don't let a long tempdir (e.g., via TMPDIR) provoke heap corruption.

[ jdike - formatting cleanups, deleted obsolete comment ]

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/os-Linux/mem.c

@@ -162,11 +162,6 @@ static void which_tmpdir(void)
 	goto out;
 }
 
-/*
- * This proc still used in tt-mode
- * (file: kernel/tt/ptproxy/proxy.c, proc: start_debugger).
- * So it isn't 'static' yet.
- */
 static int __init make_tempfile(const char *template, char **out_tempname,
 				int do_unlink)
 {
@@ -175,10 +170,13 @@ static int __init make_tempfile(const char *template, char **out_tempname,
 
 	which_tmpdir();
 	tempname = malloc(MAXPATHLEN);
-	if (!tempname)
-		goto out;
+	if (tempname == NULL)
+		return -1;
 
 	find_tempdir();
+	if ((tempdir == NULL) || (strlen(tempdir) >= MAXPATHLEN))
+		return -1;
+
 	if (template[0] != '/')
 		strcpy(tempname, tempdir);
 	else
@@ -196,9 +194,8 @@ static int __init make_tempfile(const char *template, char **out_tempname,
 	}
 	if (out_tempname) {
 		*out_tempname = tempname;
-	} else {
+	} else
 		free(tempname);
-	}
 	return fd;
 out:
 	free(tempname);