libfdt: fdt_path_offset_namelen: Reject empty path

Reject empty paths and negative lengths, according to the DT spec v0.4:

    The convention for specifying a device path is:
        /node-name-1/node-name-2/node-name-N

    The path to the root node is /.

This prevents the access to path[0] from ever being out-of-bounds.

Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
Message-ID: <20231010092822.qo2nxc3g47t26dqs@google.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_ro.c

@@ -255,6 +255,9 @@ int fdt_path_offset_namelen(const void *fdt, const char *path, int namelen)
 
 	FDT_RO_PROBE(fdt);
 
+	if (!can_assume(VALID_INPUT) && namelen <= 0)
+		return -FDT_ERR_BADPATH;
+
 	/* see if we have an alias */
 	if (*path != '/') {
 		const char *q = memchr(path, '/', end - p);

tests/path_offset.c

@@ -48,10 +48,13 @@ static void check_path_offset(void *fdt, const char *path, int offset)
 	verbose_printf("Checking offset of \"%s\" is %d...\n", path, offset);
 
 	rc = fdt_path_offset(fdt, path);
+	if (rc == offset)
+		return;
+
 	if (rc < 0)
 		FAIL("fdt_path_offset(\"%s\") failed: %s",
 		     path,  fdt_strerror(rc));
-	if (rc != offset)
+	else
 		FAIL("fdt_path_offset(\"%s\") returned incorrect offset"
 		     " %d instead of %d", path, rc, offset);
 }
@@ -102,6 +105,7 @@ int main(int argc, char *argv[])
 	check_path_offset(fdt, "/subnode@2/subsubnode", subsubnode2_offset2);
 
 	/* Test paths with extraneous separators */
+	check_path_offset(fdt, "", -FDT_ERR_BADPATH);
 	check_path_offset(fdt, "//", 0);
 	check_path_offset(fdt, "///", 0);
 	check_path_offset(fdt, "//subnode@1", subnode1_offset);
@@ -110,6 +114,8 @@ int main(int argc, char *argv[])
 	check_path_offset(fdt, "/subnode@2////subsubnode", subsubnode2_offset2);
 
 	/* Test fdt_path_offset_namelen() */
+	check_path_offset_namelen(fdt, "/subnode@1", -1, -FDT_ERR_BADPATH);
+	check_path_offset_namelen(fdt, "/subnode@1", 0, -FDT_ERR_BADPATH);
 	check_path_offset_namelen(fdt, "/subnode@1", 1, 0);
 	check_path_offset_namelen(fdt, "/subnode@1/subsubnode", 10, subnode1_offset);
 	check_path_offset_namelen(fdt, "/subnode@1/subsubnode", 11, subnode1_offset);