Check for null bytes in filenames in 'expand-file-name' (bug#49723)

* src/fileio.c (expand-file-name): Check for null bytes for both
NAME and DEFAULT-DIRECTORY arguments.  Also check for null bytes
in buffer-local default-directory, assuming it is used.
* src/coding.c (encode_file_name): Use CHECK_STRING_NULL_BYTES.
* src/lisp.h (CHECK_STRING_NULL_BYTES): Add function for checking
for null bytes in Lisp strings.
* test/src/fileio-tests.el (fileio-test--expand-file-name-null-bytes):
Add test for new changes to expand-file-name.
* etc/NEWS: Announce changes.

3 files changed, 13 insertions, 3 deletions

diff --git a/src/coding.c b/src/coding.c
index d027c7d5399..7030a53869a 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -10430,8 +10430,7 @@ encode_file_name (Lisp_Object fname)
      cause subtle bugs because the system would silently use a
      different filename than expected.  Perform this check after
      encoding to not miss NUL bytes introduced through encoding.  */
-  CHECK_TYPE (memchr (SSDATA (encoded), '\0', SBYTES (encoded)) == NULL,
-              Qfilenamep, fname);
+  CHECK_STRING_NULL_BYTES (encoded);
   return encoded;
 }
 
diff --git a/src/fileio.c b/src/fileio.c
index 0db8ed793b3..3c13d3fe416 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -945,6 +945,7 @@ the root directory.  */)
   USE_SAFE_ALLOCA;
 
   CHECK_STRING (name);
+  CHECK_STRING_NULL_BYTES (name);
 
   /* If the file name has special constructs in it,
      call the corresponding file name handler.  */
@@ -993,7 +994,10 @@ the root directory.  */)
       if (STRINGP (dir))
         {
           if (file_name_absolute_no_tilde_p (dir))
-            default_directory = dir;
+            {
+              CHECK_STRING_NULL_BYTES (dir);
+              default_directory = dir;
+            }
           else
             {
               Lisp_Object absdir
diff --git a/src/lisp.h b/src/lisp.h
index 7bfc69b647b..9716b34baee 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -1615,6 +1615,13 @@ STRING_SET_CHARS (Lisp_Object string, ptrdiff_t newsize)
   XSTRING (string)->u.s.size = newsize;
 }
 
+INLINE void
+CHECK_STRING_NULL_BYTES (Lisp_Object string)
+{
+  CHECK_TYPE (memchr (SSDATA (string), '\0', SBYTES (string)) == NULL,
+              Qfilenamep, string);
+}
+
 /* A regular vector is just a header plus an array of Lisp_Objects.  */
 
 struct Lisp_Vector