Subject: recvfrom! optional start and end parameter invalid
From: d4ryus <[email-masked]>
To: [email-masked]
Date: Fri, 1 Jan 2021 12:34:57 +0100
List-Id: <debbugs-submit.debbugs.gnu.org>

hi,

the parameter validation for the optional "start" and "end" arguments to
"recvfrom!" are off by one if "end" is passed. From libguile/socket.c
(master commit 64c89458e6):

  ...
  if (SCM_UNBNDP (end))
    cend = SCM_BYTEVECTOR_LENGTH (buf);
  else
    {
      cend = scm_to_size_t (end);
      if (SCM_UNLIKELY (cend >= SCM_BYTEVECTOR_LENGTH (buf)
                        || cend < offset))
        scm_out_of_range (FUNC_NAME, end);
    }
  ...

"end" is the optional end argument, "offset" is 0 or "start" if start
was given. The check must be:

  cend > SCM_BYTEVECTOR_LENGTH (buf) || cend <= offset

to allow filling the last byte in the buffer and verify that start is
not equal to end. A workaround to skip the validation is to not pass
end. But i think a better way would be to always validate start (and
end), if one (or both) of them are passed. A potentional fix is
attached.

If you need any additional information, please let me know.

Thank you for your great work!

-  d4ryus
diff --git a/libguile/socket.c b/libguile/socket.c
index 64354f1f1..d6e676744 100644
--- a/libguile/socket.c
+++ b/libguile/socket.c
@@ -1480,21 +1480,24 @@ SCM_DEFINE (scm_recvfrom, "recvfrom!", 2, 3, 0,
 
   SCM_VALIDATE_BYTEVECTOR (1, buf);
 
-  if (SCM_UNBNDP (start))
-    offset = 0;
-  else
-    offset = scm_to_size_t (start);
-
   if (SCM_UNBNDP (end))
     cend = SCM_BYTEVECTOR_LENGTH (buf);
   else
     {
       cend = scm_to_size_t (end);
-      if (SCM_UNLIKELY (cend >= SCM_BYTEVECTOR_LENGTH (buf)
-                        || cend < offset))
+      if (SCM_UNLIKELY (cend > SCM_BYTEVECTOR_LENGTH (buf)))
         scm_out_of_range (FUNC_NAME, end);
     }
 
+  if (SCM_UNBNDP (start))
+    offset = 0;
+  else
+    {
+      offset = scm_to_size_t (start);
+      if (SCM_UNLIKELY (cend <= offset))
+        scm_out_of_range (FUNC_NAME, start);
+    }
+
   SCM_SYSCALL (rv = recvfrom (fd,
                               SCM_BYTEVECTOR_CONTENTS (buf) + offset,
                               cend - offset, flg,