Subject: if_attach problem
To: None <>
From: None <>
List: tech-kern
Date: 11/10/2003 15:08:11
Hi ,

I faced crash in NetBSD kernel while creating and deleting interfaces when
i executed a script which creates/destroy  interfaces in succession..(say
create 128 vlan interfaces and delete 128 vlan interfaces)

I am using Netbsd version 1.6

On debugging I came across an issue in if_attach function...
Atleast preliminary study leads me to this conclusion ..

Issue with if_attach function..
It is related to extendible array approach for if ifindex2ifnet global
variable which stores pointer to ifnet structure for a particular
After fixed number of interfaces, this ifindex2ifnet array of pointers is
malloc for double number of interfaces..

see following line of code from if.c
      if (ifindex2ifnet == 0)
            while (ifindex2ifnet[ifp->if_index] != NULL) {              <--- issue at this line
                  if (if_index == 0)
                        if_index = 1;
                   * If we hit USHRT_MAX, we skip back to 0 since
                   * there are a number of places where the value
                   * of if_index or if_index itself is compared
                   * to to or stored in an unsigned short.  By
                   * jumping back, we won't botch those assignments
                   * or comparisons.
                  else if (if_index == USHRT_MAX) {
                         * However, if we have to jump back to
                         * zero *twice* without finding an empty
                         * slot in ifindex2ifnet[], then there
                         * there are too many (>65535) interfaces.
                        if (indexlim++)
                              panic("too many interfaces");
                              if_index = 1;
                  ifp->if_index = if_index;

--> crash occurs at line "while (ifindex2ifnet[ifp->if_index] != NULL) {
when ifp->if_index value is 1024 and total number of records malloc are
--> In this case index of array becomes 1024 while limit should have been
from 0 to 1023..
--> Consider iteration when before entering while loop value of ifindex is
1023.After entering if_index is incremented, and ine the end assigned to
ifp->if_index, In while statement, not-malloc area is accessed..
--> Actually this should cause problem everytime, when pointer for last
ifnet interface is to be updated ..say when array ifindex2ifnet is to be
increase dynamically from 8 records to 16 , 16 to 32 and so on..
--> But it gave problem to me only in case of 1024th record ..May be due to
some boundary condition related to paging.. (4 k memory = 1024 * 4 bytes)
-->Interesting thing is that this problem occurs rarely under normal if you try to create/delte 10000 interfaces also in any
existing netbsd won't give error/crash. But by looking at code
this problem seems logical..

Just add following check before while loop ends..


             ifp->if_index = if_index;
                       /* Change starts */
                        if(ifp->if_index == if_indexlim)
                       /* Change ends */

Just want you feedback on this issue..If I have missed anything

Thanks in advance,
Manish Aggarwal

"DISCLAIMER: This message is proprietary to Hughes Software Systems Limited
(HSS) and is intended solely for the use of the individual to whom it is
addressed. It may contain  privileged or confidential information and
should not be circulated or used for any purpose other than for what it is
intended. If you have received this message in error, please notify the
originator immediately. If you are not the intended recipient, you are
notified that you are strictly prohibited from using, copying, altering, or
disclosing the contents of this message. HSS accepts no responsibility for
loss or damage arising from the use of the information transmitted by this
email including damage from virus."