Created attachment 343134 [details]
Proposed patch

Description of problem:

The code in Xorg xfs is not 64bit safe and crashed on 64bits platforms if client-limit is set to a greater value.

Version-Release number of selected component (if applicable):

Any xfs version (including current upstream)

How reproducible:

100% reproducible

Steps to Reproduce:
1. install and configure xfs on a x86_64 machine
2. Change the default value of client-limit to 100 in /etc/X11/fs/config

   client-limit = 100
   # no-listen = tcp

2. Stress the server with Xvfb (for example)

  while $(/bin/true); do for i in $(seq 1 50); do Xvfb -fp tcp/localhost:7100 :$i & done; sleep 10; for i in $(seq 1 50); do xlsfonts -display :$i & done;sleep 10; killall Xvfb; done

Actual results:

The xfs server will die withing seconds with a segfault.

Expected results:

The xfs server handle the load.

Additional info:

The crash occurs in WaitforSomething()

193     if (XFD_ANYSET(&clientsReadable)) {
194         ClientPtr   client;
195         int         conn;
196
197         if (current_time)       /* may not have been set */
198             current_time = GetTimeInMillis();
199         for (i = 0; i < howmany(XFD_SETSIZE, NFDBITS); i++) {
200             while (clientsReadable.fds_bits[i]) {
201                 curclient = ffs(clientsReadable.fds_bits[i]) - 1;
202                 conn = ConnectionTranslation[curclient + (i << 5)];
203                 clientsReadable.fds_bits[i] &= ~(((fd_mask)1L) << curclient);
204                 client = clients[conn];
205                 if (!client)
206                     continue;
207                 pClientsReady[nready++] = conn;
208                 client->last_request_time = current_time;
209                 client->clientGone = CLIENT_ALIVE;
210             }
211         }
212     }

For two reasons:

1. fds_bits is a long on 64bit, so need to use ffsl() instead of ffs()
2. curclient + (i << 5) is not 64bit safe

Proposed patch attached. 

Note:

1. This is follow up of bug #464619 (there was more than one bug in xfs)
2. I already discussed the issue and the patch with krh on irc.
3. This bug seems to be present in el5 also, Fedora and current git upstream.