diff options
authorMartin Peres <>2015-07-20 10:37:30 +0300
committerAdam Jackson <>2015-10-27 16:12:03 -0400
commitbbf1893cc039155432a960b61d55978f1b706295 (patch)
parent6e3892045e57808b18d053288330306119ca2252 (diff)
os: make sure the clientsWritable fd_set is initialized before use
In WaitForSomething(), the fd_set clientsWritable may be used unitialized when the boolean AnyClientsWriteBlocked is set in the WakeupHandler(). This leads to a crash in FlushAllOutput() after x11proto's commit 2c94cdb453bc641246cc8b9a876da9799bee1ce7. The problem did not manifest before because both the XFD_SIZE and the maximum number of clients were set to 256. As the connectionTranslation table was initalized for the 256 clients to 0, the test on the index not being 0 was aborting before dereferencing the client #0. As of commit 2c94cdb453bc641246cc8b9a876da9799bee1ce7 in x11proto, the XFD_SIZE got bumped to 512. This lead the OutputPending fd_set to have any fd above 256 to be uninitialized which in turns lead to reading an index after the end of the ConnectionTranslation table. This index would then be used to find the client corresponding to the fd marked as pending writes and would also result to an out-of-bound access which would usually be the fatal one. Fix this by zeroing the clientsWritable fd_set at the beginning of WaitForSomething(). In this case, the bottom part of the loop, which would indirectly call FlushAllOutput, will not do any work but the next call to select will result in the execution of the right codepath. This is exactly what we want because we need to know the writable clients before handling them. In the end, it also makes sure that the fds above MaxClient are initialized, preventing the crash in FlushAllOutput(). Thanks to everyone involved in tracking this one down! Reported-by: Karol Herbst <> Reported-by: Tobias Klausmann <> Signed-off-by: Martin Peres <> Tested-by: Tobias Klausmann <> Tested-by: Martin Peres <> Bugzilla: Cc: Ilia Mirkin <> Cc: Olivier Fourdan < Cc: Adam Jackson <> Cc: Alan Coopersmith < Cc: Chris Wilson <> Reviewed-by: Alan Coopersmith <>
1 files changed, 1 insertions, 0 deletions
diff --git a/os/WaitFor.c b/os/WaitFor.c
index 431f1a6b6..993c14e52 100644
--- a/os/WaitFor.c
+++ b/os/WaitFor.c
@@ -158,6 +158,7 @@ WaitForSomething(int *pClientsReady)
Bool someReady = FALSE;
+ FD_ZERO(&clientsWritable);
if (nready)