From 6f437783919467437f19ec534a0317aef2fd2584 Mon Sep 17 00:00:00 2001 From: Stefan Richter Date: Sat, 9 Jul 2011 16:43:22 +0200 Subject: firewire: cdev: prevent race between first get_info ioctl and bus reset event queuing commit 93b37905f70083d6143f5f4dba0a45cc64379a62 upstream. Between open(2) of a /dev/fw* and the first FW_CDEV_IOC_GET_INFO ioctl(2) on it, the kernel already queues FW_CDEV_EVENT_BUS_RESET events to be read(2) by the client. The get_info ioctl is practically always issued right away after open, hence this condition only occurs if the client opens during a bus reset, especially during a rapid series of bus resets. The problem with this condition is twofold: - These bus reset events carry the (as yet undocumented) @closure value of 0. But it is not the kernel's place to choose closures; they are privat to the client. E.g., this 0 value forced from the kernel makes it unsafe for clients to dereference it as a pointer to a closure object without NULL pointer check. - It is impossible for clients to determine the relative order of bus reset events from get_info ioctl(2) versus those from read(2), except in one way: By comparison of closure values. Again, such a procedure imposes complexity on clients and reduces freedom in use of the bus reset closure. So, change the ABI to suppress queuing of bus reset events before the first FW_CDEV_IOC_GET_INFO ioctl was issued by the client. Note, this ABI change cannot be version-controlled. The kernel cannot distinguish old from new clients before the first FW_CDEV_IOC_GET_INFO ioctl. We will try to back-merge this change into currently maintained stable/ longterm series, and we only document the new behaviour. The old behavior is now considered a kernel bug, which it basically is. Signed-off-by: Stefan Richter Cc: --- include/linux/firewire-cdev.h | 3 +++ 1 file changed, 3 insertions(+) (limited to 'include') diff --git a/include/linux/firewire-cdev.h b/include/linux/firewire-cdev.h index 4ff09889c5c..55814aa33be 100644 --- a/include/linux/firewire-cdev.h +++ b/include/linux/firewire-cdev.h @@ -475,6 +475,9 @@ union fw_cdev_event { * of the bus. This does not cause a bus reset to happen. * @bus_reset_closure: Value of &closure in this and subsequent bus reset events * @card: The index of the card this device belongs to + * + * As a side effect, reception of %FW_CDEV_EVENT_BUS_RESET events to be read(2) + * is started by this ioctl. */ struct fw_cdev_get_info { __u32 version; -- cgit v1.2.3-18-g5258