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authorStefan Richter <stefanr@s5r6.in-berlin.de>2011-07-09 18:43:22 +0400
committerStefan Richter <stefanr@s5r6.in-berlin.de>2011-07-16 09:24:32 +0400
commit93b37905f70083d6143f5f4dba0a45cc64379a62 (patch)
treeb6917581cd390ed9967e1df9922940362ad4309e /Documentation/ABI
parentd873d794235efa590ab3c94d5ee22bb1fab19ac4 (diff)
downloadlinux-93b37905f70083d6143f5f4dba0a45cc64379a62.tar.xz
firewire: cdev: prevent race between first get_info ioctl and bus reset event queuing
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 <stefanr@s5r6.in-berlin.de> Cc: <stable@kernel.org>
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