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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