Le 02. 10. 15 00:57, Peter Hurley a écrit :
On 10/01/2015 12:56 PM, Jean-Christian de Rivaz wrote:
Hi Greg and Jiri,
I try to fix a kernel panic bug related to the AX25 (and probably SLIP) line
discipline when the corresponding serial device is removed [1]. I proposed some
patches [2] [3] on the linux-hams mailing list but I think there raise more
questions about how tty_ldisc_hangup() should work when a serial device is
removed [4].
I actually see the following options:
a) Let the specific line discipline set the TTY_DRIVER_RESET_TERMIOS flag in
tty->driver as in [2] but this is suspected bad practice [5].
b) Let the specific line discipline set the TTY_OTHER_CLOSED flag in tty and
check it in tty_ldisc_hangup() as in [3].
c) Let the specific line discipline set the TTY_LDISC_HALTED flag in tty and
check it in tty_ldisc_hangup().
d) Let the specific line discipline set a new flag for that purpose, for
example TTY_LDISC_RESET, and check it in tty_ldisc_hangup().
e) Close the tty earlier so that tty_ldisc_reinit() is not even called. Need
some advise on how this should be done.
f) That's all wrong, something other need to be changed.
I would appreciate some comments from tty subsystem experts about this issue.
[1] http://www.spinics.net/lists/linux-hams/msg03500.html
Hi Peter, thanks for your time,
The crash reported here appears to be related to how mkiss handles its netdev;
maybe prematurely freeing the tx/rx buffers? I'd relook at how slip handles
netdev teardown.
Yes but this is a consequence of the fact that the ax0 interface was
re-opened uninitialized while the corresponding serial device is no
longer connected to the system. I don't see any rational to create this
bogus interface: the serial device is gone.
I don't see a problem with the ACM tty/tty core side of this.
At the time the hangup occurs, there is actually still an ACM tty device.
Not physically, sorry. The physical serial device was unplugged front
the system (or in hardware forced reset in the case of my test), causing
a USB disconnect. It's important to understand that the USB disconnect
has already occurred seconds before the crash. The fact that there is
still an ACM tty structure in the kernel corresponding to nothing real
is the cause of the problem.
The line discipline is reinited as a security precaution to prevent a previous
session's data from being visible in the new session.
Pragmatically reinited to N_TTY is ok, this is in fact how my proposed
patches work. But reinited to N_AX25 while the serial device is no more
have no sense at all and cause the crash when the new uninitialized
parasitic interface try to send a packet.
The tty core does not know
at the time the vhangup() occurs that the ACM driver plans to unregister the
tty device.
That's the root problem: It must a least known that it must not call
mkiss_open(). That's the bug that must be fixed. Or maybe the option e)
fix must be developed.
Don't do any of the things you suggest above.
Can I ask what did you suggest to solve the problem ? The bug is real,
causing a kernel panic and complete crash of the system, requiring a
hardware reset to reboot.
Best Regards,
Jean-Christian de Rivaz
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