Re: [CONFIG_MULTIUSER] BUG: unable to handle kernel paging request at ffffffee

[josh@joshtriplett.org]

On Wed, May 06, 2015 at 08:44:29AM -0700, Josh Triplett wrote:
> On Wed, May 06, 2015 at 05:08:50PM +0800, Fengguang Wu wrote:
> > FYI, the reported bug is still not fixed in linux-next 20150506.
> 
> This isn't the same bug.  The previous one you mentioned was a userspace
> assertion failure in libnih, likely caused because some part of upstart
> didn't have appropriate error handling for some syscall returning
> ENOSYS; that one wasn't an issue, since CONFIG_MULTIUSER=n is not
> expected to boot a standard Linux distribution.
> 
> This one, on the other hand, is a kernel panic, and does need fixing.
> 
> > commit 2813893f8b197a14f1e1ddb04d99bce46817c84a
> > 
> > +-----------------------------------------------------------+------------+------------+------------+
> > |                                                           | c79574abe2 | 2813893f8b | cbdacaf0c1 |
> > +-----------------------------------------------------------+------------+------------+------------+
> > | boot_successes                                            | 60         | 0          | 0          |
> > | boot_failures                                             | 0          | 22         | 1064       |
> > | BUG:unable_to_handle_kernel                               | 0          | 22         | 1032       |
> > | Oops                                                      | 0          | 22         | 1032       |
> > | EIP_is_at_devpts_new_index                                | 0          | 22         | 1032       |
> > | Kernel_panic-not_syncing:Fatal_exception                  | 0          | 22         | 1032       |
> > | backtrace:do_sys_open                                     | 0          | 22         | 1032       |
> > | backtrace:SyS_open                                        | 0          | 22         | 1032       |
> > | WARNING:at_arch/x86/kernel/fpu/core.c:#fpu__clear()       | 0          | 0          | 32         |
> > | Kernel_panic-not_syncing:Attempted_to_kill_init!exitcode= | 0          | 0          | 32         |
> > +-----------------------------------------------------------+------------+------------+------------+
> 
> Is this table saying the number of times the type of error in the first
> column occurred in each commit?
> 
> In any case, investigating.  Iulia, can you look at this as well?
> 
> I'm digging through the call stack, and I'm having a hard time seeing
> how the CONFIG_MULTIUSER patch could affect anything here.

Update: it looks like init_devpts_fs is getting ERR_PTR(-EINVAL) back
from kern_mount and storing that in devpts_mnt; later, devpts_new_index
pokes at devpts_mnt and explodes.

So, there are two separate bugs here.  On the one hand, CONFIG_MULTIUSER
should not be causing kern_mount to fail with -EINVAL; tracking that
down now.  On the other hand, devpts and ptmx should handle the failure
better, without crashing; ptmx_open should have gracefully failed back
to userspace with -ENODEV or something, since ptmx doesn't make sense
without devpts.  I'll send a patch for that too.

- Josh Triplett

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