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

[Peter Hurley <peter@hurleysoftware.com>]

On 05/06/2015 07:59 PM, josh@joshtriplett.org wrote:
> 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.

The mount failure is probably from the devpts mount options specifying
gid= for devpts nodes:

devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0

The relevant code is fs/devpts/inode.c:parse_mount_options().
devpts also supports specifying the uid.

To me, kern_mount() appropriately fails with -EINVAL, since the mount
options failed.


> 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.

Yeah, crashing is bad, but I don't think we should even be init'ing
either BSD or SysV pty drivers if there is no devpts.

Regards,
Peter Hurley

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