Re: BUG: unable to handle kernel paging request from pty_write [was: Linux 4.4.2]

From: Jiri Slaby
Date: Thu Feb 25 2016 - 16:32:36 EST

On 02/25/2016, 09:51 PM, Linus Torvalds wrote:
> Jiri, can you check your try_to_wake_up() disassembly for some
> indirect "jmp" instructions?

Nope, there is none.

I will reply to all your questions tomorrow.

Just quickly, as I have to go (and don't want you to duplicate efforts)
the kernel which was used can be obtained here:

The issue is very weird, indeed, this is what I noted to our bugzilla:
The stack trace ends in call of try_to_wake_up. Then, there it has to be
some of the indirect calls:

callq *0x40(%rax)
p->sched_class->select_task_rq from select_task_rq

RAX is 0x00000000bb37e180, barely can be read with offset 0x40

callq *0xd85656(%rip) # ffffffff81e2aba0 <smp_ops+0x20>
smp_ops.smp_send_reschedule from ttwu_queue_remote

Which hardly can be it, given smp_ops is static.

So it has to be some other "call *" from a nested function :(.

Interestingly, RBP contains address inside try_to_wake_up --
ffffffff810a535a (dunno why) which is:
ffffffff810a5355: e8 66 a0 ff ff callq ffffffff8109f3c0
ffffffff810a535a: e9 9d fe ff ff jmpq ffffffff810a51fc

ttwu_stat does in the begginning:
mov $0x16e80,%r14

which is what we actually still have in r14 when it crashes. The first
ttwu_stat's "if" has to go through the true branch (otherwise r14 would
be overwritten).

Another note: we die when jmp/calling to 0xffff88023fd40000.
RSI=RDI=0xffff88023fdd6e80. RSI-RIP is 0x96e80, which is R14 + 0x80000.

suse labs