Re: [PATCH] uml/helper: Fix stack alignment

[Anton Ivanov <anton.ivanov@cambridgegreys.com>]

On 18/04/2021 07:56, YiFei Zhu wrote:
> On Sat, Apr 17, 2021 at 11:56 PM YiFei Zhu <zhuyifei1999@gmail.com> wrote:
>>    * um on um x86_64: I'm having trouble testing um within um, getting
>> a weird error ("start_userspace : expected SIGSTOP, got status = 2943"
>> when starting init, might try to debug later), but the code in
>> handle_signal also aligns the stack.
> 
> Figured this one out. The inner um, in userspace_tramp, is trying to
> mmap the syscall stub to the same syscall stub at the same location as
> the outer um, and that fails with ENOMEM. In theory, this would cause
> the printk of "mapping mmap stub at ... failed, errno = ..." to occur,
> but because:
> * call stack: vprintk_store -> printk_caller_id -> in_task -> in_nmi
> -> nmi_count -> preempt_count -> current_thread_info
> * um's current_thread_info is at the current stack pointer & mask,
> hence it is often not valid when on small temporary stacks.
> Therefore, userspace_tramp can't printk.
> 
> I'm wondering, is this issue of printk being broken in userspace_tramp
> an issue worth fixing? Has there been prior discussions on it?
> 
> YiFei Zhu
> 

Based ob experience - printk does not work correctly out of some uml 
threads. We had to kill printk use in the ubd helper thread.

I never got to the bottom of that, it was easier to kill it. In that 
case it was not particularly informative.

-- 
Anton R. Ivanov
Cambridgegreys Limited. Registered in England. Company Number 10273661
https://www.cambridgegreys.com/

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