Re: WARNING in syscall_exit_to_user_mode

[Thomas Gleixner]

On Fri, Sep 11 2020 at 03:39, syzbot wrote:
>
> ------------[ cut here ]------------
> syscall 56 left IRQs disabled

Ouch. syscall 56 == sys_clone

> WARNING: CPU: 1 PID: 31467 at kernel/entry/common.c:245 syscall_exit_to_user_mode_prepare kernel/entry/common.c:245 [inline]

<snip>

>  ret_from_fork+0x15/0x30 arch/x86/entry/entry_64.S:287

But this does not make any sense for regular fork because
ret_from_fork() invokes schedule_tail() which drops the runqueue lock
and enables interrupts.

But for kernel threads that's more interesting because after
schedule_tail() the kernel thread function is invoked and this can call
kernel_execve() and return to ret_from_fork() and then run through
syscall_exit_to_user_mode().

Let me get that reproducer...