[PATCH] uml/helper: Fix stack alignment

[YiFei Zhu]

On Sat, Apr 17, 2021 at 11:56 PM YiFei Zhu <zhuyifei1999 at 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