Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

[Eric W. Biederman]

Folks,

Digging through the guts of exit I found something I am not quite
certain what to do with.  On some architectures such as alpha, m68k, and
nios2 the kernel calls into system calls with a subset of the registers
saved on the kernel stack, and the kernel calls into signal handling and
a few other contexts with all of the registers saved on the kernel
stack.  The problem is sometimes we read all of the registers from
a context where they are not all saved.

When this was initially observed it looked just like a coredump problem
and it could be solved by tweaking the coredump code.  That change was
77f6ab8b7768 ("don't dump the threads that had been already exiting when
zapped.")

However I have looked farther and we have the location where get_signal
is called from io_uring, and we have the ptrace_stop in
PTRACE_EVENT_EXIT.  In PTRACE_EVENT_EXIT we could be called from exit(2)
which is a syscall and we definitely won't have everything saved on the
kernel stack.  I have not doubled checked create_io_thread but I don't
think create_io_threads saves all of the registers on the kernel stack.

I think at this point we need to say that the architectures that have a
do this need to be fixed to at least call do_exit and the kernel
function in create_io_thread with the deeper stack.

Is that reasonable of me to ask?  Is there some other way to deal with
this issue that I am not seeing?  Am I missing some critical detail that
makes PTRACE_EVENT_EXIT in do_exit not a problem if someone reads the
register with ptrace?

Eric