On Mon, May 3, 2021 at 4:16 PM Linus Torvalds
[off-list ref] wrote:

On Mon, May 3, 2021 at 3:56 PM Thomas Gleixner [off-list ref] wrote:

quoted

It's all fine that we have lots of blurb about GDB, but there is no
reasoning why this does not affect regular kernel threads which take the
same code path.

Actual kernel threads don't get attached to by ptrace.

quoted

This is a half setup user space thread which is assumed to behave like a
regular kernel thread, but is this assumption actually true?

No, no.

It's a *fully set up USER thread*.

Those IO threads used to be kernel threads. That didn't work out for
the reasons already mentioned earlier.

These days they really are fully regular user threads, they just don't
return to user space because they continue to do the IO work that they
were created for.

Maybe instead of Stefan's patch, we could do something like this:

   diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
   index 43cbfc84153a..890f3992e781 100644
   --- a/arch/x86/kernel/process.c
   +++ b/arch/x86/kernel/process.c
   @@ -156,7 +156,7 @@ int copy_thread(unsigned long clone_flags,
unsigned long sp, unsigned long arg,
    #endif

        /* Kernel thread ? */
   -    if (unlikely(p->flags & (PF_KTHREAD | PF_IO_WORKER))) {
   +    if (unlikely(p->flags & PF_KTHREAD)) {
                memset(childregs, 0, sizeof(struct pt_regs));
                kthread_frame_init(frame, sp, arg);
                return 0;
   @@ -168,6 +168,17 @@ int copy_thread(unsigned long clone_flags,
unsigned long sp, unsigned long arg,
        if (sp)
                childregs->sp = sp;

   +    /*
   +     * An IO thread is a user space thread, but it doesn't
   +     * return to ret_after_fork(), it does the same kernel
   +     * frame setup to return to a kernel function that
   +     * a kernel thread does.
   +     */
   +    if (unlikely(p->flags & PF_IO_WORKER)) {
   +            kthread_frame_init(frame, sp, arg);
   +            return 0;
   +    }
   +
    #ifdef CONFIG_X86_32
        task_user_gs(p) = get_user_gs(current_pt_regs());
    #endif

does that clarify things and make people happier?

Maybe the compiler might even notice that the

                kthread_frame_init(frame, sp, arg);
                return 0;

part is common code and then it will result in less generated code too.

NOTE! The above is - as usual - COMPLETELY UNTESTED. It looks obvious
enough, and it builds cleanly. But that's all I'm going to guarantee.

It's whitespace-damaged on purpose.

I like this patch considerably more than I liked the previous patch.

FWIW, I have this fixlet sitting around:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/kentry&id=1eef07ae5b236112c9a0c5d880d7f9bb13e73761

Your patch fixes the same bug for the specific case of io_uring.