Re: [GIT PULL] x86/shstk for 6.4

Linus Torvalds
2023-05-16T20:40:01Z

On Mon, May 15, 2023 at 3:40 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So I *think* fork() could do something like this:
>
> struct fork_cookie; // dummy type purely for type checking
> static struct fork_cookie *is_singe_threaded(void)
> {
> struct mm_struct *mm = current->mm;
> mutex_lock(&mm->fork_lock);

Actually, let's not even bother with a new fork_lock.

I for some reason thought that fork only took the mmap_lock for
reading (because it kind of does just read the old VM data), but when
I actually look at dup_mmap() I notice that what it does is actually
just

if (mmap_write_lock_killable(oldmm)) ..

and so I think we can just use that as the lock.

So then all we'd need is to use mmap_read_lock(mm) in kthread_use_mm()
around the mmgrab.

I don't think we even need it in kthread_unuse_mm(), because
*decrementing* the mm counters isn't even something we need to worry
about.

How does that sound?

Linus