Re: [PATCH] uprobes: use vm_special_mapping close() functionality

[Oleg Nesterov <oleg@redhat.com>]

I didn't have time to write a full reply yesterday.

On 09/03, Linus Torvalds wrote:
>
> On Tue, 3 Sept 2024 at 02:09, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > but with or without this fix __create_xol_area() also needs
> >
> >         area->xol_mapping.mremap = NULL;
>
> I think the whole thing needs to be zeroed out.

Again, this is what Sven did and I agree with this fix for now.

> It was always horribly buggy. The close thing just made it more
> *obviously* buggy, because closing a vma is a lot more common than
> mremap'ing it.

Well, this code is very old, I don't think it was "always" buggy.
But at least today it is horribly ugly, and

> Either use kzalloc(), or do a proper initializer something like this:
>
> -       area->xol_mapping.name = "[uprobes]";
> -       area->xol_mapping.fault = NULL;
> -       area->xol_mapping.pages = area->pages;
> +       area->xol_mapping = (struct vm_special_mapping) {
> +               .name = "[uprobes]",
> +               .pages = area->pages,
> +               .close = uprobe_clear_state,
> +       };

either way the code is still ugly, imo.

How about the (untested) patch below?

I am not going to send this patch right now, it conflicts with the ongoing
close/mremap changes, but what do you think?

We do not need to add the instance of vm_special_mapping into mm_struct,
a single instance (xol_mapping below) with .fault = xol_fault() is enough.
And, with this change xol_area no longer needs "struct page *pages[2]", it
can be turned into "struct page *page".

Oleg.
---

--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -101,7 +101,6 @@ struct xol_area {
 	atomic_t 			slot_count;	/* number of in-use slots */
 	unsigned long 			*bitmap;	/* 0 = free slot */
 
-	struct vm_special_mapping	xol_mapping;
 	struct page 			*pages[2];
 	/*
 	 * We keep the vma's vm_start rather than a pointer to the vma
@@ -1453,6 +1452,21 @@ void uprobe_munmap(struct vm_area_struct *vma, unsigned long start, unsigned lon
 		set_bit(MMF_RECALC_UPROBES, &vma->vm_mm->flags);
 }
 
+static vm_fault_t xol_fault(const struct vm_special_mapping *sm,
+			    struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	struct xol_area *area = vma->vm_mm->uprobes_state.xol_area;
+
+	vmf->page = area->pages[0];
+	get_page(vmf->page);
+	return 0;
+}
+
+static const struct vm_special_mapping xol_mapping = {
+	.name = "[uprobes]",
+	.fault = xol_fault,
+};
+
 /* Slot allocation for XOL */
 static int xol_add_vma(struct mm_struct *mm, struct xol_area *area)
 {
@@ -1479,7 +1493,7 @@ static int xol_add_vma(struct mm_struct *mm, struct xol_area *area)
 
 	vma = _install_special_mapping(mm, area->vaddr, PAGE_SIZE,
 				VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO,
-				&area->xol_mapping);
+				&xol_mapping);
 	if (IS_ERR(vma)) {
 		ret = PTR_ERR(vma);
 		goto fail;
@@ -1518,9 +1532,6 @@ static struct xol_area *__create_xol_area(unsigned long vaddr)
 	if (!area->bitmap)
 		goto free_area;
 
-	area->xol_mapping.name = "[uprobes]";
-	area->xol_mapping.fault = NULL;
-	area->xol_mapping.pages = area->pages;
 	area->pages[0] = alloc_page(GFP_HIGHUSER);
 	if (!area->pages[0])
 		goto free_bitmap;