On Sun, Aug 27, 2023 at 10:21:14AM +0100, Lorenzo Stoakes wrote: [..]
/*
- Flags used by change_protection(). For now we make it a bitmap so
diff --git a/mm/mremap.c b/mm/mremap.c index 035fbf542a8f..06baa13bd2c8 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -490,12 +490,13 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma, }
/*
- A helper to check if a previous mapping exists. Required for
- move_page_tables() and realign_addr() to determine if a previous mapping
- exists before we can do realignment optimizations.
- A helper to check if aligning down is OK. The aligned address should fall
- on *no mapping*. For the stack moving down, that's a special move within
- the VMA that is created to span the source and destination of the move,
*/
- so we make an exception for it.
static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_align,
unsigned long mask)
unsigned long mask, bool for_stack){ unsigned long addr_masked = addr_to_align & mask;
@@ -504,7 +505,7 @@ static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_ali * of the corresponding VMA, we can't align down or we will destroy part * of the current mapping. */
- if (vma->vm_start != addr_to_align)
- if (!for_stack && vma->vm_start != addr_to_align) return false;
I'm a little confused by this exception, is it very specifically for the shift_arg_pages() case where can assume we are safe to just discard the lower portion of the stack?
Wouldn't the find_vma_intersection() line below fail in this case? I may be missing something here :)
I think you are right. In v4, this was not an issue as we did this:
+ if (!for_stack && vma->vm_start != addr_to_align) + return false; + + cur = find_vma_prev(vma->vm_mm, vma->vm_start, &prev); + if (WARN_ON_ONCE(cur != vma)) + return false;
Which essentially means this patch is a NOOP in v5 for the stack case.
So what we really want is the VMA previous to @vma and whether than subsumes the masked address.
Should I just change it back to the v4 version then as above for both patch 1 and 2 and carry your review tags?
This is also hard to test as it requires triggering the execve stack move case. Though it is not a bug (as it is essentially a NOOP), it still would be nice to test it. This is complicated by also the fact that mremap(2) itself does not allow overlapping moves. I could try to hardcode the unfavorable situation as I have done in the past to force that mremap warning.
thanks,
- Joel