Re: [PATCH v4 1/3] mm/mprotect: Fix soft-dirty check in can_change_pte_writable()

Muhammad Usama Anjum
2022-12-28T14:14:25Z

On 12/19/22 5:19 PM, Muhammad Usama Anjum wrote:
> Addition of vma_soft_dirty_enabled() has tinkered with the soft-dirty PTE
> bit status setting. The internal behavior has changed. The test case was
> shared by David
> (https://lore.kernel.org/all/bfcae708-db21-04b4-0bbe-712badd03071@redhat.com/).
> The explanation is as following:
>
> _Before_ addition of this patch(76aefad628aae),
> m = mmap(2 pages)
> clear_softdirty()
> mremap(m + pag_size)
> mprotect(READ)
> mprotect(READ | WRITE);
> memset(m)
> After memset(),
> PAGE-1 PAGE-2
> VM_SOFTDIRTY set set
> PTE softdirty flag set set
> /proc//pagemap view set set
>
>
> _After_ addition of this patch(76aefad628aae)
> m = mmap(2 pages)
> clear_softdirty()
> mremap(m + page_size)
> mprotect(READ)
> mprotect(READ | WRITE);
> memset(m)
> After memset(),
> PAGE-1 PAGE-2
> VM_SOFTDIRTY set set
> PTE softdirty flag *not set* set
> /proc//pagemap view set set
>
> The user's point of view hasn't changed. But internally after this patch,
> the soft-dirty tracking in PTEs gets turn off if VM_SOFTDIRTY is set. The
> soft-dirty tracking in the PTEs shouldn't be just turned off when mprotect
> is used. Why? Because soft-dirty tracking in the PTEs is always enabled
> regardless of VM_SOFTDIRTY is set or not. Example:
>
> m = mem(2 pages)
> At this point:
> PAGE-1 PAGE-2
> VM_SOFTDIRTY set set
> PTE softdirty flag not set not set
> /proc//pagemap view set set
> memset(m)
> At this point:
> PAGE-1 PAGE-2
> VM_SOFTDIRTY set set
> PTE softdirty flag set set
> /proc//pagemap view set set
>
> This example proves that soft-dirty flag on the PTE is set regardless of
> the VM_SOFTDIRTY.

Hi Andrew and Cyrill,

Peter doesn't agree with me here that this change in behavior should be
reverted etc. Please comment.

--
BR,
Muhammad Usama Anjum