On Tue, Jul 11, 2023 at 6:27 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> Smatch has observed that pte_offset_map_lock() is now allowed to fail,
> and then ptl should not be unlocked. Use -EAGAIN here like elsewhere.
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Looks correct to me, thanks Hugh! If it's useful, feel free to take:
Reviewed-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> ---
> Axel, Peter: this seems right as a fix to the patch in mm-unstable;
> but in preparing this, I noticed mfill_atomic()'s code before calling
> mfill_atomic_pte(), and think that my original choice of -EFAULT was
> therefore better than -EAGAIN for all of these; and that mfill_atomic()'s
> BUG_ONs there would be better deleted (and is its BUG_ON(folio) safe??).
> Something one of us should address, after this fixup is in akpm's tree.
Agreed, dealing with the BUG_ONs is something I have been meaning to
do as checkpatch bothers me about it frequently.
What this code is trying to do is to enforce the contract that: if
mfill_atomic_pte failed, then it must have released *folio and set it
to NULL. But, you're exactly right that if we had such a bug, it would
mean we leaked one page in an unusual error case - not great, but not
worth crashing the kernel either.
For UFFDIO_POISON this doesn't really apply because it doesn't
allocate or get a reference to a page in any case.
For other cases like UFFDIO_COPY where it does matter, I think it's
fine as is regardless of the error code returned (as long as it's not
-ENOENT :) ). mfill_atomic_pte_copy() is sure to set `*foliop = NULL`
before it attempts this lock (in mfill_atomic_install_pte). So -EAGAIN
or -EFAULT should work equally well at least from that perspective. I
somewhat prefer -EAGAIN, but maybe I'm missing something.
>
> mm/userfaultfd.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -300,7 +300,10 @@ static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
> spinlock_t *ptl;
>
> _dst_pte = make_pte_marker(PTE_MARKER_POISONED);
> + ret = -EAGAIN;
> dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> + if (!dst_pte)
> + goto out;
>
> if (mfill_file_over_size(dst_vma, dst_addr)) {
> ret = -EFAULT;
> @@ -319,6 +322,7 @@ static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
> ret = 0;
> out_unlock:
> pte_unmap_unlock(dst_pte, ptl);
> +out:
> return ret;
> }
>
