Re: [PATCH] mm/shmem: fix uninitialized folio in shmem_symlink

[Barry Song]

On Thu, Dec 25, 2025 at 6:01 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Wed, Dec 24, 2025 at 10:40:27PM +1300, Barry Song wrote:
> > From: Barry Song <baohua@xxxxxxxxxx>
> >
> > Uninitialized folio allocated in shmem_symlink() may be accessed
> > during swap-out, causing KMSAN BUG:
>
> This would be an unfortunate way to fix it.  The vast majority of
> symlinks are short, and we'll never access past the \0 in normal
> operation, so we'll be dirtying a lot of cachelines essentially to (1)
> shut up an automated tool and (2) optimise a corner case.
>
> How about this instead which delays zeroing to swapout?

Matthew, thank you very much for your review, even during Christmas.
I would like to wish you a happy holiday!

I am not quite sure, as shm symlinks do not seem very common. Since
allocating a folio requires a symname longer than 128 bytes (where
128 == SHORT_SYMLINK_LEN), such cases appear even rarer.

BTW, do we need to migrate the owner_2 flag in folio_migrate_flags()?
If so, I am not quite sure it is worth changing the hotpath to
accommodate this.

>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index ec6c01378e9d..f3b3be1b50fe 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1636,6 +1636,13 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>                 folio_mark_uptodate(folio);
>         }
>
> +       /* Zero out symlink tails to help with compression */
> +       if (folio_test_owner_2(folio)) {
> +               struct inode *inode = folio->mapping->host;
> +               folio_zero_segment(folio, inode->i_size, folio_size(folio));
> +               folio_clear_owner_2(folio);
> +       }
> +
>         if (!folio_alloc_swap(folio)) {
>                 bool first_swapped = shmem_recalc_inode(inode, 0, nr_pages);
>                 int error;
> @@ -4133,6 +4140,7 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
>                 memcpy(folio_address(folio), symname, len);
>                 folio_mark_uptodate(folio);
>                 folio_mark_dirty(folio);
> +               folio_set_owner_2(folio);
>                 folio_unlock(folio);
>                 folio_put(folio);
>         }

Thanks
Barry