Re: [PATCH RFC 13/29] iomap: integrate fs-verity verification into iomap's read path

Darrick J. Wong
2025-07-31T15:08:19Z

On Thu, Jul 31, 2025 at 05:01:48PM +0200, Andrey Albershteyn wrote:
> On 2025-07-31 07:52:33, Darrick J. Wong wrote:
> > On Thu, Jul 31, 2025 at 01:34:24PM +0200, Andrey Albershteyn wrote:
> > > On 2025-07-29 16:21:52, Darrick J. Wong wrote:
> > > > On Mon, Jul 28, 2025 at 10:30:17PM +0200, Andrey Albershteyn wrote:
> > > > > From: Andrey Albershteyn <aalbersh@redhat.com>
> > > > >
> > > > > This patch adds fs-verity verification into iomap's read path. After
> > > > > BIO's io operation is complete the data are verified against
> > > > > fs-verity's Merkle tree. Verification work is done in a separate
> > > > > workqueue.
> > > > >
> > > > > The read path ioend iomap_read_ioend are stored side by side with
> > > > > BIOs if FS_VERITY is enabled.
> > > > >
> > > > > [djwong: fix doc warning]
> > > > > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > > > > Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
> > > > > ---
> > > > > fs/iomap/buffered-io.c | 151 +++++++++++++++++++++++++++++++++++++++++++++++--
> > > > > fs/iomap/ioend.c | 41 +++++++++++++-
> > > > > include/linux/iomap.h | 13 +++++
> > > > > 3 files changed, 198 insertions(+), 7 deletions(-)
> > > > >
> > > > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > > > > index e959a206cba9..87c974e543e0 100644
> > > > > --- a/fs/iomap/buffered-io.c
> > > > > +++ b/fs/iomap/buffered-io.c
> > > > > @@ -6,6 +6,7 @@
> > > > > #include <linux/module.h>
> > > > > #include <linux/compiler.h>
> > > > > #include <linux/fs.h>
> > > > > +#include <linux/fsverity.h>
> > > > > #include <linux/iomap.h>
> > > > > #include <linux/pagemap.h>
> > > > > #include <linux/uio.h>
> > > > > @@ -363,6 +364,116 @@ static inline bool iomap_block_needs_zeroing(const struct iomap_iter *iter,
> > > > > pos >= i_size_read(iter->inode);
> > > > > }
> > > > >
> > > > > +#ifdef CONFIG_FS_VERITY
> > > > > +int iomap_init_fsverity(struct super_block *sb, unsigned int wq_flags,
> > > > > + int max_active)
> > > > > +{
> > > > > + int ret;
> > > > > +
> > > > > + if (!iomap_fsverity_bioset) {
> > > > > + ret = iomap_fsverity_init_bioset();
> > > > > + if (ret)
> > > > > + return ret;
> > > > > + }
> > > > > +
> > > > > + return fsverity_init_wq(sb, wq_flags, max_active);
> > > > > +}
> > > > > +EXPORT_SYMBOL_GPL(iomap_init_fsverity);
> > > > > +
> > > > > +static void
> > > > > +iomap_read_fsverify_end_io_work(struct work_struct *work)
> > > > > +{
> > > > > + struct iomap_fsverity_bio *fbio =
> > > > > + container_of(work, struct iomap_fsverity_bio, work);
> > > > > +
> > > > > + fsverity_verify_bio(&fbio->bio);
> > > > > + iomap_read_end_io(&fbio->bio);
> > > > > +}
> > > > > +
> > > > > +static void
> > > > > +iomap_read_fsverity_end_io(struct bio *bio)
> > > > > +{
> > > > > + struct iomap_fsverity_bio *fbio =
> > > > > + container_of(bio, struct iomap_fsverity_bio, bio);
> > > > > +
> > > > > + INIT_WORK(&fbio->work, iomap_read_fsverify_end_io_work);
> > > > > + queue_work(bio->bi_private, &fbio->work);
> > > > > +}
> > > > > +
> > > > > +static struct bio *
> > > > > +iomap_fsverity_read_bio_alloc(struct inode *inode, struct block_device *bdev,
> > > > > + int nr_vecs, gfp_t gfp)
> > > > > +{
> > > > > + struct bio *bio;
> > > > > +
> > > > > + bio = bio_alloc_bioset(bdev, nr_vecs, REQ_OP_READ, gfp,
> > > > > + iomap_fsverity_bioset);
> > > > > + if (bio) {
> > > > > + bio->bi_private = inode->i_sb->s_verity_wq;
> > > > > + bio->bi_end_io = iomap_read_fsverity_end_io;
> > > > > + }
> > > > > + return bio;
> > > > > +}
> > > > > +
> > > > > +/*
> > > > > + * True if tree is not aligned with fs block/folio size and we need zero tail
> > > > > + * part of the folio
> > > > > + */
> > > > > +static bool
> > > > > +iomap_fsverity_tree_end_align(struct iomap_iter *iter, struct folio *folio,
> > > > > + loff_t pos, size_t plen)
> > > > > +{
> > > > > + int error;
> > > > > + u8 log_blocksize;
> > > > > + u64 tree_size, tree_mask, last_block_tree, last_block_pos;
> > > > > +
> > > > > + /* Not a Merkle tree */
> > > > > + if (!(iter->iomap.flags & IOMAP_F_BEYOND_EOF))
> > > > > + return false;
> > > > > +
> > > > > + if (plen == folio_size(folio))
> > > > > + return false;
> > > > > +
> > > > > + if (iter->inode->i_blkbits == folio_shift(folio))
> > > > > + return false;
> > > > > +
> > > > > + error = fsverity_merkle_tree_geometry(iter->inode, &log_blocksize, NULL,
> > > > > + &tree_size);
> > > > > + if (error)
> > > > > + return false;
> > > > > +
> > > > > + /*
> > > > > + * We are beyond EOF reading Merkle tree. Therefore, it has highest
> > > > > + * offset. Mask pos with a tree size to get a position whare are we in
> > > > > + * the tree. Then, compare index of a last tree block and the index of
> > > > > + * current pos block.
> > > > > + */
> > > > > + last_block_tree = (tree_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > > > > + tree_mask = (1 << fls64(tree_size)) - 1;
> > > > > + last_block_pos = ((pos & tree_mask) >> PAGE_SHIFT) + 1;
> > > > > +
> > > > > + return last_block_tree == last_block_pos;
> > > > > +}
> > > > > +#else
> > > > > +# define iomap_fsverity_read_bio_alloc(...) (NULL)
> > > > > +# define iomap_fsverity_tree_end_align(...) (false)
> > > > > +#endif /* CONFIG_FS_VERITY */
> > > > > +
> > > > > +static struct bio *iomap_read_bio_alloc(struct inode *inode,
> > > > > + const struct iomap *iomap, int nr_vecs, gfp_t gfp)
> > > > > +{
> > > > > + struct bio *bio;
> > > > > + struct block_device *bdev = iomap->bdev;
> > > > > +
> > > > > + if (fsverity_active(inode) && !(iomap->flags & IOMAP_F_BEYOND_EOF))
> > > > > + return iomap_fsverity_read_bio_alloc(inode, bdev, nr_vecs, gfp);
> > > > > +
> > > > > + bio = bio_alloc(bdev, nr_vecs, REQ_OP_READ, gfp);
> > > > > + if (bio)
> > > > > + bio->bi_end_io = iomap_read_end_io;
> > > > > + return bio;
> > > > > +}
> > > > > +
> > > > > static int iomap_readpage_iter(struct iomap_iter *iter,
> > > > > struct iomap_readpage_ctx *ctx)
> > > > > {
> > > > > @@ -375,6 +486,10 @@ static int iomap_readpage_iter(struct iomap_iter *iter,
> > > > > sector_t sector;
> > > > > int ret;
> > > > >
> > > > > + /* Fail reads from broken fsverity files immediately. */
> > > > > + if (IS_VERITY(iter->inode) && !fsverity_active(iter->inode))
> > > > > + return -EIO;
> > > > > +
> > > > > if (iomap->type == IOMAP_INLINE) {
> > > > > ret = iomap_read_inline_data(iter, folio);
> > > > > if (ret)
> > > > > @@ -391,6 +506,11 @@ static int iomap_readpage_iter(struct iomap_iter *iter,
> > > > > if (iomap_block_needs_zeroing(iter, pos) &&
> > > > > !(iomap->flags & IOMAP_F_BEYOND_EOF)) {
> > > > > folio_zero_range(folio, poff, plen);
> > > > > + if (fsverity_active(iter->inode) &&
> > > > > + !fsverity_verify_blocks(folio, plen, poff)) {
> > > > > + return -EIO;
> > > > > + }
> > > > > +
> > > > > iomap_set_range_uptodate(folio, poff, plen);
> > > > > goto done;
> > > > > }
> > > > > @@ -408,32 +528,51 @@ static int iomap_readpage_iter(struct iomap_iter *iter,
> > > > > !bio_add_folio(ctx->bio, folio, plen, poff)) {
> > > > > gfp_t gfp = mapping_gfp_constraint(folio->mapping, GFP_KERNEL);
> > > > > gfp_t orig_gfp = gfp;
> > > > > - unsigned int nr_vecs = DIV_ROUND_UP(length, PAGE_SIZE);
> > > > >
> > > > > if (ctx->bio)
> > > > > submit_bio(ctx->bio);
> > > > >
> > > > > if (ctx->rac) /* same as readahead_gfp_mask */
> > > > > gfp |= __GFP_NORETRY | __GFP_NOWARN;
> > > > > - ctx->bio = bio_alloc(iomap->bdev, bio_max_segs(nr_vecs),
> > > > > - REQ_OP_READ, gfp);
> > > > > +
> > > > > + ctx->bio = iomap_read_bio_alloc(iter->inode, iomap,
> > > > > + bio_max_segs(DIV_ROUND_UP(length, PAGE_SIZE)),
> > > > > + gfp);
> > > > > +
> > > > > /*
> > > > > * If the bio_alloc fails, try it again for a single page to
> > > > > * avoid having to deal with partial page reads. This emulates
> > > > > * what do_mpage_read_folio does.
> > > > > */
> > > > > if (!ctx->bio) {
> > > > > - ctx->bio = bio_alloc(iomap->bdev, 1, REQ_OP_READ,
> > > > > - orig_gfp);
> > > > > + ctx->bio = iomap_read_bio_alloc(iter->inode,
> > > > > + iomap, 1, orig_gfp);
> > > > > }
> > > > > if (ctx->rac)
> > > > > ctx->bio->bi_opf |= REQ_RAHEAD;
> > > > > ctx->bio->bi_iter.bi_sector = sector;
> > > > > - ctx->bio->bi_end_io = iomap_read_end_io;
> > > > > bio_add_folio_nofail(ctx->bio, folio, plen, poff);
> > > > > }
> > > > >
> > > > > done:
> > > > > + /*
> > > > > + * For post EOF region, zero part of the folio which won't be read. This
> > > > > + * happens at the end of the region. So far, the only user is
> > > > > + * fs-verity which stores continuous data region.
> > > >
> > > > Is it ever the case that the zeroed region actually has merkle tree
> > > > content on disk? Or if this region truly was never written by the
> > > > fsverity construction code, then why would it access the unwritten
> > > > region later?
> > > >
> > > > Or am I misunderstanding something here?
> > > >
> > > > (Probably...)
> > >
> > > The zeroed region is never written. With 1k fs block and 1k merkle
> > > tree block and 4k page size we could end up reading only single
> > > block, at the end of the tree. But we have to pass PAGE to the
> > > fsverity. So, only the 1/4 of the page is read. This if-case zeroes
> > > the rest of the page to make it uptodate. In the normal read path
> > > this is bound by EOF, but here I use tree size. So, we don't read
> > > this unwritten region but zero the folio.
> > >
> > > The fsverity does zeroing of unused space while construction, but
> > > this works only for full fs blocks, therefore, 4k fs block and 1k
> > > merkle tree block.
> >
> > But if the regions you have to zero are outside the merkle tree, then
> > fsverity shouldn't ever see those bytes, so why zero them? Or does it
> > actually check the uptodate bit? So then you want the folio to have
> > well defined contents?
>
> verity doesn't check uptodate, but I do in xfs_fsverity_read_merkle().
> I think we need to check as it tells that iomap is done reading,
> but that's true that zeroing is not necessary.

Ok sounds good to me.

--D

> >
> > (At this point I'm picking at nits :P)
> >
> > --D
> >
> > > --
> > > - Andrey
> > >
> > >
> >
>
> --
> - Andrey
>
>