Re: [PATCH 03/13] fsverity: support block-based Merkle tree caching

Darrick J. Wong
2024-04-24T21:25:24Z

On Thu, Apr 04, 2024 at 10:31:45PM -0400, Eric Biggers wrote:
> On Fri, Mar 29, 2024 at 05:33:27PM -0700, Darrick J. Wong wrote:
> > diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
> > index b3506f56e180b..c9d97c2bebd84 100644
> > --- a/fs/verity/fsverity_private.h
> > +++ b/fs/verity/fsverity_private.h
> > @@ -154,4 +154,41 @@ static inline void fsverity_init_signature(void)
> >
> > void __init fsverity_init_workqueue(void);
> >
> > +static inline bool fsverity_caches_blocks(const struct inode *inode)
> > +{
> > + const struct fsverity_operations *vops = inode->i_sb->s_vop;
> > +
> > + WARN_ON_ONCE(vops->read_merkle_tree_block &&
> > + !vops->drop_merkle_tree_block);
> > +
> > + return vops->read_merkle_tree_block != NULL;
> > +}
> > +
> > +static inline bool fsverity_uses_bitmap(const struct fsverity_info *vi,
> > + const struct inode *inode)
> > +{
> > + if (fsverity_caches_blocks(inode))
> > + return false;
> > +
> > + /*
> > + * If fs uses block-based Merkle tree caching, then fs-verity must use
> > + * hash_block_verified bitmap as there's no page to mark it with
> > + * PG_checked.
> > + */
> > + return vi->tree_params.block_size != PAGE_SIZE;
> > +}
>
> For checking whether the bitmap is in use, it's simpler and more efficient to
> just directly check whether vi->hash_block_verified is NULL, as the existing
> code does. Only the code that allocates the bitmap actually needs to be aware
> of the details of when the bitmap gets enabled.
>
> fsverity_caches_blocks() has a similar issue, where it could just be replaced
> with checking vops->read_merkle_tree_block != NULL directly (or equivalently
> vops->drop_merkle_tree_block, which works well in
> fsverity_drop_merkle_tree_block() since that's the function pointer it's
> calling). The WARN_ON_ONCE() should be done in fsverity_create_info(), not
> inlined multiple times into the verification code.

Ok, I'll move the WARN_ON_ONCE there:

/*
* If the filesystem implementation supplies Merkle tree content on a
* per-block basis, it must implement both the read and drop functions.
* If it supplies content on a per-page basis, neither should be
* provided.
*/
if (vops->read_merkle_tree_block)
WARN_ON_ONCE(vops->drop_merkle_tree_block == NULL);
else
WARN_ON_ONCE(vops->drop_merkle_tree_block != NULL);

> (I think the name "fsverity_caches_blocks()" is also confusing because the
> fsverity support layer does not cache blocks, but all the filesystems do. What
> it's actually trying to convey is a difference in how the filesystem caches the
> blocks, which I don't think it does a good job at.)

Ok, I'll drop both and open-code them with nicer comments.

> > +int fsverity_read_merkle_tree_block(struct inode *inode,
> > + const struct merkle_tree_params *params,
> > + u64 pos, unsigned long ra_bytes,
> > + struct fsverity_blockbuf *block);
> > +
> > +/*
> > + * Drop 'block' obtained with ->read_merkle_tree_block(). Calls out back to
> > + * filesystem if ->drop_merkle_tree_block() is set, otherwise, drop the
> > + * reference in the block->context.
>
> "drop the reference" => "drops the reference"

Fixed.

> > + /*
> > + * If fs uses block-based Merkle tree cachin, then fs-verity must use
> > + * hash_block_verified bitmap as there's no page to mark it with
> > + * PG_checked.
> > + */
> > + if (fsverity_uses_bitmap(vi, inode)) {
>
> The comment contradicts the code.

Urrrk, yes, it does. Sorry about that.

/*
* If the fs supplies Merkle tree blocks on a per-page basis and the
* page size doesn't match the block size, fs-verity must use the
* hash_block_verified bitmap instead of PG_checked.
*/
if (vops->read_merkle_tree_block == NULL &&
vi->tree_params.block_size != PAGE_SIZE) {

...allocate bitmap...
}

>
> > diff --git a/fs/verity/read_metadata.c b/fs/verity/read_metadata.c
> > index f58432772d9ea..94fffa060f829 100644
> > --- a/fs/verity/read_metadata.c
> > +++ b/fs/verity/read_metadata.c
> > @@ -14,65 +14,60 @@
> >
> > static int fsverity_read_merkle_tree(struct inode *inode,
> > const struct fsverity_info *vi,
> > - void __user *buf, u64 offset, int length)
> > + void __user *buf, u64 pos, int length)
> > {
> > - const struct fsverity_operations *vops = inode->i_sb->s_vop;
> > - u64 end_offset;
> > - unsigned int offs_in_page;
> > - pgoff_t index, last_index;
> > + const u64 end_pos = min(pos + length, vi->tree_params.tree_size);
> > + const struct merkle_tree_params *params = &vi->tree_params;
> > + unsigned int offs_in_block = pos & (params->block_size - 1);
> > int retval = 0;
> > int err = 0;
> >
> > - end_offset = min(offset + length, vi->tree_params.tree_size);
> > - if (offset >= end_offset)
> > + if (pos >= end_pos)
> > return 0;
>
> The above 'pos >= end_pos' check is no longer necessary.

Aha, good catch.

> > /*
> > - * Iterate through each Merkle tree page in the requested range and copy
> > - * the requested portion to userspace. Note that the Merkle tree block
> > - * size isn't important here, as we are returning a byte stream; i.e.,
> > - * we can just work with pages even if the tree block size != PAGE_SIZE.
> > + * Iterate through each Merkle tree block in the requested range and
> > + * copy the requested portion to userspace. Note that we are returning
> > + * a byte stream.
> > */
> > - for (index = offset >> PAGE_SHIFT; index <= last_index; index++) {
> > - unsigned long num_ra_pages =
> > - min_t(unsigned long, last_index - index + 1,
> > - inode->i_sb->s_bdi->io_pages);
> > - unsigned int bytes_to_copy = min_t(u64, end_offset - offset,
> > - PAGE_SIZE - offs_in_page);
> > - struct page *page;
> > - const void *virt;
> > + while (pos < end_pos) {
> > + unsigned long ra_bytes;
> > + unsigned int bytes_to_copy;
> > + struct fsverity_blockbuf block = {
> > + .size = params->block_size,
> > + };
> >
> > - page = vops->read_merkle_tree_page(inode, index, num_ra_pages);
> > - if (IS_ERR(page)) {
> > - err = PTR_ERR(page);
> > + ra_bytes = min_t(unsigned long, end_pos - pos + 1,
> > + inode->i_sb->s_bdi->io_pages << PAGE_SHIFT);
>
> This introduces an off-by-one error in the calculation of ra_bytes. end_pos
> is exclusive, but the calculation of ra_bytes assumes it is inclusive.
>
> Also, might io_pages << PAGE_SHIFT overflow an unsigned long?

I hope not -- max io and readahead should never be 4GB. I'm a little
surprised this isn't s_bdi->ra_pages.

struct backing_dev_info *bdi = inode->i_sb->s_bdi;
const u64 max_ra_bytes = min((u64)bdi->io_pages << PAGE_SHIFT,
ULONG_MAX);

while (pos < end_pos) {
ra_bytes = min_t(unsigned long, end_pos - pos, max_ra_bytes);

> > + bytes_to_copy = min_t(u64, end_pos - pos,
> > + params->block_size - offs_in_block);
> > +
> > + err = fsverity_read_merkle_tree_block(inode, &vi->tree_params,
> > + pos - offs_in_block, ra_bytes, &block);
> > + if (err) {
> > fsverity_err(inode,
> > - "Error %d reading Merkle tree page %lu",
> > - err, index);
> > + "Error %d reading Merkle tree block %llu",
> > + err, pos);
> > break;
>
> The error message should go into fsverity_read_merkle_tree_block() so that it
> does not need to be duplicated in its two callers. This would, additionally,
> eliminate the need to introduce the 'err' variable in verify_data_block().

Do you want that to be a separate cleanup patch?

> > diff --git a/fs/verity/verify.c b/fs/verity/verify.c
> > index 4fcad0825a120..0b5e11073e883 100644
> > --- a/fs/verity/verify.c
> > +++ b/fs/verity/verify.c
> > @@ -13,14 +13,27 @@
> > static struct workqueue_struct *fsverity_read_workqueue;
> >
> > /*
> > - * Returns true if the hash block with index @hblock_idx in the tree, located in
> > - * @hpage, has already been verified.
> > + * Returns true if the hash block with index @hblock_idx in the tree has
> > + * already been verified.
> > */
> > -static bool is_hash_block_verified(struct fsverity_info *vi, struct page *hpage,
> > +static bool is_hash_block_verified(struct inode *inode,
> > + struct fsverity_blockbuf *block,
> > unsigned long hblock_idx)
>
> The comment should be updated to mention @block.

I'll mention @inode too:

/*
* Returns true if the hash @block with index @hblock_idx in the merkle tree
* for @inode has already been verified.
*/

> > @@ -143,11 +156,9 @@ verify_data_block(struct inode *inode, struct fsverity_info *vi,
> > for (level = 0; level < params->num_levels; level++) {
> > unsigned long next_hidx;
> > unsigned long hblock_idx;
> > - pgoff_t hpage_idx;
> > - unsigned int hblock_offset_in_page;
> > + u64 hblock_pos;
> > unsigned int hoffset;
> > - struct page *hpage;
> > - const void *haddr;
> > + struct fsverity_blockbuf *block = &hblocks[level].block;
> >
> > /*
> > * The index of the block in the current level; also the index
> > @@ -158,36 +169,34 @@ verify_data_block(struct inode *inode, struct fsverity_info *vi,
> > /* Index of the hash block in the tree overall */
> > hblock_idx = params->level_start[level] + next_hidx;
> >
> > - /* Index of the hash page in the tree overall */
> > - hpage_idx = hblock_idx >> params->log_blocks_per_page;
> > -
> > - /* Byte offset of the hash block within the page */
> > - hblock_offset_in_page =
> > - (hblock_idx << params->log_blocksize) & ~PAGE_MASK;
> > + /* Byte offset of the hash block in the tree overall */
> > + hblock_pos = hblock_idx << params->log_blocksize;
>
> 'hblock_idx << params->log_blocksize' may overflow an unsigned long, so
> 'hblock_idx' needs to be cast to u64 before doing the shift.

Fixed.

> > + if (level == 0)
> > + ra_bytes = min(max_ra_bytes,
> > + params->tree_size - hblock_pos);
> > + else
> > + ra_bytes = 0;
>
> The first argument to min() has type unsigned long, and the second has type u64.
> Doesn't this generate a warning on 32-bit systems?

Probably. I'll turn it into a min_t(u64...).

> > @@ -325,7 +333,7 @@ void fsverity_verify_bio(struct bio *bio)
> >
> > bio_for_each_folio_all(fi, bio) {
> > if (!verify_data_blocks(fi.folio, fi.length, fi.offset,
> > - max_ra_pages)) {
> > + max_ra_pages << PAGE_SHIFT)) {
> > bio->bi_status = BLK_STS_IOERR;
> > break;
>
> This function should calculate max_ra_bytes as bio->bi_iter.bi_size >> 2. It's
> not necessary to convert to pages, then back to bytes again.

Aha, Andrey and I missed that. Fixed.

> > +/**
> > + * fsverity_read_merkle_tree_block() - read Merkle tree block
> > + * @inode: inode to which this Merkle tree blocks belong
>
> "this Merkle tree blocks belong" => "this Merkle tree block belongs"
>
> > + * @params: merkle tree parameters
> > + * @pos: byte position within merkle tree
> > + * @ra_bytes: try to read ahead this many btes
>
> "btes" => "bytes"

Both fixed.

> > +int fsverity_read_merkle_tree_block(struct inode *inode,
> > + const struct merkle_tree_params *params,
> > + u64 pos, unsigned long ra_bytes,
> > + struct fsverity_blockbuf *block)
> > +{
> > + const struct fsverity_operations *vops = inode->i_sb->s_vop;
> > + unsigned long page_idx;
> > + struct page *page;
> > + unsigned long index;
> > + unsigned int offset_in_page;
> > +
> > + if (fsverity_caches_blocks(inode)) {
> > + block->verified = false;
> > + return vops->read_merkle_tree_block(inode, pos, ra_bytes,
> > + params->log_blocksize, block);
> > + }
> > +
> > + index = pos >> params->log_blocksize;
>
> Should the fourth parameter to ->read_merkle_tree_block be the block index
> (which is computed above) instead of log_blocksize? XFS only uses
> params->log_blocksize to compute the block index anyway.

I don't know. XFS is infamous for everyone having a different opinion
and developers being unable to drive a consensus and being able to get a
patchset to completion. So:

Andrey wrote an implementation that used the buffer cache and used block
indexes to load/store from the xattr structure. I didn't like that
because layering violation.

willy suggested hanging an xarray off struct xfs_inode and using that to
cache merkle tree blocks. For that design, we want the xarray indexes
in units of blocks to conserve space, which means passing in (pos,
log_blocksize) or a direct block index.

Christoph then suggested using a per-AG rhashtable to reduce per-inode
memory overhead, in which case the lookup key can be either (inumber,
pos) or (inumber, block index). This is a better design if there aren't
really going to be that many xfs inodes with verity enabled, though we
lose per-inode sharding of the merkle blocks.

Dave thinks that verity inodes could constitute the majority of xfs
inodes some day and it should be in the inode instead.

Andrey and I do not have crystal balls, we have no idea how this
dchinner/hch disagreement will play out. Earlier I got the sense that
you wanted to move towards expressing all the merkle tree info in units
of bytes.

In a lot of ways it would be preferable to move to block indexes
instead, since there's never going to be a meaningful access to merkle
position 1337. But you're the maintainer, so it's up to you. :)

> > +/**
> > + * struct fsverity_blockbuf - Merkle Tree block buffer
> > + * @kaddr: virtual address of the block's data
> > + * @offset: block's offset into Merkle tree
> > + * @size: the Merkle tree block size
> > + * @context: filesystem private context
> > + * @verified: has this buffer been validated?
> > + *
> > + * Buffer containing single Merkle Tree block. These buffers are passed
> > + * - to filesystem, when fs-verity is building merkel tree,
> > + * - from filesystem, when fs-verity is reading merkle tree from a disk.
> > + * Filesystems sets kaddr together with size to point to a memory which contains
> > + * Merkle tree block. Same is done by fs-verity when Merkle tree is need to be
> > + * written down to disk.
>
> This comment still incorrectly claims that fsverity_blockbuf is being used for
> writes.

Oops, I forgot to delete that.

--D

>
> - Eric
>