On Wed, May 12, 2021 at 07:57:54PM +0200, David Sterba wrote:
> On Wed, May 05, 2021 at 12:20:41PM -0700, Boris Burkov wrote:
> > The majority of reads receive a verity check after the bio is complete
> > as the page is marked uptodate. However, there is a class of reads which
> > are handled with btrfs logic in readpage, rather than by submitting a
> > bio. Specifically, these are inline extents, preallocated extents, and
> > holes. Tweak readpage so that if it is going to mark such a page
> > uptodate, it first checks verity on it.
>
> So verity works with inline extents and fills the unused space by zeros
> before hashing?
There is no special logic to zero the unused space for verity, we just
ship the page off to the VFS verity code before marking it Uptodate. The
inline extent logic in btrfs_get_extent does zero the parts of the page
past the data copied in.
>
> > Now if a veritied file has corruption to this class of EXTENT_DATA
> > items, it will be detected at read time.
> >
> > There is one annoying edge case that requires checking for start <
> > last_byte: if userspace reads to the end of a file with page aligned
> > size and then tries to keep reading (as cat does), the buffered read
> > code will try to read the page past the end of the file, and expects it
> > to be filled with 0s and marked uptodate. That bogus page is not part of
> > the data hashed by verity, so we have to ignore it.
> >
> > Signed-off-by: Boris Burkov <boris@xxxxxx>
> > ---
> > fs/btrfs/extent_io.c | 26 +++++++-------------------
> > 1 file changed, 7 insertions(+), 19 deletions(-)
> >
> > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > index d1f57a4ad2fb..d1493a876915 100644
> > --- a/fs/btrfs/extent_io.c
> > +++ b/fs/btrfs/extent_io.c
> > @@ -2202,18 +2202,6 @@ int test_range_bit(struct extent_io_tree *tree, u64 start, u64 end,
> > return bitset;
> > }
> >
> > -/*
> > - * helper function to set a given page up to date if all the
> > - * extents in the tree for that page are up to date
> > - */
> > -static void check_page_uptodate(struct extent_io_tree *tree, struct page *page)
> > -{
> > - u64 start = page_offset(page);
> > - u64 end = start + PAGE_SIZE - 1;
> > - if (test_range_bit(tree, start, end, EXTENT_UPTODATE, 1, NULL))
> > - SetPageUptodate(page);
> > -}
> > -
> > int free_io_failure(struct extent_io_tree *failure_tree,
> > struct extent_io_tree *io_tree,
> > struct io_failure_record *rec)
> > @@ -3467,14 +3455,14 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
> > &cached, GFP_NOFS);
> > unlock_extent_cached(tree, cur,
> > cur + iosize - 1, &cached);
> > - end_page_read(page, true, cur, iosize);
> > + ret = end_page_read(page, true, cur, iosize);
>
> Latest version of end_page_read does not return any value.
In case you missed it, I modified it to return a value in the second
patch (btrfs: initial support for fsverity)
>
> > break;
> > }
> > em = __get_extent_map(inode, page, pg_offset, cur,
> > end - cur + 1, em_cached);
> > if (IS_ERR_OR_NULL(em)) {
> > unlock_extent(tree, cur, end);
> > - end_page_read(page, false, cur, end + 1 - cur);
> > + ret = end_page_read(page, false, cur, end + 1 - cur);
> > break;
> > }
> > extent_offset = cur - em->start;
> > @@ -3555,9 +3543,10 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
> >
> > set_extent_uptodate(tree, cur, cur + iosize - 1,
> > &cached, GFP_NOFS);
> > +
> > unlock_extent_cached(tree, cur,
> > cur + iosize - 1, &cached);
> > - end_page_read(page, true, cur, iosize);
> > + ret = end_page_read(page, true, cur, iosize);
>
> And if it would, you'd have to check it in all cases when it's not
> followed by break, like here.
Agreed. I think I got "lucky" because the continues all break the loop in
the cases I've tried. Thinking about it more, it looks like I need to set
the error bit on the page too, so that might work without end_page_read
having a return value.
>
> > cur = cur + iosize;
> > pg_offset += iosize;
> > continue;
> > @@ -3565,9 +3554,8 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
> > /* the get_extent function already copied into the page */
> > if (test_range_bit(tree, cur, cur_end,
> > EXTENT_UPTODATE, 1, NULL)) {
> > - check_page_uptodate(tree, page);
> > unlock_extent(tree, cur, cur + iosize - 1);
> > - end_page_read(page, true, cur, iosize);
> > + ret = end_page_read(page, true, cur, iosize);
> > cur = cur + iosize;
> > pg_offset += iosize;
> > continue;
> > @@ -3577,7 +3565,7 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
> > */
> > if (block_start == EXTENT_MAP_INLINE) {
> > unlock_extent(tree, cur, cur + iosize - 1);
> > - end_page_read(page, false, cur, iosize);
> > + ret = end_page_read(page, false, cur, iosize);
> > cur = cur + iosize;
> > pg_offset += iosize;
> > continue;
> > @@ -3595,7 +3583,7 @@ int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
> > *bio_flags = this_bio_flag;
> > } else {
> > unlock_extent(tree, cur, cur + iosize - 1);
> > - end_page_read(page, false, cur, iosize);
> > + ret = end_page_read(page, false, cur, iosize);
> > goto out;
> > }
> > cur = cur + iosize;
> > --
> > 2.30.2
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