Re: [PATCHv3 4/6] ext4: Make ext4_ind_map_blocks work with fiemap

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On 2/26/20 9:41 PM, Darrick J. Wong wrote:
On Wed, Feb 26, 2020 at 03:27:06PM +0530, Ritesh Harjani wrote:
For indirect block mapping if the i_block > max supported block in inode
then ext4_ind_map_blocks may return a -EIO error. But in case of fiemap
this could be a valid query to ext4_map_blocks.
So in case if !create then return 0. This also makes ext4_warning to
ext4_debug in ext4_block_to_path() for the same reason.

Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
---
  fs/ext4/indirect.c | 11 +++++++++--
  1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index 3a4ab70fe9e0..e1ab495dd900 100644
--- a/fs/ext4/indirect.c
+++ b/fs/ext4/indirect.c
@@ -102,7 +102,11 @@ static int ext4_block_to_path(struct inode *inode,
  		offsets[n++] = i_block & (ptrs - 1);
  		final = ptrs;
  	} else {
-		ext4_warning(inode->i_sb, "block %lu > max in inode %lu",
+		/*
+		 * It's not yet an error to just query beyond max
+		 * block in inode. Fiemap callers may do so.
+		 */
+		ext4_debug("block %lu > max in inode %lu",
  			     i_block + direct_blocks +
  			     indirect_blocks + double_blocks, inode->i_ino);

Does that mean fiemap callers can spamflood dmesg with this message just
by setting the query start range to a huge value?

Not in the old implementation. But This could happen with indirect
block mapping with new implementation in iomap (as there is no check in place before calling ext4_map_blocks()).
Previously __generic_block_fiemap() used to not query beyond
i_size_read(), so we were safe there.

So yes now as Jan also suggested, will add a check in place in
ext4_iomap_begin_report() itself, so that this flooding wont happen.


Thanks for the review!!

-ritesh


--D

  	}
@@ -537,8 +541,11 @@ int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
  	depth = ext4_block_to_path(inode, map->m_lblk, offsets,
  				   &blocks_to_boundary);
- if (depth == 0)
+	if (depth == 0) {
+		if (!(flags & EXT4_GET_BLOCKS_CREATE))
+			err = 0;
  		goto out;
+	}
partial = ext4_get_branch(inode, depth, offsets, chain, &err); --
2.21.0





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