Re: [PATCH 3/3] ext4: prevent getting empty inode buffer

On Wed 18-08-21 20:15:59, Zhang Yi wrote:

On 2021/8/17 1:14, Jan Kara wrote:

quoted

On Mon 16-08-21 22:29:01, Zhang Yi wrote:

quoted

On 2021/8/13 21:44, Jan Kara wrote:

quoted

On Tue 10-08-21 22:27:22, Zhang Yi wrote:

quoted

In ext4_get_inode_loc(), we may skip IO and get an zero && uptodate
inode buffer when the inode monopolize an inode block for performance
reason. For most cases, ext4_mark_iloc_dirty() will fill the inode
buffer to make it fine, but we could miss this call if something bad
happened. Finally, __ext4_get_inode_loc_noinmem() may probably get an
empty inode buffer and trigger ext4 error.

For example, if we remove a nonexistent xattr on inode A,
ext4_xattr_set_handle() will return ENODATA before invoking
ext4_mark_iloc_dirty(), it will left an uptodate but zero buffer. We
will get checksum error message in ext4_iget() when getting inode again.

  EXT4-fs error (device sda): ext4_lookup:1784: inode #131074: comm cat: iget: checksum invalid

Even worse, if we allocate another inode B at the same inode block, it
will corrupt the inode A on disk when write back inode B.

So this patch clear uptodate flag and mark buffer new if we get an empty
buffer, clear it after we fill inode data or making read IO.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>

Thanks for the fix! Really good catch! The patch looks correct but
honestly, I'm not very happy about the special buffer_new handling. It
looks correct but I'm a bit uneasy that e.g. the block device code can
access this buffer and manipulate its state. Cannot we instead e.g. check
whether the buffer is uptodate in ext4_mark_iloc_dirty(), if not, lock it,
if still not uptodate, zero it, mark as uptodate, unlock it and then call
ext4_do_update_inode()? That would seem like a bit more foolproof solution
to me. Basically the fact that the buffer is not uptodate in
ext4_mark_iloc_dirty() would mean that nobody else is past
__ext4_get_inode_loc() for another inode in that buffer and so zeroing is
safe.

Thanks for your suggestion! I understand what you're concerned and your
approach looks fine except mark buffer uptodate just behind zero buffer
in ext4_mark_iloc_dirty(). Because I think (1) if ext4_do_update_inode()
return error before filling the inode, it will still left an uptodate
but zero buffer, and it's not easy to handle the error path. (2) it is
still not conform the semantic of buffer uptodate because it it not
contain an uptodate inode information. How about move mark as uptodate
into ext4_do_update_inode(), something like that（not tested）？

OK, but this way could loading of buffer from the disk race with
ext4_do_update_inode() and overwrite its updates? You have to have buffer
uptodate before you start modifying it or you have to keep the buffer
locked all the time while you are updating it to avoid such races.

Indeed.

quoted

Luckily the only place where ext4_do_update_inode() can fail before copying
data to the buffer is due to ext4_inode_blocks_set() which should never
happen because we set s_maxsize so that i_blocks cannot overflow. So maybe
we can just get rid of that case and keep the uptodate setting with the
zeroing?

It's fine, Let's fix it this way now.(But I guess it's fragile because we
have to prevent modify ext4_do_update_inode() return before filling data
into inode buffer cautiously in the future.)

I guess can have "bring buffer uptodate" code in ext4_do_update_inode() to
have it closer to the code filling the buffer with correct data and comment
there that once we mark the buffer as uptodate, we rely on filling in the
correct information.

BTW, could we also add a patch to just remove the
ext4_has_feature_huge_file() check in ext4_inode_blocks_set() or move it
to ext4_mark_iloc_dirty() before ext4_mark_iloc_dirty()? Or else we may
get confused and have to add comments to explain it.

Yes, I would transform that check to WARN_ON_ONCE() with a comment that
sb->s_maxbytes should not have allowed this.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR