在 2021/9/22 16:24, Huang Jianan 写道:

在 2021/9/22 16:06, Chengguang Xu 写道:

quoted

在 2021/9/22 15:23, Huang Jianan 写道:

quoted

From: Huang Jianan <redacted>

At present, overlayfs provides overlayfs inode to users. Overlayfs
inode provides ovl_aops with noop_direct_IO to avoid open failure
with O_DIRECT. But some compressed filesystems, such as erofs and
squashfs, don't support direct_IO.

Users who use f_mapping->a_ops->direct_IO to check O_DIRECT support,
will read file through this way. This will cause overlayfs to access
a non-existent direct_IO function and cause panic due to null pointer:

I just looked around the code more closely, in open_with_fake_path(),

do_dentry_open() has already checked O_DIRECT open flag and 
a_ops->direct_IO of underlying real address_space.

Am I missing something?

It seems that loop_update_dio will set lo->use_dio after open file 
without set O_DIRECT.
loop_update_dio will check f_mapping->a_ops->direct_IO but it deal 
with ovl_aops with
noop _direct_IO.

So I think we still need a new aops?

It means we should only set ->direct_IO for overlayfs inodes whose 
underlying fs has DIRECT IO ability.


Hi Miklos,

Is it right solution for this kind of issue? What do you think?


Thanks,

Chengguang

Thanks,
Jianan

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Thanks,

Chengguang

quoted

Kernel panic - not syncing: CFI failure (target: 0x0)
CPU: 6 PID: 247 Comm: loop0
Call Trace:
  panic+0x188/0x45c
  __cfi_slowpath+0x0/0x254
  __cfi_slowpath+0x200/0x254
  generic_file_read_iter+0x14c/0x150
  vfs_iocb_iter_read+0xac/0x164
  ovl_read_iter+0x13c/0x2fc
  lo_rw_aio+0x2bc/0x458
  loop_queue_work+0x4a4/0xbc0
  kthread_worker_fn+0xf8/0x1d0
  loop_kthread_worker_fn+0x24/0x38
  kthread+0x29c/0x310
  ret_from_fork+0x10/0x30

The filesystem may only support direct_IO for some file types. For
example, erofs supports direct_IO for uncompressed files. So return
-EINVAL when the file doesn't support direct_IO to fix this problem.

Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from 
overlayfs over xfs")
Signed-off-by: Huang Jianan <redacted>
---
change since v2:
  - Return error in ovl_open directly. (Chengguang Xu)

Change since v1:
  - Return error to user rather than fall back to buffered io. 
(Chengguang Xu)

  fs/overlayfs/file.c | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index d081faa55e83..a0c99ea35daf 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c

@@ -157,6 +157,10 @@ static int ovl_open(struct inode *inode, struct 

file *file)
      if (IS_ERR(realfile))
          return PTR_ERR(realfile);
  +    if ((f->f_flags & O_DIRECT) && (!realfile->f_mapping->a_ops ||
+        !realfile->f_mapping->a_ops->direct_IO))
+        return -EINVAL;
+
      file->private_data = realfile;
        return 0;