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    SubjectRe: [PATCH] ovl: fix null pointer when filesystem doesn't support direct IO
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    在 2021/9/22 9:56, Chengguang Xu 写道:
    > 在 2021/9/18 20:13, Huang Jianan 写道:
    >> From: Huang Jianan <huangjianan@oppo.com>
    >>
    >> 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:
    >>
    >> 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 fall
    >> back to buffered io only when the file doesn't support direct_IO to
    >> fix this problem.
    >
    >
    > IMO, return error to user seems better option than fall back to
    >
    > buffered io directly.
    >
    Agreed, I will send v2 to fix it.

    Thanks,
    Jianan
    >
    > Thanks,
    >
    > Chengguang
    >
    >
    >>
    >> Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from
    >> overlayfs over xfs")
    >> Signed-off-by: Huang Jianan <huangjianan@oppo.com>
    >> ---
    >>   fs/overlayfs/file.c | 4 ++++
    >>   1 file changed, 4 insertions(+)
    >>
    >> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
    >> index d081faa55e83..998c60770b81 100644
    >> --- a/fs/overlayfs/file.c
    >> +++ b/fs/overlayfs/file.c
    >> @@ -296,6 +296,10 @@ static ssize_t ovl_read_iter(struct kiocb *iocb,
    >> struct iov_iter *iter)
    >>       if (ret)
    >>           return ret;
    >>   +    if ((iocb->ki_flags & IOCB_DIRECT) &&
    >> (!real.file->f_mapping->a_ops ||
    >> +        !real.file->f_mapping->a_ops->direct_IO))
    >> +        iocb->ki_flags &= ~IOCB_DIRECT;
    >> +
    >>       old_cred = ovl_override_creds(file_inode(file)->i_sb);
    >>       if (is_sync_kiocb(iocb)) {
    >>           ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,
    >

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