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SubjectRe: [PATCH 14/39] ovl: stack file ops
On Tue, May 29, 2018 at 04:43:14PM +0200, Miklos Szeredi wrote:
> Implement file operations on a regular overlay file. The underlying file
> is opened separately and cached in ->private_data.
>
> It might be worth making an exception for such files when accounting in
> nr_file to confirm to userspace expectations. We are only adding a small
> overhead (248bytes for the struct file) since the real inode and dentry are
> pinned by overlayfs anyway.
>
> This patch doesn't have any effect, since the vfs will use d_real() to find
> the real underlying file to open. The patch at the end of the series will
> actually enable this functionality.

> +static struct file *ovl_open_realfile(const struct file *file)
> +{
> + struct inode *inode = file_inode(file);
> + struct inode *upperinode = ovl_inode_upper(inode);
> + struct inode *realinode = upperinode ?: ovl_inode_lower(inode);
> + struct file *realfile;
> + const struct cred *old_cred;
> +
> + old_cred = ovl_override_creds(inode->i_sb);
> + realfile = path_open(&file->f_path, file->f_flags | O_NOATIME,
> + realinode, current_cred(), false);
> + revert_creds(old_cred);
> +
> + pr_debug("open(%p[%pD2/%c], 0%o) -> (%p, 0%o)\n",
> + file, file, upperinode ? 'u' : 'l', file->f_flags,
> + realfile, IS_ERR(realfile) ? 0 : realfile->f_flags);
> +
> + return realfile;
> +}

IDGI. OK, you open a file in the layer you want; good, but why the hell do you
*not* use the dentry/vfsmount from the same layer?

IOW, why does your path_open() get an explicit inode argument at all? With the
rest of the work done in that series it looks like you should be able to use
vfs_open() instead... Sure, for ovlfs file you want ->f_path on overlayfs and
not in a layer, but why do the same for those?

And why bother with override_creds at all? What's wrong with simply passing
->creator_cred to path_open()/vfs_open()/whatnot?

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