[Virtio-fs] [PATCH] overlayfs: Pass O_TRUNC flag to underlying filesystem

[Miklos Szeredi]

On Tue, Apr 21, 2020 at 8:41 PM Vivek Goyal <vgoyal at redhat.com> wrote:
>
> As of now during open(), we don't pass bunch of flags to underlying
> filesystem. O_TRUNC is one of these. Normally this is not a problem as VFS
> calls ->setattr() with zero size and underlying filesystem sets file size
> to 0.
>
> But when overlayfs is running on top of virtiofs, it has an optimization
> where it does not send setattr request to server if dectects that
> truncation is part of open(O_TRUNC). It assumes that server already zeroed
> file size as part of open(O_TRUNC).
>
> fuse_do_setattr() {
>         if (attr->ia_valid & ATTR_OPEN) {
>                 /*
>                  * No need to send request to userspace, since actual
>                  * truncation has already been done by OPEN.  But still
>                  * need to truncate page cache.
>                  */
>         }
> }
>
> IOW, fuse expects O_TRUNC to be passed to it as part of open flags.
>
> But currently overlayfs does not pass O_TRUNC to underlying filesystem
> hence fuse/virtiofs breaks. Setup overlayfs on top of virtiofs and
> following does not zero the file size of a file is either upper only
> or has already been copied up.
>
> fd = open(foo.txt, O_TRUNC | O_WRONLY);
>
> Fix it by passing O_TRUNC to underlying filesystem.


Or clear ATTR_OPEN in ovl_setattr()

Need to think about side effects of passing O_TRUNC down to underlying
fs.   Clearing ATTR_OPEN seems obviously safe, so as a quick fix I'd
rather go with that for now.

Thanks,
Miklos