On Thu, Jan 09, 2025 at 04:41:50PM +0900, Akihiko Odaki wrote: > On 2025/01/09 16:31, Michael S. Tsirkin wrote: > > On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote: > > > tun used to simply advance iov_iter when it needs to pad virtio header, > > > which leaves the garbage in the buffer as is. This is especially > > > problematic when tun starts to allow enabling the hash reporting > > > feature; even if the feature is enabled, the packet may lack a hash > > > value and may contain a hole in the virtio header because the packet > > > arrived before the feature gets enabled or does not contain the > > > header fields to be hashed. If the hole is not filled with zero, it is > > > impossible to tell if the packet lacks a hash value. > > > > > > In theory, a user of tun can fill the buffer with zero before calling > > > read() to avoid such a problem, but leaving the garbage in the buffer is > > > awkward anyway so fill the buffer in tun. > > > > > > Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> > > > > But if the user did it, you have just overwritten his value, > > did you not? > > Yes. but that means the user expects some part of buffer is not filled after > read() or recvmsg(). I'm a bit worried that not filling the buffer may break > assumptions others (especially the filesystem and socket infrastructures in > the kernel) may have. > > If we are really confident that it will not cause problems, this behavior > can be opt-in based on a flag or we can just write some documentation > warning userspace programmers to initialize the buffer. It's been like this for years, I'd say we wait until we know there's a problem? > > > > > --- > > > drivers/net/tun_vnet.c | 3 ++- > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c > > > index fe842df9e9ef..ffb2186facd3 100644 > > > --- a/drivers/net/tun_vnet.c > > > +++ b/drivers/net/tun_vnet.c > > > @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter, > > > if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr)) > > > return -EFAULT; > > > - iov_iter_advance(iter, sz - sizeof(*hdr)); > > > + if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr)) > > > + return -EFAULT; > > > return 0; > > > } > > > > > > -- > > > 2.47.1 > >
