[OpenWrt-Devel] [PATCH v2] ubus: use network order in ubus message header fields

[Eyal Birger]

Hi Felix,

Thanks again for your responses.

On Mon, Feb 15, 2016 at 2:14 PM Felix Fietkau <nbd at openwrt.org> wrote:

> On 2016-02-15 12:54, Eyal Birger wrote:
> >     >     >       if (offset < sizeof(ub->hdr)) {
> >     >     > -             iov[0].iov_base = ((char *) &ub->hdr) + offset;
> >     >     > -             iov[0].iov_len = sizeof(ub->hdr) - offset;
> >     >     > +             struct ubus_msghdr hdr;
> >     >     > +
> >     >     > +             hdr.version = ub->hdr.version;
> >     >     > +             hdr.type = ub->hdr.type;
> >     >     > +             hdr.seq = cpu_to_be16(ub->hdr.seq);
> >     >     > +             hdr.peer = cpu_to_be32(ub->hdr.peer);
> >     >     > +
> >     >     > +             iov[0].iov_base = ((char *) &hdr) + offset;
> >     >     > +             iov[0].iov_len = sizeof(hdr) - offset;
> >     The corner case is this: You changed the iov to point at stack space
> >     instead of ub->hdr. If the code receives a part of the header in one
> >     call, and another one in the next (offset > 0), the contents of hdr
> will
> >     be corrupt, as it will be a mix of uninitialized stack space + the
> >     received data from the last call.
> > Interesting... I initialize the iov_base every time to a newly created
> and
> > calculated hdr variable before the sendmsg() call, and iov is never used
> > otherwise - so I wonder how it could be reused in subsequent calls?
> Before your change, iov[0].iov_base points at ub->hdr, which is on heap
> and is preserved across calls.
> After your change, iov[0].iov_base points at the on-stack struct hdr,
> which is not preserved across calls.
>
>
The thing is, I don't see why the area pointed to in iov_base is required
to be preserved between calls - the sendmsg() call never uses the cached
values in iov. They are always re-armed with new pointers.




> - Felix
>
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