Re: [RFC PATCH] net, sock.h: Make sure accesses to a fullsock when it is indeed one

[Ze Gao]

On Wed, Feb 28, 2024 at 4:34 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Wed, Feb 28, 2024 at 8:46 AM Ze Gao <zegao2021@xxxxxxxxx> wrote:
> >
> > We know a pointer that has type struct sock* can actually points to
> > one of some different sock types which have different memory layouts,
> > take req_to_sk() for example, and whether a sock is full or not
> > depends upon ->sk_state which is a shared field among them so that we
> > see some repeated code pattern similar to this:
> >
> >         if (sk && sk fullsock(sk) && sk->field_not_shared)
> >
> > which seems to have no problem at the first glance, but it is actually
> > unsound in a way that ->field_not_shared is likely uninitialized (or
> > unmapped) when it's not a full sock, and a compiler is free to reorder
> > accesses to fields of a struct sock when it can, that is, it could
> > reorder accesses to ->field_not_shared across ->sk_state or load them
> > all before the branch test, which leads to unexpected behavior, although
> > most of them won't do this.
> >
> > So leave a barrier() in between and force the compiler to keep the
> > obvious program order.
> >
> > Cc: Honglin Li <honglinli@xxxxxxxxxxx>
> > Signed-off-by: Ze Gao <zegao@xxxxxxxxxxx>
> > ---
> >
> > IIUC, casting a pointer to refer to a bigger object in size is
> > technically UB, which may lead to unsound code. From the POV of
> > a compiler, when it is allowed to assume that one struct member
> > is valid, they all are through a pointer, and thus it's likely
> > for the compiler to do such optimizations and reorder what we
> > want to keep in order.
> >
> > Note this is not a typical way to use barrier(), which only
> > acts an ok fix to what's already unsound, at least IMO.
> >
> > Comments are welcome, since I'm not an expert in C and I know
> > most of compilers won't do this reorder, but I'm being pessimistic
> > here.
>
> Well, my suggestion is to have evidence first...

Fair point!  my initial purpose is to raise my question here to check if
there is UB here and see if C experts have insights on this.

> We are not going to add barriers just because we do not trust
> compilers handling of sequence points.

Makes sense to me as well if this is indeed not an issue here.

Thanks,
        -- Ze