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From: Eric Dumazet <edumazet@google.com>
Date: Wed, 28 Feb 2024 09:34:28 +0100
Message-ID: <CANn89iLQ8r3e7=RP9F7wO=b+bZbGucbuitOswkVwmijbd1Fzig@mail.gmail.com>
Subject: Re: [RFC PATCH] net, sock.h: Make sure accesses to a fullsock when it
 is indeed one
To: Ze Gao <zegao2021@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>, Jakub Kicinski <kuba@kernel.org>, 
	Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, 
	Ze Gao <zegao@tencent.com>, Honglin Li <honglinli@tencent.com>
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On Wed, Feb 28, 2024 at 8:46=E2=80=AFAM Ze Gao <zegao2021@gmail.com> 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@tencent.com>
> Signed-off-by: Ze Gao <zegao@tencent.com>
> ---
>
> 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...

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