netdev - Re: [PATCH v2] net/smc: fix refcount bug in sk_psock

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Date:   Tue,  2 Aug 2022 22:32:14 +0800
From:   Hawkins Jiawei <yin31149@...il.com>
To:     jakub@...udflare.com
Cc:     18801353760@....com, andrii@...nel.org, ast@...nel.org,
        borisp@...dia.com, bpf@...r.kernel.org, daniel@...earbox.net,
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        kgraul@...ux.ibm.com, kpsingh@...nel.org, kuba@...nel.org,
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        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        netdev@...r.kernel.org, pabeni@...hat.com, paskripkin@...il.com,
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Subject: Re: [PATCH v2] net/smc: fix refcount bug in sk_psock_get (2)

Thanks for your suggestion!

On Mon, 1 Aug 2022 at 17:16, Jakub Sitnicki <jakub@...udflare.com> wrote:
> This way we would also avoid some confusion. With the change below, the
> SK_USER_DATA_NOTPSOCK is not *always* set when sk_user_data holds a
> non-psock pointer. Only when SMC sets it.
>
> If we go with the current approach, the rest of sites, execpt for psock,
> that assign to sk_user_data should be updated to set
> SK_USER_DATA_NOTPSOCK as well, IMO.
Yes, as you point out, in this patch, this flag's name should be
*SK_USER_DATA_NEEDCHECK_NOTPSOCK*, which is more clearly.

To be more specific, we don't need to set this flag for
every sk_user_data who holds non-psock pointer. Only set the flag for
the site that has been reported involved with the type-mismatch bug
like this bug.
> > During SMC fallback process in connect syscall, kernel will
> > replaces TCP with SMC. In order to forward wakeup
> > smc socket waitqueue after fallback, kernel will sets
> > clcsk->sk_user_data to origin smc socket in
> > smc_fback_replace_callbacks().
> > 
> > Later, in shutdown syscall, kernel will calls
> > sk_psock_get(), which treats the clcsk->sk_user_data
> > as sk_psock type, triggering the refcnt warning.

For other sites, this patch is actually transparent to them, because
the *SK_USER_DATA_NEEDCHECK_NOTPSOCK* flag is always unset. So this
patch won't affect them, which won't introduce any extra
potential bugs.
> +/**
> + * rcu_dereference_sk_user_data_psock - return psock if sk_user_data points
> + * to the psock
> + * @sk: socket
> + */
> +static inline
> +struct sk_psock *rcu_dereference_sk_user_data_psock(const struct sock *sk)
> +{
> +	uintptr_t __tmp = (uintptr_t)rcu_dereference(__sk_user_data((sk)));
> +
> +	if (__tmp & SK_USER_DATA_NOTPSOCK)
> +		return NULL;
> +	return (struct sk_psock *)(__tmp & SK_USER_DATA_PTRMASK);
> +}

>
> Hi,
> Since using psock is not the common case, I'm wondering if it makes more
> sense to have an inverse flag - SK_USER_DATA_PSOCK. Flag would be set by
> the psock code on assignment to sk_user_data.
However, your suggestion seems more elegant. For my patch, as you point
out, when anyone reports a new type-mismatch bug, the relative assign to
sk_user_data should be updated to set *SK_USER_DATA_NEEDCHECK_NOTPSOCK*
flag.

For your suggestion, you seems avoid above situation. What's more, as I
use git grep to search, there seems no direct access to sk_user_data,
all via a small amount macros and wrapper functions. So we can keep
transparent by only update those macros and wrapper functions, which
also won't introduce any extra potential bugs.

I will patch as you suggest in v3 patch.