linux-kernel - Re: [PATCH 1/1] nospec: Add documentation for array_index

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Message-ID: <5d517282-9dfe-5508-8dbc-a20f862245e1@intel.com>
Date:   Fri, 16 Jun 2023 09:18:28 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jordy Zomer <jordyzomer@...gle.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc:     corbet@....net, dave.hansen@...ux.intel.com, daniel@...earbox.net,
        tglx@...utronix.de, rdunlap@...radead.org,
        pawan.kumar.gupta@...ux.intel.com
Subject: Re: [PATCH 1/1] nospec: Add documentation for array_index_nospec

On 6/16/23 06:37, Jordy Zomer wrote:
> array_index_nospec() should only be used if the upper boundary is a built
> time constant. Otherwise the boundary could be speculated on as well.
> While it might be more difficult to control two loads, it doesn't rule
> out the problem. Adding this to the documentation so people won't mis-use
> it instead of barrier_nospec().

Then shouldn't we be using __builtin_constant_p() to enforce this?