linux-kernel - Re: Build warnings in Xen 5.15.y and 5.10.y with retbleed backports

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Message-ID: <7e075c90-5adb-856a-36fb-73a8d424cd0b@oracle.com>
Date:   Mon, 18 Jul 2022 09:36:26 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, Nicolai Stange <nstange@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        Ben Hutchings <ben@...adent.org.uk>
Subject: Re: Build warnings in Xen 5.15.y and 5.10.y with retbleed backports


On 7/17/22 1:20 AM, Juergen Gross wrote:
>
> What about filling the complete hypercall page just with "int 3" or "ud2"?
>
> Any try to do a hypercall before the hypercall page has been initialized
> is a bug anyway. What good can come from calling a function which will
> return a basically random value instead of doing a privileged operation?
>

This is all about objtool, that's why 'ret' was added there originally by f4b4bc10b0b8 ("x86/xen: Support objtool vmlinux.o validation in xen-head.S").


Before that it was all 'nop' which is similar to what you are suggesting ('int3' or 'ud2' would of course be better)


-boris