linux-kernel - Re: [PATCH 0/5] KVM: selftests: Fix "fix hypercall test" build errors

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Message-ID: <CALzav=cZBsmtD3+hBko3kW6bh2+RC7j533r94576ExuPx7SgEQ@mail.gmail.com>
Date:   Thu, 22 Sep 2022 10:20:05 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Jones <andrew.jones@...ux.dev>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [PATCH 0/5] KVM: selftests: Fix "fix hypercall test" build errors

On Thu, Sep 8, 2022 at 4:34 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> After a toolchain upgrade (I think), the x86 fix_hypercall_test started
> throwing warnings due to -Werror=array-bounds rightly complaining that
> the test is generating an out-of-bounds array access.
>
> The "obvious" fix is to replace the memcpy() with a memcmp() and compare
> only the exact size of the hypercall instruction.  That worked, until I
> fiddled with the code a bit more and suddenly the test started jumping into
> the weeds due to gcc generating a call to the external memcmp() through the
> PLT, which isn't supported in the selftests.
>
> To fix that mess, which has been a pitfall for quite some time, provide
> implementations of memcmp(), memcpy(), and memset() to effectively override
> the compiler built-ins.  My thought is to start with the helpers that are
> most likely to be used in guest code, and then add more as needed.

Ah ha! This also fixes an issue I've long since noticed and finally
got around to debugging this morning. userspace_io_test fails for me
when built with Clang but passes with GCC. It turns out Clang
generates a call to <memset@plt>, whereas GCC directly generates rep
stos, to clear @buffer in guest_code().