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

Sean Christopherson
2022-09-22T18:16:28Z

On Thu, Sep 22, 2022, Jim Mattson wrote:
> On Thu, Sep 22, 2022 at 10:20 AM David Matlack <dmatlack@google.com> wrote:
> >
> > On Thu, Sep 8, 2022 at 4:34 PM Sean Christopherson <seanjc@google.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().
>
> Hey! Did I miss a revert of commit ed290e1c20da ("KVM: selftests: Fix
> nested SVM tests when built with clang") in that patch set?

LOL, no, no you did not. I'll do that in v2.