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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 23 Mar 2020 12:30:53 -0700
Message-ID: <CAKwvOdnwhoHe8ouao2VBo1meRd8H4EOC7Nr8hnFkbXBACWRm9w@mail.gmail.com>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in handle_external_interrupt_irqoff
To:     Alexander Potapenko <glider@google.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        syzbot <syzbot+3f29ca2efb056a761e38@syzkaller.appspotmail.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, KVM list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>
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On Mon, Mar 23, 2020 at 11:49 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> So maybe we can find why
> commit 76b043848fd2 ("x86/retpoline: Add initial retpoline support")
> added THUNK_TARGET with and without "m" constraint, and either:
> - remove "m" from THUNK_TARGET. (Maybe this doesn't compile somewhere)
> or
> - use my above recommendation locally avoiding THUNK_TARGET.  We can
> use "r" rather than "a" (what Clang would have picked) or "b (what GCC
> would have picked) to give the compilers maximal flexibility.

So I've sent a patch for the latter; my reason for not pursuing the former is:
1. I assume that the thunk target could be spilled, or a pointer, and
we'd like to keep flexibility for the general case of inline asm that
doesn't modify the stack pointer.
2. `entry` is local to `handle_external_interrupt_irqoff`; it's not
being passed in via pointer as a function parameter.
3. register pressure is irrelevant if the resulting code is incorrect.
-- 
Thanks,
~Nick Desaulniers