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Date:   Tue, 19 Oct 2021 09:47:26 +0000
To:     Peter Zijlstra <peterz@infradead.org>
From:   Alexander Lobakin <alobakin@pm.me>
Cc:     Alexander Lobakin <alobakin@pm.me>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, jpoimboe@redhat.com, andrew.cooper3@citrix.com,
        linux-kernel@vger.kernel.org, alexei.starovoitov@gmail.com,
        ndesaulniers@google.com
Reply-To: Alexander Lobakin <alobakin@pm.me>
Subject: Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support
Message-ID: <20211019094038.80569-1-alobakin@pm.me>
In-Reply-To: <20211019001845.85256-1-alobakin@pm.me>
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From: Alexander Lobakin <alobakin@pm.me>
Date: Tue, 19 Oct 2021 00:25:30 +0000

> From: Alexander Lobakin <alobakin@pm.me>
> Date: Mon, 18 Oct 2021 23:06:35 +0000
>
> Sorry for double posting, should've include this from the start.
>
> > Hi,
> >
> > Gave it a spin with Clang/LLVM, and
> >
> > > On Fri, Oct 15, 2021 at 04:24:08PM +0200, Borislav Petkov wrote:
> > > > On Wed, Oct 13, 2021 at 02:22:21PM +0200, Peter Zijlstra wrote:
> > > > > +static int patch_retpoline(void *addr, struct insn *insn, u8 *by=
tes)
> > > > > +{
> > > > > +=09void (*target)(void);
> > > > > +=09int reg, i =3D 0;
> > > > > +
> > > > > +=09if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
> > > > > +=09=09return -1;
> > > > > +
> > > > > +=09target =3D addr + insn->length + insn->immediate.value;
> > > > > +=09reg =3D (target - &__x86_indirect_thunk_rax) /
> > > > > +=09      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax)=
;
> >
> > this triggers
> >
> > > > I guess you should compute those values once so that it doesn't hav=
e to
> > > > do them for each function invocation. And it does them here when I =
look
> > > > at the asm it generates.
> > >
> > > Takes away the simplicity of the thing. It can't know these values at
> > > compile time (due to external symbols etc..) although I suppose LTO
> > > might be able to fix that.
> > >
> > > Other than that, the above is the trivial form of reverse indexing an
> > > array.
> > >
> > > > > +
> > > > > +=09if (WARN_ON_ONCE(reg & ~0xf))
> > > > > +=09=09return -1;
> >
> > this:
> >
> > WARN in patch_retpoline:408: addr pcibios_scan_specific_bus+0x196/0x200=
, op 0xe8, reg 0xb88ca
> > WARN in patch_retpoline:408: addr xen_pv_teardown_msi_irqs+0x8d/0x120, =
op 0xe8, reg 0xb88ca
> > WARN in patch_retpoline:408: addr __mptcp_sockopt_sync+0x7e/0x200, op 0=
xe8, reg 0xb88ca
> > [...]
> > (thousands of them, but op =3D=3D 0xe8 && reg =3D=3D 0xb88ca are always=
 the same)
>
> SMP alternatives: WARN in patch_retpoline:408: addr __strp_unpause+0x62/0=
x1b0/0xffffffff92d20a12, op 0xe8, reg 0xb88ca
> SMP alternatives: insn->length: 5, insn->immediate.value: 0xffae0989
> SMP alternatives: target: 0xffffffff928013a0/__x86_indirect_thunk_r11+0x0=
/0x20
> SMP alternatives: rax: 0xffffffff9223cd50, target - rax: 0x5c4650
> SMP alternatives: rcx - rax: 0x8
>
> Imm value and addr are different each time, the rest are the same.
> target is correct and even %pS works on it, but this distance
> between r11 and rax thunks (0x5c4650) doesn't look fine, as well as
> rcx - rax being 0x8. Thunks are 0x11 sized + alignment, should be
> 0x20, and it is, according to vmlinux.map. Weird. Amps/&s?

Oh okay, it's because of ClangCFI:

SMP alternatives: You were looking for __typeid__ZTSFvvE_global_addr+0x370/=
0x1410 at 0xffffffffa523cd60,>
SMP alternatives: rax is __typeid__ZTSFvvE_global_addr+0x360/0x1410 at 0xff=
ffffffa523cd50

Sorry for confusing, seems like it's a side effect of using it on
Clang 12 while the original series supports only 13+. I'll double
check and let know if find something.

[ snip ]

Thanks,
Al