linux-kernel - Re: [PATCH 4/9] x86/alternative: Implement .retpoline

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Date:   Mon, 18 Oct 2021 23:06:35 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexander Lobakin <alobakin@...me>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support

From: Peter Zijlstra <peterz@...radead.org>
Date: Fri, 15 Oct 2021 18:56:35 +0200

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 *bytes)
> > > +{
> > > +	void (*target)(void);
> > > +	int reg, i = 0;
> > > +
> > > +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
> > > +		return -1;
> > > +
> > > +	target = addr + insn->length + insn->immediate.value;
> > > +	reg = (target - &__x86_indirect_thunk_rax) /
> > > +	      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);

this triggers

> > I guess you should compute those values once so that it doesn't have 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.
>
> > > +
> > > +	if (WARN_ON_ONCE(reg & ~0xf))
> > > +		return -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 0xe8, reg 0xb88ca
[...]
(thousands of them, but op == 0xe8 && reg == 0xb88ca are always the same)

I know this reg calculation is about to be replaced, but anyway ;)

> > Sanity-checking the alignment of those thunks?
>
> Nah, the target address of the instruction; if that's not a retpoline
> thunk (for whatever raisin) then the computation will not result in a
> valid reg and we should bail.
>
> > > +
> > > +	i = emit_indirect(insn->opcode.bytes[0], reg, bytes);
> > > +	if (i < 0)
> > > +		return i;
> > > +
> > > +	for (; i < insn->length;)
> > > +		bytes[i++] = BYTES_NOP1;
> >
> > Why not:
> >
> >         nop_len = insn->length - i;
> >         if (nop_len) {
> >                 memcpy(&bytes[i], x86_nops[nop_len], nop_len);
> >                 i += nop_len;
> >         }
> >
> > and then you save yourself the optimize_nops() call because it'll take
> > the right-sized NOP directly.
>
> That's not immediately safe; if for some reason or other the original
> instrucion is 15 bytes long, and we generated 2 bytes, then we need 13
> nop bytes, the above will then do an out-of-bound array access (due to
> the nops array only doing 8 byte nops at max).
>
> I wanted this code to be simple and obvious.

Thanks,
Al