linux-kernel - Re: [PATCH v3 10/16] x86/alternative: Implement .retpoline

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Date:   Wed, 27 Oct 2021 21:10:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com, bpf@...r.kernel.org
Subject: Re: [PATCH v3 10/16] x86/alternative: Implement .retpoline_sites
 support

On Wed, Oct 27, 2021 at 07:38:59PM +0200, Borislav Petkov wrote:
> On Tue, Oct 26, 2021 at 02:01:42PM +0200, Peter Zijlstra wrote:
> > +static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
> > +{
> > +	retpoline_thunk_t *target;
> > +	int reg, i = 0;
> > +
> > +	target = addr + insn->length + insn->immediate.value;
> > +	reg = target - __x86_indirect_thunk_array;
> > +
> > +	if (WARN_ON_ONCE(reg & ~0xf))
> > +		return -1;
> > +
> > +	/* If anyone ever does: CALL/JMP *%rsp, we're in deep trouble. */
> > +	BUG_ON(reg == 4);
> > +
> > +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
> > +		return -1;
> 
> I wanna say this should be the first thing being checked on function
> entry but I get the feeling you'll be looking at other X86_FEATURE bits
> in future patches... /me goes into the future...
> 
> yap, you do. Lemme look at the whole thing first then.

I wanted the sanity checks done unconditionally.