linux-kernel - Re: [PATCH 2/3] x86/insn-eval: Introduce insn_decode

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Date:   Tue, 19 Oct 2021 10:34:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH 2/3] x86/insn-eval: Introduce insn_decode_mmio()

On Mon, Oct 18, 2021 at 07:49:41PM +0300, Kirill A. Shutemov wrote:
> On Mon, Oct 18, 2021 at 05:53:49PM +0200, Peter Zijlstra wrote:
> > On Mon, Oct 18, 2021 at 06:33:32PM +0300, Kirill A. Shutemov wrote:
> > 
> > > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> > > index fbaa3fa24bde..2ab29d8d6731 100644
> > > --- a/arch/x86/lib/insn-eval.c
> > > +++ b/arch/x86/lib/insn-eval.c
> > > @@ -1559,3 +1559,85 @@ bool insn_decode_from_regs(struct insn *insn, struct pt_regs *regs,
> > >  
> > >  	return true;
> > >  }
> > > +
> > > +/**
> > > + * insn_decode_mmio() - Decode a MMIO instruction
> > > + * @insn:	Structure to store decoded instruction
> > > + * @bytes:	Returns size of memory operand
> > > + *
> > > + * Decodes instruction that used for Memory-mapped I/O.
> > > + *
> > > + * Returns:
> > > + *
> > > + * Type of the instruction. Size of the memory operand is stored in
> > > + * @bytes. If decode failed, MMIO_DECODE_FAILED returned.
> > > + */
> > > +enum mmio_type insn_decode_mmio(struct insn *insn, int *bytes)
> > > +{
> > > +	int type = MMIO_DECODE_FAILED;
> > > +
> > > +	*bytes = 0;
> > > +
> > > +	insn_get_opcode(insn);
> > 
> > insn_get_opcode() can fail. Either you assume it's already called and
> > don't call it, or you can't assume anything and get to do error
> > handling.
> 
> Fair enough. I will return MMIO_DECODE_FAILED if insn_get_opcode() fails.
> 
> BTW, looks like is_string_insn() suffers from the same issue. Not sure how
> to fix it though.

AFAICT all callers of insn_get_addr_ref() (which is what
is_string_insn() seems to be part of) do a insn_decode_*() call with
error checking before.

So it looks like that insn_get_opcode() in there is superfluous.
probably same for insn_has_rep_prefix() / get_seg_reg_override_idx() /
get_eff_addr_modrm_*(). That all wants cleaning up.

The esaiest way is probably to push those things up the callchains into
the !static function and have it fail early there.