Re: [PATCH 7/8] tools/objtool: Check for use of the ENQCMD instruction in the kernel

[Fenghua Yu]

Hi, Peter,

On Thu, Sep 23, 2021 at 09:17:01AM +0200, Peter Zijlstra wrote:
> On Wed, Sep 22, 2021 at 11:44:41PM +0000, Fenghua Yu wrote:
> 
> > > Since you're making it a fatal error, before doing much of anything
> > > else, you might at well fail decode and keep it all in the x86/decode.c
> > > file, no need to spread this 'knowledge' any further.
> 
> > Is the following updated patch a right one?
> 
> Yes, that's what I was thinking of.
> 
> > diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> > index bc821056aba9..3e0f928e28a5 100644
> > --- a/tools/objtool/arch/x86/decode.c
> > +++ b/tools/objtool/arch/x86/decode.c
> > @@ -110,7 +110,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
> >  {
> >  	struct insn insn;
> >  	int x86_64, ret;
> > -	unsigned char op1, op2,
> > +	unsigned char op1, op2, op3,
> >  		      rex = 0, rex_b = 0, rex_r = 0, rex_w = 0, rex_x = 0,
> >  		      modrm = 0, modrm_mod = 0, modrm_rm = 0, modrm_reg = 0,
> >  		      sib = 0, /* sib_scale = 0, */ sib_index = 0, sib_base = 0;
> > @@ -137,6 +137,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
> >  
> >  	op1 = insn.opcode.bytes[0];
> >  	op2 = insn.opcode.bytes[1];
> > +	op3 = insn.opcode.bytes[2];
> >  
> >  	if (insn.rex_prefix.nbytes) {
> >  		rex = insn.rex_prefix.bytes[0];
> > @@ -489,6 +490,16 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
> >  			/* nopl/nopw */
> >  			*type = INSN_NOP;
> >  
> > +		} else if (op2 == 0x38 && op3 == 0xf8) {
> > +			if (insn.prefixes.nbytes == 1 &&
> > +			    insn.prefixes.bytes[0] == 0xf2) {
> > +				/* ENQCMD cannot be used in the kernel. */
> > +				WARN("ENQCMD instruction at %s:%lx", sec->name,
> > +				     offset);
> > +
> > +				return -1;
> > +			}
> 
> The only concern here is if we want it to be fatal or not. But otherwise
> this seems to be all that's required.

objtool doesn't fail kernel build on this fatal warning.

Returning -1 here stops checking the rest of the file and won't report any
further warnings unless this ENQCMD warning is fixed. Not returning -1
continues checking the rest of the file and may report more warnings.
Seems that's the only difference b/w them.

Should I keep this "return -1" or not? Please advice.

-Fenghua