linux-kernel - Re: [RFC][PATCH 07/22] x86,extable: Extend extable functionality

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Date:   Fri, 5 Nov 2021 08:54:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mark.rutland@....com,
        dvyukov@...gle.com, seanjc@...gle.com, pbonzini@...hat.com,
        mbenes@...e.cz
Subject: Re: [RFC][PATCH 07/22] x86,extable: Extend extable functionality

On Thu, Nov 04, 2021 at 02:49:35PM -0700, Josh Poimboeuf wrote:
> On Thu, Nov 04, 2021 at 05:47:36PM +0100, Peter Zijlstra wrote:
> > +asm(
> > +"	.macro extable_type_reg type:req reg:req\n"
> > +"	.set regnr, 0\n"
> > +"	.irp rs,rax,rcx,rdx,rbx,rsp,rbp,rsi,rdi,r8,r9,r10,r11,r12,r13,r14,r15\n"
> > +"	.ifc \\reg, %\\rs\n"
> > +"	.long \\type + (regnr << 8)\n"
> > +"	.endif\n"
> > +"	.set regnr, regnr+1\n"
> > +"	.endr\n"
> > +"	.set regnr, 0\n"
> > +"	.irp rs,eax,ecx,edx,ebx,esp,ebp,esi,edi,r8d,r9d,r10d,r11d,r12d,r13d,r14d,r15d\n"
> > +"	.ifc \\reg, %\\rs\n"
> > +"	.long \\type + (regnr << 8)\n"
> > +"	.endif\n"
> > +"	.set regnr, regnr+1\n"
> > +"	.endr\n"
> > +"	.endm\n"
> > +);
> 
> How about some error checking to detect a typo, or a forgotten '%':
> 
> diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
> index 5d0ff8c60983..95bb23082b87 100644
> --- a/arch/x86/include/asm/asm.h
> +++ b/arch/x86/include/asm/asm.h
> @@ -154,9 +154,11 @@
>  
>  asm(
>  "	.macro extable_type_reg type:req reg:req\n"
> +"	.set found, 0\n"
>  "	.set regnr, 0\n"
>  "	.irp rs,rax,rcx,rdx,rbx,rsp,rbp,rsi,rdi,r8,r9,r10,r11,r12,r13,r14,r15\n"
>  "	.ifc \\reg, %\\rs\n"
> +"	.set found, found+1\n"
>  "	.long \\type + (regnr << 8)\n"
>  "	.endif\n"
>  "	.set regnr, regnr+1\n"
> @@ -164,10 +166,14 @@ asm(
>  "	.set regnr, 0\n"
>  "	.irp rs,eax,ecx,edx,ebx,esp,ebp,esi,edi,r8d,r9d,r10d,r11d,r12d,r13d,r14d,r15d\n"
>  "	.ifc \\reg, %\\rs\n"
> +"	.set found, found+1\n"
>  "	.long \\type + (regnr << 8)\n"
>  "	.endif\n"
>  "	.set regnr, regnr+1\n"
>  "	.endr\n"
> +"	.if (found != 1)\n"
> +"	.error \"extable_type_reg: bad register argument\"\n"
> +"	.endif\n"
>  "	.endm\n"
>  );

Ooh, nice! I'd actually triggered that once. At the time it was objtool
complaining .extable size wasn't a multiple of 12. Took me a while to
figure out which one had gone missing.

> > +#define EX_FLAG_CLR_AX			EX_TYPE_FLAG(1)
> > +#define EX_FLAG_CLR_DX			EX_TYPE_FLAG(2)
> > +#define EX_FLAG_CLR_AX_DX		EX_TYPE_FLAG(3)
> 
> I'd like to buy two vowels: CL̲E̲AR

Yes, can do. The macro name was longer earlier on, but in this form we
can add the two characters.

> (I hope that Wheel of Fortune reference isn't too US-centric.)

Sadly not, TV was infested with crap like that here in .nl as well.

> > +static inline unsigned long *pt_regs_nr(struct pt_regs *regs, int nr)
> > +{
> > +	/* because having pt_regs in machine order was too much to ask */
> > +	switch (nr) {
> > +	case 0:		return &regs->ax;
> > +	case 1:		return &regs->cx;
> > +	case 2:		return &regs->dx;
> > +	case 3:		return &regs->bx;
> > +	case 4:		return &regs->sp;
> > +	case 5:		return &regs->bp;
> > +	case 6:		return &regs->si;
> > +	case 7:		return &regs->di;
> > +#ifdef CONFIG_X86_64
> > +	case 8:		return &regs->r8;
> > +	case 9:		return &regs->r9;
> > +	case 10:	return &regs->r10;
> > +	case 11:	return &regs->r11;
> > +	case 12:	return &regs->r12;
> > +	case 13:	return &regs->r13;
> > +	case 14:	return &regs->r14;
> > +	case 15:	return &regs->r15;
> > +#endif
> > +	default:	return NULL;
> > +	}
> > +}
> 
> Instead of all this craziness, why not just admit defeat and put them in
> pt_regs order in the 'extable_type_reg' macro?

That makes the macro different between 32bit and 64bit :/ Also, I just
found another, extant, copy of this function, so I can get rid of it and
use that one, see get_reg_offset() in insn-eval.c

> > +static bool ex_handler_imm_reg(const struct exception_table_entry *fixup,
> > +			       struct pt_regs *regs, int reg, int imm)
> > +{
> > +	*pt_regs_nr(regs, reg) = (long)imm;
> > +	return ex_handler_default(fixup, regs);
> > +}
> > +
> > +#define EX_TYPE_MASK	0x000000FF
> > +#define EX_REG_MASK	0x00000F00
> > +#define EX_FLAG_MASK	0x0000F000
> > +#define EX_IMM_MASK	0xFFFF0000
> 
> To avoid mismatches these should probably be in the header file next to
> EX_TYPE_*_SHIFT?

Can do.

> > +
> >  int ex_get_fixup_type(unsigned long ip)
> >  {
> >  	const struct exception_table_entry *e = search_exception_tables(ip);
> >  
> > -	return e ? e->type : EX_TYPE_NONE;
> > +	return e ? FIELD_GET(EX_TYPE_MASK, e->type) : EX_TYPE_NONE;
> 
> Maybe the 'type' field should be renamed, to better represent its new
> use, and to try to discourage direct access.  Not that I have any good
> ideas.  Some not-so-good ideas: "handler", "flags", "_type".

How about the non-descript: "data" ?