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

Peter Zijlstra
2021-11-05T07:54:20Z

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" ?