Re: [RFC] gcc feature request: Moving blocks into sections

Mathieu Desnoyers
2013-08-06T04:14:43Z

* H. Peter Anvin (hpa@linux.intel.com) wrote:
> On 08/05/2013 02:28 PM, Mathieu Desnoyers wrote:
> > * Linus Torvalds (torvalds@linux-foundation.org) wrote:
> >> On Mon, Aug 5, 2013 at 12:54 PM, Mathieu Desnoyers
> >> <mathieu.desnoyers@efficios.com> wrote:
> >>>
> >>> I remember that choosing between 2 and 5 bytes nop in the asm goto was
> >>> tricky: it had something to do with the fact that gcc doesn't know the
> >>> exact size of each instructions until further down within compilation
> >>
> >> Oh, you can't do it in the coompiler, no. But you don't need to. The
> >> assembler will pick the right version if you just do "jmp target".
> >
> > Yep.
> >
> > Another thing that bothers me with Steven's approach is that decoding
> > jumps generated by the compiler seems fragile IMHO.
> >
> > x86 decoding proposed by https://lkml.org/lkml/2012/3/8/464 :
> >
> > +static int make_nop_x86(void *map, size_t const offset)
> > +{
> > + unsigned char *op;
> > + unsigned char *nop;
> > + int size;
> > +
> > + /* Determine which type of jmp this is 2 byte or 5. */
> > + op = map + offset;
> > + switch (*op) {
> > + case 0xeb: /* 2 byte */
> > + size = 2;
> > + nop = ideal_nop2_x86;
> > + break;
> > + case 0xe9: /* 5 byte */
> > + size = 5;
> > + nop = ideal_nop;
> > + break;
> > + default:
> > + die(NULL, "Bad jump label section (bad op %x)\n", *op);
> > + __builtin_unreachable();
> > + }
> >
> > My though is that the code above does not cover all jump encodings that
> > can be generated by past, current and future x86 assemblers.
> >
>
> For unconditional jmp that should be pretty safe barring any fundamental
> changes to the instruction set, in which case we can enable it as
> needed, but for extra robustness it probably should skip prefix bytes.

On x86-32, some prefixes are actually meaningful. AFAIK, the 0x66 prefix
is used for:

E9 cw jmp rel16 relative jump, only in 32-bit

Other prefixes can probably be safely skipped.

Another question is whether anything prevents the assembler from
generating a jump near (absolute indirect), or far jump. The code above
seems to assume that we have either a short or near relative jump.

Thoughts ?

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.