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

H. Peter Anvin
2013-08-05T21:43:55Z

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.

-hpa