* H. Peter Anvin (h...@linux.intel.com) wrote: > On 08/05/2013 02:28 PM, Mathieu Desnoyers wrote: > > * Linus Torvalds (torva...@linux-foundation.org) wrote: > >> On Mon, Aug 5, 2013 at 12:54 PM, Mathieu Desnoyers > >> <mathieu.desnoy...@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. http://www.efficios.com
