On 04/10, Oleg Nesterov wrote:
>
> On 04/10, Masami Hiramatsu wrote:
> >
> > (2014/04/10 22:41), Denys Vlasenko wrote:
> > > There is this monstrosity, "16-bit override for branches" in 64-mode:
> > >
> > > 66 e8 nn nn callw <offset16>
> > >
> > > Nobody sane uses it because it truncates instruction pointer.
> >
> > No problem, insn.c can handle that too. :)
>
> Does it?
>
> "callw 1f; 1:\n"
> "rep; nop\n"
>
> objdump:
>
> 66 e8 00 00 callw 485 <_init-0x3ffed3>
> f3 90 pause
>
>
> if we probe this "callw", we copy MAX_INSN_BYTES into auprobe->insn,
> and after insn_get_length() (insn_complete() == T)
>
> // this is correct
> OPCODE1() == e8
>
> // this all looks wrong
> insn->length == 6
> insn->immediate.value == -1863122944
> insn->immediate.nbytes == 4
>
> so it seems that lib/insn.c treats the next "pause" insn as the high
> 16 bits of address.
Or perhaps lib/insn.c is fine but objdump is wrong? And everything
should work correctly? Although in this case I do not understand what
this "callw" actually does.
int main(void)
{
asm (
"nop\n"
"callw 1f; 1:\n"
".byte 0\n"
".byte 0\n"
);
return 0;
}
this runs just fine. With or without gdb. And gdb shows that ->ip is
incremented by 6 after "callw".
int main(void)
{
asm (
"nop\n"
"callw 1f; 1:\n"
".byte 10\n"
".byte 20\n"
);
return 0;
}
objdump:
000000000040047c <main>:
40047c: 55 push %rbp
40047d: 48 89 e5 mov %rsp,%rbp
400480: 90 nop
400481: 66 e8 00 00 callw 485 <_init-0x3ffed3>
400485: 0a 14 b8 or (%rax,%rdi,4),%dl
400488: 00 00 add %al,(%rax)
40048a: 00 00 add %al,(%rax)
40048c: c9 leaveq
40048d: c3 retq
run:
$ ./t
Segmentation fault (core dumped)
$ gdb ./t core.*
...
#0 0x00000000144a0487 in ?? ()
0x144a0487 - 0x400481 == 0x140a0006, this matches the additional 2 .bytes
treated
as offset.
So I am totally confused.
Oleg.
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