On Thu, 30 Jul 2015, Andy Lutomirski wrote:
> On Thu, Jul 30, 2015 at 8:41 AM, Paolo Bonzini <[email protected]> wrote: > > > > > > On 24/07/2015 23:08, Andy Lutomirski wrote: > >> user_icebp is set if int $0x01 happens, except it isn't because user > >> code can't actually do that -- it'll cause #GP instead. > >> > >> user_icebp is also set if the user has a bloody in-circuit emulator, > >> given the name. But who on Earth has one of those on a system new > >> enough to run Linux and, even if they have one, why on Earth are they > >> using it to send SIGTRAP. > > > > You do not need either "int $0x01" or an ICE to set user_icebp = 1. You > > can use the 0xf1 opcode, which is kinda like 0xcc but generates #DB > > instead of #BP. > > Great. There's an opcode that invokes an interrupt gate that's not > marked as allowing unprivileged access, and that opcode doesn't appear > in the SDM. It appears in the APM opcode map with no explanation at > all. The only SDM reference I found is: "The opcodes D6 and F1 are undefined opcodes reserved by the Intel 64 and IA-32 architectures. These opcodes, even though undefined, do not generate an invalid opcode exception." D6 is actually something useful: if (carry flag set) AL = FF else AL = 0 It's been there since i386. It has been conveniant for return code magic from ASM to C. I haven't thought of it for at least a decade :) So all we need to worry about is F1, but thats bad enough :( Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
