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SubjectRe: [RFC PATCH v2 5/6] uprobes/x86: Emulate rip-relative conditional "short" jmp's
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On Mon, 2014-04-07 at 16:27 +0200, Oleg Nesterov wrote:
> On 04/06, Oleg Nesterov wrote:
> >
> > Incomplete, lacks "jcxz". Simple to fix. Anything else?
>
> Please see v2 below. Simplify the preprocessor hacks.
>
> -------------------------------------------------------------------------------
> Subject: [RFC PATCH v2 5/6] uprobes/x86: Emulate rip-relative conditional "short" jmp's
>
> Incomplete, lacks "jcxz". Simple to fix. Anything else?
>
> Reported-by: Jonathan Lebon <jlebon@redhat.com>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
> arch/x86/kernel/uprobes.c | 61 +++++++++++++++++++++++++++++++++++++++++++-
> 1 files changed, 59 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 9283024..3865d8b 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -466,18 +466,72 @@ static bool ttt_is_call(struct arch_uprobe *auprobe)
> return auprobe->ttt.opc1 == 0xe8;
> }
>
> +#define CASE_COND \
> + COND(70, 71, XF(OF)) \
> + COND(72, 73, XF(CF)) \
> + COND(74, 75, XF(ZF)) \
> + COND(78, 79, XF(SF)) \
> + COND(7a, 7b, XF(PF)) \
> + COND(76, 77, XF(CF) || XF(ZF)) \
> + COND(7c, 7d, XF(SF) != XF(OF)) \
> + COND(7e, 7f, XF(ZF) || XF(SF) != XF(OF))
> +
> +#define COND(op_y, op_n, expr) \
> + case 0x ## op_y: DO((expr) != 0) \
> + case 0x ## op_n: DO((expr) == 0)
> +
> +#define XF(xf) (!!(flags & X86_EFLAGS_ ## xf))

All this macro magic seems way more clever than it is legible.

Given that you're mapping 0f 8x to 7x (patch #6), is_cond_jmp_opcode()
could just be
return (0x70 <= opcode && opcode <= 0x7f);

I would keep the XF macro (although the !! operation to convert non-zero
to 1 isn't strictly needed) and just do an explicit 16-case switch for
check_jmp_cond().

> +
> +static bool is_cond_jmp_opcode(u8 opcode)
> +{
> + switch (opcode) {
> + #define DO(expr) \
> + return true;
> + CASE_COND
> + #undef DO
> +
> + default:
> + return false;
> + }
> +}
> +
> +static bool check_jmp_cond(struct arch_uprobe *auprobe, struct pt_regs *regs)
> +{
> + unsigned long flags = regs->flags;
> +
> + switch (auprobe->ttt.opc1) {
> + case 0x00: /* not a conditional jmp */
> + return true;
> +
> + #define DO(expr) \
> + return expr;
> + CASE_COND
> + #undef DO
> +
> + default:
> + BUG();
> + }
> +}
> +
> +#undef XF
> +#undef COND
> +#undef CASE_COND
> +
> static bool ttt_emulate_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
> {
> unsigned long new_ip = regs->ip += auprobe->ttt.ilen;
> + unsigned long disp = auprobe->ttt.disp;

Looks like a negative ttt.disp will get sign-extended like you want, but
still, making disp unsigned here doesn't seem quite right.

>
> if (ttt_is_call(auprobe)) {
> unsigned long new_sp = regs->sp - sizeof_long();
> if (copy_to_user((void __user *)new_sp, &new_ip, sizeof_long()))
> return false;
> regs->sp = new_sp;
> + } else if (!check_jmp_cond(auprobe, regs)) {
> + disp = 0;
> }
>
> - regs->ip = new_ip + auprobe->ttt.disp;
> + regs->ip = new_ip + disp;
> return true;
> }
>
> @@ -536,8 +590,11 @@ static int ttt_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
> ttt_clear_displacement(auprobe, insn);
> auprobe->ttt.opc1 = opc1;
> break;
> +
> default:
> - return -ENOSYS;
> + if (!is_cond_jmp_opcode(opc1))
> + return -ENOSYS;
> + auprobe->ttt.opc1 = opc1;
> }
>
> auprobe->ttt.ilen = insn->length;




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