Current kprobe copies the original instruction at the probe point and replaces
it with a breakpoint instruction (int3).  When the kernel hits the probe
point, kprobe handler is invoked.  And the copied instruction is single-step
executed on the copied buffer (not on the original address) by kprobe.  After
that, the kprobe checks registers and modify it (if need) as if the
instructions was executed on the original address.

My proposal is based on the fact there are many instructions which do NOT
require the register modification after the single-step execution.  When the
copied instruction is a kind of them, kprobe just jumps back to the next
instruction after single-step execution.  If so, why don't we execute those
instructions directly?

With kprobe-booster patch, kprobes will execute a copied instruction directly
and (if need) jump back to original code.  This direct execution is executed
when the kprobe don't have both post_handler and break_handler, and the copied
instruction can be executed directly.

I sorted instructions which can be executed directly or not;

- Call instructions are NG(can not be executed directly).
  We should correct the return address pushed into top of stack.
- Indirect instructions except for absolute indirect-jumps
  are NG. Those instructions changes EIP randomly. We should
  check EIP and correct it.
- Instructions that change EIP beyond the range of the
  instruction buffer are NG.
- Instructions that change EIP to tail 5 bytes of the
  instruction buffer (it is the size of a jump instruction).
  We must write a jump instruction which backs to original
  kernel code in the instruction buffer.
- Break point instruction is NG. We should not touch EIP and
  pass to other handlers.
- Absolute direct/indirect jumps are OK.- Conditional Jumps are NG.
- Halt and software-interruptions are NG. Because it will stay on
  the instruction buffer of kprobes.
- Prefixes are NG.
- Unknown/reserved opcode is NG.
- Other 1 byte instructions are OK. But those instructions need a
  jump back code.
- 2 bytes instructions are mapped sparsely. So, in this release,
  this patch don't boost those instructions.

>From Intel's IA-32 opcode map described in IA-32 Intel Architecture Software
Developer's Manual Vol.2 B, I determined that following opcodes are not
boostable.

- 0FH (2byte escape)
- 70H - 7FH (Jump on condition)
- 9AH (Call) and 9CH (Pushf)
- C0H-C1H (Grp 2: includes reserved opcode)
- C6H-C7H (Grp11: includes reserved opcode)
- CCH-CEH (Software-interrupt)
- D0H-D3H (Grp2: includes reserved opcode)
- D6H (Reserved)
- D8H-DFH (Coprocessor)
- E0H-E3H (loop/conditional jump)
- E8H (Call)
- F0H-F3H (Prefixes and reserved)
- F4H (Halt)
- F6H-F7H (Grp3: includes reserved opcode)
- FEH-FFH(Grp4,5: includes reserved opcode)

Kprobe-booster checks whether target instruction can be boosted (can be
executed directly) at arch_copy_kprobe() function.  If the target instruction
can be boosted, it clears "boostable" flag.  If not, it sets "boostable" flag
-1.  This is disabled status.  In resume_execution() function, If "boostable"
flag is cleared, kprobe-booster measures the size of the target instruction
and sets "boostable" flag 1.

In kprobe_handler(), kprobe checks the "boostable" flag.  If the flag is 1, it
resets current kprobe and executes instruction buffer directly instead of
single stepping.

When unregistering a boosted kprobe, it calls synchronize_sched()
after "int3" is removed. So we can ensure followings after
the synchronize_sched() called.
- interrupt handlers are finished on all CPUs.
- instruction buffer is not executed on all CPUs.
And we can release the boosted kprobe safely.

And also, on preemptible kernel, the booster is not enabled where the kernel
preemption is enabled.  So, there are no preempted threads on the instruction
buffer.

The description of kretprobe-booster:
====================================

In the normal operation, kretprobe make a target function return to trampoline
code.  And a kprobe (called trampoline_probe) have been inserted at the
trampoline code.  When the kernel hits this kprobe, it calls kretprobe's
handler and it returns to original return address.

Kretprobe-booster patch removes the trampoline_probe.  It allows the
trampoline code to call kretprobe's handler directly instead of invoking
kprobe.  And tranpoline code returns to original return address.

This new trampoline code stores and restores registers, so the kretprobe
handler is still able to access those registers.

Current kprobe has about 1.3 usec/probe(*) overhead, and kprobe-booster patch
reduces it to 0.6 usec/probe(*).  Also current kretprobe has about 2.0
usec/probe(*) overhead.  Kprobe-booster patch reduces it to 1.3 usec/probe(*),
and the combination of both kprobe-booster patch and kretprobe-booster patch
reduces it to 0.9 usec/probe(*).

I expect the combination of both patches can reduce half of a probing
overhead.

Performance numbers strongly depend on the processor model.

Andrew Morton wrote:
> These preempt tricks look rather nasty.  Can you please describe what the
> problem is, precisely?  And how this code avoids it?  Perhaps we can find
> something cleaner.

The problem is how to remove the copied instructions of the
kprobe *safely* on the preemptable kernel (CONFIG_PREEMPT=y).

Kprobes basically executes the following actions;

(1)int3
(2)preempt_disable()
(3)kprobe_prehandler()
(4)copied instructioin(single step)
(5)kprobe_posthandler()
(6)preempt_enable()
(7)return to the original code

During the execution of copied instruction, preemption is
disabled (from step (2) to (6)).
When unregistering the probes, Kprobe waits for RCU
quiescent state by using synchronize_sched() after removing
int3 instruction.
Thus we can ensure the copied instruction is not executed.

On the other hand, kprobe-booster executes the following actions;

(1)int3
(2)preempt_disable()
(3)kprobe_prehandler()
(4)preempt_enable()             <-- this one is added by my patch
(5)copied instruction(direct execution)
(6)jmp back to the original code

The problem is that we have no way to prevent preemption on
step (5) or (6). We cannot call preempt_disable() after step (6),
because there are no rooms to do that. Thus, some other
processes may be preempted at step(5) or (6) on preemptable kernel.
And I couldn't find the easy way to ensure that other processes'
stack do *not* have the address of them. (I thought some way
to do that, but those are very costly.)

So currently, I simply boost the kprobe only when the probe
point is already preemption disabled.

> Also, the patch adds a preempt_enable() but I don't see a corresponding
> preempt_disable().  Am I missing something?

It is corresponding to the preempt_disable() in the top of
kprobe_handler().
I copied the code of kprobe_handler() here:

static int __kprobes kprobe_handler(struct pt_regs *regs)
{
        struct kprobe *p;
        int ret = 0;
        kprobe_opcode_t *addr = NULL;
        unsigned long *lp;
        struct kprobe_ctlblk *kcb;

        /*
         * We don't want to be preempted for the entire
         * duration of kprobe processing
         */
        preempt_disable();             <-- HERE
        kcb = get_kprobe_ctlblk();
Signed-off-by: Masami Hiramatsu <hiramatu@sdl.hitachi.co.jp>
Cc: Prasanna S Panchamukhi <prasanna@in.ibm.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>