Re: [RFC][Patch 2/2][take3]kprobe: kprobe-booster against 2.6.15-rc5-mm3 for i386

[Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>]

On Thu, Dec 22, 2005 at 10:24:04PM +0900, Masami Hiramatsu wrote:
> Hi,
> 
> Here is a patch of kprobe-booster for i386 arch.
> 
> This patch works with kernel preemption. On preemptable kernel,
> kprobe-booster works only when the preemption is disabled.

I don't know the intricacies of the IA32 instruction set as well as the
opcode tricks you could play. However, some comments (^^^) below.

Ananth

> 
> -- 
> Masami HIRAMATSU
> 2nd Research Dept.
> Hitachi, Ltd., Systems Development Laboratory
> E-mail: hiramatu@sdl.hitachi.co.jp
> 
>  arch/i386/kernel/kprobes.c |   83 +++++++++++++++++++++++++++++++++++++++++++--
>  include/asm-i386/kprobes.h |    4 ++
>  2 files changed, 85 insertions(+), 2 deletions(-)
> diff -Narup a/arch/i386/kernel/kprobes.c b/arch/i386/kernel/kprobes.c
> --- a/arch/i386/kernel/kprobes.c	2005-12-20 19:57:56.000000000 +0900
> +++ b/arch/i386/kernel/kprobes.c	2005-12-20 20:05:01.000000000 +0900
> @@ -41,6 +41,49 @@ void jprobe_return_end(void);
>  DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
>  DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
> 
> +/* insert a jmp code */
> +static inline void set_jmp_op(void *from, void *to)
> +{
> +	struct __arch_jmp_op {
> +		char op;
> +		long raddr;
> +	} __attribute__((packed)) *jop;
> +	jop = (struct __arch_jmp_op *)from;
> +	jop->raddr = (long)(to) - ((long)(from) + 5);
> +	jop->op = RELATIVEJUMP_INSTRUCTION;
> +}
> +
> +/*
> + * returns non-zero if opcodes can be boosted.
> + */
> +static inline int can_boost(kprobe_opcode_t opcode)
> +{
> +	switch (opcode & 0xf0 ) {
> +	case 0x70:
> +		return 0; /* can't boost conditional jump */
> +	case 0x90:
> +		/* can't boost call and pushf */
> +		return opcode != 0x9a && opcode != 0x9c;
> +	case 0xc0:
> +		/* can't boost undefined opcodes and soft-interruptions */
> +		return (0xc1 < opcode && opcode < 0xc6) ||
> +			(0xc7 < opcode && opcode < 0xcc) || opcode == 0xcf;
> +	case 0xd0:
> +		/* can boost AA* and XLAT */
> +		return (opcode == 0xd4 || opcode == 0xd5 || opcode == 0xd7);
> +	case 0xe0:
> +		/* can boost in/out and (may be) jmps */
> +		return (0xe3 < opcode && opcode != 0xe8);
> +	case 0xf0:
> +		/* clear and set flags can be boost */
> +		return (opcode == 0xf5 || (0xf7 < opcode && opcode < 0xfe));
> +	default:
> +		/* currently, can't boost 2 bytes opcodes */
> +		return opcode != 0x0f;
> +	}
> +}
> +
> +
>  /*
>   * returns non-zero if opcode modifies the interrupt flag.
>   */
> @@ -60,6 +103,11 @@ int __kprobes arch_prepare_kprobe(struct
>  {
>  	memcpy(p->ainsn.insn, p->addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
>  	p->opcode = *p->addr;
> +	if (can_boost(p->opcode)) {
> +		p->ainsn.boostable = 0;
> +	} else {
> +		p->ainsn.boostable = -1;
> +	}
>  	return 0;
>  }
> 
> @@ -227,6 +275,19 @@ static int __kprobes kprobe_handler(stru
>  		/* handler has already set things up, so skip ss setup */
>  		return 1;
> 
> +	if (p->ainsn.boostable == 1 &&
> +#ifdef CONFIG_PREEMPT
> +	    preempt_count() != 1 && /* This enables booster when the
> +				     direct execution path aren't preempted. */
			     ^^^^^
Why is the comparison against an absolute value? preempt_disable/enable()
can be nested and on each such invocation, the count is
incremented/decremented respectively - this number can be 0 or hold
any positive value.

And multiline comments in
	/*
	 *
	 */
format please

> +#endif /* CONFIG_PREEMPT */
> +	    !p->post_handler && !p->break_handler ) {
> +		/* Boost up -- we can execute copied instructions directly */
> +		reset_current_kprobe();
> +		regs->eip = (unsigned long)&p->ainsn.insn;
> +		preempt_enable_no_resched();
> +		return 1;
> +	}
> +
>  ss_probe:
>  	prepare_singlestep(p, regs);
>  	kcb->kprobe_status = KPROBE_HIT_SS;
> @@ -332,6 +393,8 @@ int __kprobes trampoline_probe_handler(s
>   * 2) If the single-stepped instruction was a call, the return address
>   * that is atop the stack is the address following the copied instruction.
>   * We need to make it the address following the original instruction.
> + *
> + * This function also checks instruction size for preparing direct execution.
>   */
>  static void __kprobes resume_execution(struct kprobe *p,
>  		struct pt_regs *regs, struct kprobe_ctlblk *kcb)
> @@ -352,6 +415,7 @@ static void __kprobes resume_execution(s
>  	case 0xca:
>  	case 0xea:		/* jmp absolute -- eip is correct */
>  		/* eip is already adjusted, no more changes required */
> +		p->ainsn.boostable = 1;
>  		goto no_change;
>  	case 0xe8:		/* call relative - Fix return addr */
>  		*tos = orig_eip + (*tos - copy_eip);
> @@ -359,18 +423,33 @@ static void __kprobes resume_execution(s
>  	case 0xff:
>  		if ((p->ainsn.insn[1] & 0x30) == 0x10) {
>  			/* call absolute, indirect */
> -			/* Fix return addr; eip is correct. */
> +			/* Fix return addr; eip is correct
> +			   But this is not boostable */
			^^^ Please fix comment style

>  			*tos = orig_eip + (*tos - copy_eip);
>  			goto no_change;
>  		} else if (((p->ainsn.insn[1] & 0x31) == 0x20) ||	/* jmp near, absolute indirect */
>  			   ((p->ainsn.insn[1] & 0x31) == 0x21)) {	/* jmp far, absolute indirect */
> -			/* eip is correct. */
> +			/* eip is correct. And this is boostable */
> +			p->ainsn.boostable = 1;
>  			goto no_change;
>  		}
>  	default:
>  		break;
>  	}
> 
> +	if (p->ainsn.boostable == 0) {
> +		if ( regs->eip > copy_eip &&
		   ^^^ Coding-style please :)
		   
> +		     (regs->eip - copy_eip) + 5 < MAX_INSN_SIZE) {
> +			/* these instructions can be executed directly if it
> +			 jumps back to correct address. */
			^^^ Comment style fix please
			
> +			set_jmp_op((void *)regs->eip,
> +				   (void *)orig_eip + (regs->eip - copy_eip));
> +			p->ainsn.boostable = 1;
> +		} else {
> +			p->ainsn.boostable = -1;
> +		}
> +	}
> +
>  	regs->eip = orig_eip + (regs->eip - copy_eip);
> 
>         no_change:
> diff -Narup a/include/asm-i386/kprobes.h b/include/asm-i386/kprobes.h
> --- a/include/asm-i386/kprobes.h	2005-12-20 19:55:51.000000000 +0900
> +++ b/include/asm-i386/kprobes.h	2005-12-20 20:03:11.000000000 +0900
> @@ -31,6 +31,7 @@ struct pt_regs;
> 
>  typedef u8 kprobe_opcode_t;
>  #define BREAKPOINT_INSTRUCTION	0xcc
> +#define RELATIVEJUMP_INSTRUCTION 0xe9
>  #define MAX_INSN_SIZE 16
>  #define MAX_STACK_SIZE 64
>  #define MIN_STACK_SIZE(ADDR) (((MAX_STACK_SIZE) < \
> @@ -48,6 +49,9 @@ void kretprobe_trampoline(void);
>  struct arch_specific_insn {
>  	/* copy of the original instruction */
>  	kprobe_opcode_t insn[MAX_INSN_SIZE];
> +	/* If this flag is not 0, this kprobe can be boost when its
> +	   post_handler and break_handler is not set. */
	^^^
	Pls fix comment style

> +	int boostable;
>  };
> 
>  struct prev_kprobe {
> 
>