Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again

[Masami Hiramatsu]

On 2015/06/02 1:32, Eugene Shatokhin wrote:
> Commit 91e5ed49fca0 ("x86/asm/decoder: Fix and enforce max instruction
> size in the insn decoder") has changed MAX_INSN_SIZE from 16 to 15 bytes
> on x86.
> 
> As a side effect, the slots Kprobes use to store the instructions became
> 1 byte shorter. This is unfortunate because, for example, the Kprobes'
> "boost" feature can not be used now for the instructions of length 11,
> like a quite common kind of MOV:
> * movq $0xffffffffffffffff,-0x3fe8(%rax) (48 c7 80 18 c0 ff ff ff ff ff ff)
> * movq $0x0,0x88(%rdi)                   (48 c7 87 88 00 00 00 00 00 00 00)
> and so on.
> 
> This patch makes the insn slots 16 bytes long, like they were before while
> keeping MAX_INSN_SIZE intact.
> 
> Other tools may benefit from this change as well.
> 
> Signed-off-by: Eugene Shatokhin <eugene.shatokhin@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kprobes.h | 1 +
>  arch/x86/kernel/kprobes/core.c | 2 +-
>  kernel/kprobes.c               | 8 ++++++--
>  3 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kprobes.h b/arch/x86/include/asm/kprobes.h
> index 4421b5d..f3f0b4e 100644
> --- a/arch/x86/include/asm/kprobes.h
> +++ b/arch/x86/include/asm/kprobes.h
> @@ -27,6 +27,7 @@
>  #include <asm/insn.h>
>  
>  #define  __ARCH_WANT_KPROBES_INSN_SLOT

Please add a comment here why the slot size is bigger than MAX_INSN_SIZE.

> +#define KPROBE_INSN_SLOT_SIZE 16

And make sure that KPROBE_INSN_SLOT_SIZE >= MAX_INSN_SIZE.

Other parts are OK for me.

Thanks!

>  
>  struct pt_regs;
>  struct kprobe;
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 0a42b76..1067f90 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -881,7 +881,7 @@ static void resume_execution(struct kprobe *p, struct pt_regs *regs,
>  
>  	if (p->ainsn.boostable == 0) {
>  		if ((regs->ip > copy_ip) &&
> -		    (regs->ip - copy_ip) + 5 <= MAX_INSN_SIZE) {
> +		    (regs->ip - copy_ip) + 5 <= KPROBE_INSN_SLOT_SIZE) {
>  			/*
>  			 * These instructions can be executed directly if it
>  			 * jumps back to correct address.
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index c90e417..1dc074d 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -57,7 +57,6 @@
>  #define KPROBE_HASH_BITS 6
>  #define KPROBE_TABLE_SIZE (1 << KPROBE_HASH_BITS)
>  
> -
>  /*
>   * Some oddball architectures like 64bit powerpc have function descriptors
>   * so this must be overridable.
> @@ -90,6 +89,11 @@ static raw_spinlock_t *kretprobe_table_lock_ptr(unsigned long hash)
>  static LIST_HEAD(kprobe_blacklist);
>  
>  #ifdef __ARCH_WANT_KPROBES_INSN_SLOT
> +
> +#ifndef KPROBE_INSN_SLOT_SIZE
> +#define KPROBE_INSN_SLOT_SIZE MAX_INSN_SIZE
> +#endif
> +
>  /*
>   * kprobe->ainsn.insn points to the copy of the instruction to be
>   * single-stepped. x86_64, POWER4 and above have no-exec support and
> @@ -135,7 +139,7 @@ struct kprobe_insn_cache kprobe_insn_slots = {
>  	.alloc = alloc_insn_page,
>  	.free = free_insn_page,
>  	.pages = LIST_HEAD_INIT(kprobe_insn_slots.pages),
> -	.insn_size = MAX_INSN_SIZE,
> +	.insn_size = KPROBE_INSN_SLOT_SIZE,
>  	.nr_garbage = 0,
>  };
>  static int collect_garbage_slots(struct kprobe_insn_cache *c);
> 


-- 
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
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