From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753227AbbFCHys (ORCPT ); Wed, 3 Jun 2015 03:54:48 -0400 Received: from collab.rosalab.ru ([195.19.76.181]:60879 "EHLO collab.rosalab.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752713AbbFCHyl (ORCPT ); Wed, 3 Jun 2015 03:54:41 -0400 From: Eugene Shatokhin To: Masami Hiramatsu , Ingo Molnar Cc: Andy Lutomirski , Ingo Molnar , LKML , Eugene Shatokhin Subject: [PATCH v2 2/2] kprobes/x86: Use 16 bytes for each instruction slot again Date: Wed, 3 Jun 2015 10:54:11 +0300 Message-Id: <1433318051-12509-1-git-send-email-eugene.shatokhin@rosalab.ru> X-Mailer: git-send-email 2.3.2 In-Reply-To: <1433176331-479-1-git-send-email-eugene.shatokhin@rosalab.ru> References: <1433176331-479-1-git-send-email-eugene.shatokhin@rosalab.ru> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Changes in v2: * Explained in the comments what KPROBE_INSN_SLOT_SIZE is and why it may be larger than MAX_INSN_SIZE. * Added a compile-time check that KPROBE_INSN_SLOT_SIZE is not less than MAX_INSN_SIZE. Signed-off-by: Eugene Shatokhin --- arch/x86/include/asm/kprobes.h | 15 +++++++++++++++ arch/x86/kernel/kprobes/core.c | 2 +- kernel/kprobes.c | 20 ++++++++++++++++++-- 3 files changed, 34 insertions(+), 3 deletions(-) diff --git a/arch/x86/include/asm/kprobes.h b/arch/x86/include/asm/kprobes.h index 4421b5d..ab6e6a0 100644 --- a/arch/x86/include/asm/kprobes.h +++ b/arch/x86/include/asm/kprobes.h @@ -28,6 +28,21 @@ #define __ARCH_WANT_KPROBES_INSN_SLOT +/* + * The size of the instruction slot is greater than the maximum length of + * an instruction (15 bytes) for Kprobes to be able to use "boost" for + * longer instructions. + * + * "Boost" allows to avoid single-stepping over an instruction if the Kprobe + * has no post handler. A jump to the next instruction is placed after the + * copied instruction in the slot for that to work. + * + * The length of the relative jump instruction is 5 bytes. With the slot + * size of 16, "boost" can be used for the instructions up to 11 bytes long, + * including rather common kinds of "MOV r/m64, imm32" (opcode 0xc7). + */ +#define KPROBE_INSN_SLOT_SIZE 16 + 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..92788a4 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 +89,23 @@ static raw_spinlock_t *kretprobe_table_lock_ptr(unsigned long hash) static LIST_HEAD(kprobe_blacklist); #ifdef __ARCH_WANT_KPROBES_INSN_SLOT + +/* + * An instruction slot contains a copy of the probed instruction, relocated + * if needed. In some cases, it may be necessary to place additional + * instructions into that slot, so, the size of the slot may be larger than + * the maximum length of an instruction. + * Currently, the slots larger than MAX_INSN_SIZE may only be needed on x86 + * to implement Kprobe "boost". Other architectures do not need to define + * KPROBE_INSN_SLOT_SIZE explicitly. + */ +#ifndef KPROBE_INSN_SLOT_SIZE +#define KPROBE_INSN_SLOT_SIZE MAX_INSN_SIZE +#endif +#if (KPROBE_INSN_SLOT_SIZE < MAX_INSN_SIZE) +#error "Size of an instruction slot must not be less than 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 +151,7 @@ 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); -- 2.3.2