linux-kernel - [RFC PATCH tip/master V3 4/8] kprobes/x86: Do not modify singlestep buffer while resuming

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Message-Id: <149076361560.22469.1610155860343077495.stgit@devbox>
Date:   Wed, 29 Mar 2017 14:00:25 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ye Xiaolong <xiaolong.ye@...el.com>, mhiramat@...nel.org
Subject: [RFC PATCH tip/master V3 4/8] kprobes/x86: Do not modify singlestep buffer while resuming

Do not modify singlestep execution buffer (kprobe.ainsn.insn)
while resuming from single-stepping, instead, modifies
the buffer to add a jump back instruction at preparing
buffer.

Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
---
  Changes in v3:
   - Fix imbalance curly braces.
   - Remove unneeded (void *) casting.
---
 arch/x86/kernel/kprobes/core.c |   42 +++++++++++++++++++---------------------
 1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 6327f95..a654054 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -399,23 +399,36 @@ int __copy_instruction(u8 *dest, u8 *src)
 	return length;
 }
 
+/* Prepare reljump right after instruction to boost */
+static void prepare_boost(struct kprobe *p, int length)
+{
+	if (can_boost(p->ainsn.insn, p->addr) &&
+	    MAX_INSN_SIZE - length >= RELATIVEJUMP_SIZE) {
+		/*
+		 * These instructions can be executed directly if it
+		 * jumps back to correct address.
+		 */
+		synthesize_reljump(p->ainsn.insn + length, p->addr + length);
+		p->ainsn.boostable = 1;
+	} else {
+		p->ainsn.boostable = -1;
+	}
+}
+
 static int arch_copy_kprobe(struct kprobe *p)
 {
-	int ret;
+	int len;
 
 	/* Copy an instruction with recovering if other optprobe modifies it.*/
-	ret = __copy_instruction(p->ainsn.insn, p->addr);
-	if (!ret)
+	len = __copy_instruction(p->ainsn.insn, p->addr);
+	if (!len)
 		return -EINVAL;
 
 	/*
 	 * __copy_instruction can modify the displacement of the instruction,
 	 * but it doesn't affect boostable check.
 	 */
-	if (can_boost(p->ainsn.insn, p->addr))
-		p->ainsn.boostable = 0;
-	else
-		p->ainsn.boostable = -1;
+	prepare_boost(p, len);
 
 	/* Check whether the instruction modifies Interrupt Flag or not */
 	p->ainsn.if_modifier = is_IF_modifier(p->ainsn.insn);
@@ -878,21 +891,6 @@ static void resume_execution(struct kprobe *p, struct pt_regs *regs,
 		break;
 	}
 
-	if (p->ainsn.boostable == 0) {
-		if ((regs->ip > copy_ip) &&
-		    (regs->ip - copy_ip) + 5 < MAX_INSN_SIZE) {
-			/*
-			 * These instructions can be executed directly if it
-			 * jumps back to correct address.
-			 */
-			synthesize_reljump((void *)regs->ip,
-				(void *)orig_ip + (regs->ip - copy_ip));
-			p->ainsn.boostable = 1;
-		} else {
-			p->ainsn.boostable = -1;
-		}
-	}
-
 	regs->ip += orig_ip - copy_ip;
 
 no_change: