linux-kernel - [BUGFIX PATCH -tip 2/2] x86/kprobes: Identify far indirect JMP correctly

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Date:   Thu, 25 Feb 2021 21:02:18 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Ananth N Mavinakayanahalli <ananth@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org, x86@...nel.org
Subject: [BUGFIX PATCH -tip 2/2] x86/kprobes: Identify far indirect JMP correctly

Since Grp5 far indirect JMP is FF "mod 101 r/m", it should be
(modrm & 0x38) == 0x28, and near indirect JMP is also 0x38 == 0x20.
So we can mask modrm with 0x30 and check 0x20.
This is actually what the original code does, it also doesn't care
the last bit. So the result code is same.

Thus, I think this is just a cosmetic cleanup.

Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
---
 arch/x86/kernel/kprobes/core.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 08674e7a5d7b..be76568d57a5 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -462,8 +462,7 @@ static void set_resume_flags(struct kprobe *p, struct insn *insn)
 			p->ainsn.is_call = 1;
 			p->ainsn.is_abs_ip = 1;
 			break;
-		} else if (((opcode & 0x31) == 0x20) ||
-			   ((opcode & 0x31) == 0x21)) {
+		} else if ((opcode & 0x30) == 0x20) {
 			/*
 			 * jmp near and far, absolute indirect
 			 * ip is correct.