linux-kernel - [tip: x86/core] x86/kprobes: Identify far indirect JMP correctly

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Date:   Tue, 23 Mar 2021 15:15:09 -0000
From:   "tip-bot2 for Masami Hiramatsu" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/core] x86/kprobes: Identify far indirect JMP correctly

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     a194acd316f93f3435a64de3b37dca2b5a77b338
Gitweb:        https://git.kernel.org/tip/a194acd316f93f3435a64de3b37dca2b5a77b338
Author:        Masami Hiramatsu <mhiramat@...nel.org>
AuthorDate:    Wed, 03 Mar 2021 00:25:34 +09:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Tue, 23 Mar 2021 16:07:56 +01:00

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/161469873475.49483.13257083019966335137.stgit@devnote2
---
 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 9b31790..f6ec57f 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -467,8 +467,7 @@ 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.