linux-kernel - [PATCH v2] uprobes/x86: allow to probe a "nop" insn with 0x66 prefix

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Date:   Sun, 4 Dec 2022 18:39:33 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Denys Vlasenko <dvlasenk@...hat.com>,
        Seiji Nishikawa <snishika@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2] uprobes/x86: allow to probe a "nop" insn with 0x66 prefix

Intel icc -hotpatch inserts 2-byte "0x66 0x90" NOP at the start of each
function to reserve extra space for hot-patching, and currently it is not
possible to probe these functions because branch_setup_xol_ops() wrongly
nacks NOP with REP prefix.

Fixes: 250bbd12c2fe ("uprobes/x86: Refuse to attach uprobe to "word-sized" branch insns")
Reported-by: Seiji Nishikawa <snishika@...hat.com>
Suggested-by: Denys Vlasenko <dvlasenk@...hat.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 arch/x86/kernel/uprobes.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index b63cf8f7745e..6c07f6daaa22 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -722,8 +722,9 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
 	switch (opc1) {
 	case 0xeb:	/* jmp 8 */
 	case 0xe9:	/* jmp 32 */
-	case 0x90:	/* prefix* + nop; same as jmp with .offs = 0 */
 		break;
+	case 0x90:	/* prefix* + nop; same as jmp with .offs = 0 */
+		goto setup;
 
 	case 0xe8:	/* call relative */
 		branch_clear_offset(auprobe, insn);
@@ -753,6 +754,7 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
 			return -ENOTSUPP;
 	}
 
+setup:
 	auprobe->branch.opc1 = opc1;
 	auprobe->branch.ilen = insn->length;
 	auprobe->branch.offs = insn->immediate.value;
-- 
2.25.1.362.g51ebf55