linux-kernel - [PATCH v2 57/59] x86/ftrace: Rebalance RSB

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Message-ID: <20220902130952.269277054@infradead.org>
Date:   Fri, 02 Sep 2022 15:07:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH v2 57/59] x86/ftrace: Rebalance RSB

From: Peter Zijlstra (Intel) <peterz@...radead.org>

ftrace_regs_caller() uses a PUSH;RET pattern to tail-call into a
direct-call function, this unbalances the RSB, fix that.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/kernel/ftrace_64.S |   11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -271,6 +271,17 @@ SYM_INNER_LABEL(ftrace_regs_caller_end,
 	/* Restore flags */
 	popfq
 	UNWIND_HINT_FUNC
+
+	/*
+	 * The above left an extra return value on the stack; effectively
+	 * doing a tail-call without using a register. This PUSH;RET
+	 * pattern unbalances the RSB, inject a pointless CALL to rebalance.
+	 */
+	ANNOTATE_INTRA_FUNCTION_CALL
+	CALL .Ldo_rebalance
+	int3
+.Ldo_rebalance:
+	add $8, %rsp
 	RET
 
 SYM_FUNC_END(ftrace_regs_caller)