linux-kernel - [RFC PATCH for 5.2 10/10] rseq/selftests: mips: use break instruction for RSEQ

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Date:   Wed, 24 Apr 2019 11:25:02 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org
Subject: [RFC PATCH for 5.2 10/10] rseq/selftests: mips: use break instruction for RSEQ_SIG

Use break as guard instruction for the restartable sequence abort
handler.

Previously, the chosen signature was simply data, based on the
assumption that it could always sit in a literal pool. However,
some compilation environments favor disabling literal pool. Therefore,
ensure the signature is a valid uncommon trap instruction.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC: Paul Burton <paul.burton@...s.com>
CC: James Hogan <jhogan@...nel.org>
Cc: Ralf Baechle <ralf@...ux-mips.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: linux-mips@...ux-mips.org
Cc: linux-kernel@...r.kernel.org
---
 tools/testing/selftests/rseq/rseq-mips.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/rseq/rseq-mips.h b/tools/testing/selftests/rseq/rseq-mips.h
index fe3eabcdcbe5..eb53a6adfbbb 100644
--- a/tools/testing/selftests/rseq/rseq-mips.h
+++ b/tools/testing/selftests/rseq/rseq-mips.h
@@ -7,7 +7,11 @@
  * (C) Copyright 2016-2018 - Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
  */
 
-#define RSEQ_SIG	0x53053053
+/*
+ * RSEQ_SIG uses the break instruction. The instruction pattern is
+ *	0350000d        break    0x350
+ */
+#define RSEQ_SIG	0x0350000d
 
 #define rseq_smp_mb()	__asm__ __volatile__ ("sync" ::: "memory")
 #define rseq_smp_rmb()	rseq_smp_mb()
-- 
2.11.0