linux-kernel - [tip: sched/core] rseq: Deprecate RSEQ_CS_FLAG_NO_RESTART_ON

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Message-ID: <165936033979.15455.14072233165429721291.tip-bot2@tip-bot2>
Date:   Mon, 01 Aug 2022 13:25:39 -0000
From:   "tip-bot2 for Mathieu Desnoyers" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/core] rseq: Deprecate RSEQ_CS_FLAG_NO_RESTART_ON_* flags

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

Commit-ID:     0190e4198e47fe99d002d72588f34fd62c9ab570
Gitweb:        https://git.kernel.org/tip/0190e4198e47fe99d002d72588f34fd62c9ab570
Author:        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
AuthorDate:    Wed, 22 Jun 2022 15:46:16 -04:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Mon, 01 Aug 2022 15:21:29 +02:00

rseq: Deprecate RSEQ_CS_FLAG_NO_RESTART_ON_* flags

The pretty much unused RSEQ_CS_FLAG_NO_RESTART_ON_* flags introduce
complexity in rseq, and are subtly buggy [1]. Solving those issues
requires introducing additional complexity in the rseq implementation
for each supported architecture.

Considering that it complexifies the rseq ABI, I am proposing that we
deprecate those flags. [2]

So far there appears to be consensus from maintainers of user-space
projects impacted by this feature that its removal would be a welcome
simplification. [3]

The deprecation approach proposed here is to issue WARN_ON_ONCE() when
encountering those flags and kill the offending process with sigsegv.
This should allow us to quickly identify whether anyone yells at us for
removing this.

Link: https://lore.kernel.org/lkml/20220618182515.95831-1-minhquangbui99@gmail.com/ [1]
Link: https://lore.kernel.org/lkml/258546133.12151.1655739550814.JavaMail.zimbra@efficios.com/ [2]
Link: https://lore.kernel.org/lkml/87pmj1enjh.fsf@email.froward.int.ebiederm.org/ [3]
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/lkml/20220622194617.1155957-1-mathieu.desnoyers@efficios.com
---
 kernel/rseq.c | 23 ++++++++---------------
 1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 97ac20b..81d7dc8 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -18,8 +18,9 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/rseq.h>
 
-#define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE | \
-				       RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT)
+#define RSEQ_CS_NO_RESTART_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT | \
+				  RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
+				  RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)
 
 /*
  *
@@ -175,23 +176,15 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
 	u32 flags, event_mask;
 	int ret;
 
+	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS))
+		return -EINVAL;
+
 	/* Get thread flags. */
 	ret = get_user(flags, &t->rseq->flags);
 	if (ret)
 		return ret;
 
-	/* Take critical section flags into account. */
-	flags |= cs_flags;
-
-	/*
-	 * Restart on signal can only be inhibited when restart on
-	 * preempt and restart on migrate are inhibited too. Otherwise,
-	 * a preempted signal handler could fail to restart the prior
-	 * execution context on sigreturn.
-	 */
-	if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) &&
-		     (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) !=
-		     RSEQ_CS_PREEMPT_MIGRATE_FLAGS))
+	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS))
 		return -EINVAL;
 
 	/*
@@ -203,7 +196,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
 	t->rseq_event_mask = 0;
 	preempt_enable();
 
-	return !!(event_mask & ~flags);
+	return !!event_mask;
 }
 
 static int clear_rseq_cs(struct task_struct *t)