[PATCH 1/2] rseq: Deprecate RSEQ_CS_FLAG_NO_RESTART_ON_* flags

Mathieu Desnoyers
2022-06-22T19:47:11Z

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@efficios.com>
---
kernel/rseq.c | 23 ++++++++---------------
1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 97ac20b4f738..81d7dc80787b 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)
--
2.30.2