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Subject[PATCH for 5.5 1/2] rseq: Fix: Clarify rseq.h UAPI rseq_cs memory reclaim requirements
Date
The rseq.h UAPI documents that the rseq_cs field must be cleared
before reclaiming memory that contains the targeted struct rseq_cs.

We should extend this comment to also dictate that the rseq_cs field
must be cleared before reclaiming memory of the code pointed to by
the rseq_cs start_ip and post_commit_offset fields.

While we can expect that use of dlclose(3) will typically unmap
both struct rseq_cs and its associated code at once, nothing would
theoretically prevent a JIT from reclaiming the code without
reclaiming the struct rseq_cs, which would erroneously allow the
kernel to consider new code which is not a rseq critical section
as a rseq critical section following a code reclaim.

Suggested-by: Florian Weimer <fw@deneb.enyo.de>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Florian Weimer <fw@deneb.enyo.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Paul Turner <pjt@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Neel Natu <neelnatu@google.com>
Cc: linux-api@vger.kernel.org
---
include/uapi/linux/rseq.h | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 9a402fdb60e9..6f26b0b148a6 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -100,7 +100,9 @@ struct rseq {
* instruction sequence block, as well as when the kernel detects that
* it is preempting or delivering a signal outside of the range
* targeted by the rseq_cs. Also needs to be set to NULL by user-space
- * before reclaiming memory that contains the targeted struct rseq_cs.
+ * before reclaiming memory that contains the targeted struct rseq_cs
+ * or reclaiming memory that contains the code refered to by the
+ * start_ip and post_commit_offset fields of struct rseq_cs.
*
* Read and set by the kernel. Set by user-space with single-copy
* atomicity semantics. This field should only be updated by the
--
2.17.1
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