rseq/selftests: Clarify rseq_prepare_unload() helper requirements

The rseq.h UAPI now documents that the rseq_cs field must be cleared
before reclaiming memory that contains the targeted struct rseq_cs, but
also 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 <[email protected]>
Signed-off-by: Mathieu Desnoyers <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra (Intel) <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: "H . Peter Anvin" <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>

tools/testing/selftests/rseq/rseq.h

@@ -149,11 +149,13 @@ static inline void rseq_clear_rseq_cs(void)
 /*
  * rseq_prepare_unload() should be invoked by each thread executing a rseq
  * critical section at least once between their last critical section and
- * library unload of the library defining the rseq critical section
- * (struct rseq_cs). This also applies to use of rseq in code generated by
- * JIT: rseq_prepare_unload() should be invoked at least once by each
- * thread executing a rseq critical section before reclaim of the memory
- * holding the struct rseq_cs.
+ * library unload of the library defining the rseq critical section (struct
+ * rseq_cs) or the code referred to by the struct rseq_cs start_ip and
+ * post_commit_offset fields. This also applies to use of rseq in code
+ * generated by JIT: rseq_prepare_unload() should be invoked at least once by
+ * each thread executing a rseq critical section before reclaim of the memory
+ * holding the struct rseq_cs or reclaim of the code pointed to by struct
+ * rseq_cs start_ip and post_commit_offset fields.
  */
 static inline void rseq_prepare_unload(void)
 {