Introduce unwind_user_trace_cached which copies the stack trace if it
was previously stored into the cache since the last reset of the cache.

The expected use-case is sampling a stack trace from a faultable context
at system call entry (coping it into the cache), and then copying the
stack trace from the cache from non-faultable context to a ring buffer.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Indu Bhagat <indu.bhagat@oracle.com>
Cc: "Jose E. Marchesi" <jemarch@gnu.org>
Cc: Beau Belgrave <beaub@linux.microsoft.com>
Cc: Jens Remus <jremus@linux.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 include/linux/unwind_deferred.h |  2 ++
 kernel/unwind/deferred.c        | 31 +++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/include/linux/unwind_deferred.h b/include/linux/unwind_deferred.h
index a9d5b100d6b2..ddc985ba982a 100644
--- a/include/linux/unwind_deferred.h
+++ b/include/linux/unwind_deferred.h
@@ -34,6 +34,7 @@ void unwind_task_init(struct task_struct *task);
 void unwind_task_free(struct task_struct *task);
 
 int unwind_user_faultable(struct unwind_stacktrace *trace);
+int unwind_user_trace_cached(struct unwind_stacktrace *trace);
 
 int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func);
 int unwind_deferred_request(struct unwind_work *work, u64 *cookie);
@@ -67,6 +68,7 @@ static inline void unwind_task_init(struct task_struct *task) {}
 static inline void unwind_task_free(struct task_struct *task) {}
 
 static inline int unwind_user_faultable(struct unwind_stacktrace *trace) { return -ENOSYS; }
+static inline int unwind_user_trace_cached(struct unwind_stacktrace *trace) { return -ENOSYS; }
 static inline int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func) { return -ENOSYS; }
 static inline int unwind_deferred_request(struct unwind_work *work, u64 *timestamp) { return -ENOSYS; }
 static inline void unwind_deferred_cancel(struct unwind_work *work) {}
diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index 039e12700d49..02ab1d2afc21 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -145,6 +145,37 @@ int unwind_user_faultable(struct unwind_stacktrace *trace)
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(unwind_user_faultable);
+
+/**
+ * unwind_user_trace_cached - Copy user stack trace from cache
+ * @trace: The descriptor that will store the user stacktrace
+ *
+ * Copy user stack trace from cache if the cache was populated by
+ * unwind_user_faultable prior to this call. The returned trace
+ * entries are only valid until the cache is reset.
+ *
+ * Return: 0 on success and negative on error
+ *         On success @trace will contain the user space stacktrace
+ */
+int unwind_user_trace_cached(struct unwind_stacktrace *trace)
+{
+	struct unwind_task_info *info = &current->unwind_info;
+	struct unwind_cache *cache;
+
+	if (!current->mm)
+		return -EINVAL;
+
+	cache = info->cache;
+	if (!cache || !cache->nr_entries)
+		return -ENOENT;
+
+	trace->nr = cache->nr_entries;
+	trace->entries = cache->entries;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(unwind_user_trace_cached);
 
 static void process_unwind_deferred(struct task_struct *task)
 {
-- 
2.43.0