[PATCH v7 08/17] unwind_user/deferred: Add unwind cache

[Steven Rostedt <rostedt@goodmis.org>]

From: Josh Poimboeuf <jpoimboe@kernel.org>

Cache the results of the unwind to ensure the unwind is only performed
once, even when called by multiple tracers.

The cache nr_entries gets cleared every time the task exits the kernel.
When a stacktrace is requested, nr_entries gets set to the number of
entries in the stacktrace. If another stacktrace is requested, if
nr_entries is not zero, then it contains the same stacktrace that would be
retrieved so it is not processed again and the entries is given to the
caller.

Co-developed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 include/linux/entry-common.h          |  2 ++
 include/linux/unwind_deferred.h       |  7 +++++++
 include/linux/unwind_deferred_types.h |  7 ++++++-
 kernel/unwind/deferred.c              | 27 ++++++++++++++++++++-------
 4 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index f94f3fdf15fc..6e850c9d3f0c 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -12,6 +12,7 @@
 #include <linux/resume_user_mode.h>
 #include <linux/tick.h>
 #include <linux/kmsan.h>
+#include <linux/unwind_deferred.h>
 
 #include <asm/entry-common.h>
 #include <asm/syscall.h>
@@ -362,6 +363,7 @@ static __always_inline void exit_to_user_mode(void)
 	lockdep_hardirqs_on_prepare();
 	instrumentation_end();
 
+	unwind_exit_to_user_mode();
 	user_enter_irqoff();
 	arch_exit_to_user_mode();
 	lockdep_hardirqs_on(CALLER_ADDR0);
diff --git a/include/linux/unwind_deferred.h b/include/linux/unwind_deferred.h
index 5064ebe38c4f..c2d760e5e257 100644
--- a/include/linux/unwind_deferred.h
+++ b/include/linux/unwind_deferred.h
@@ -12,6 +12,11 @@ void unwind_task_free(struct task_struct *task);
 
 int unwind_deferred_trace(struct unwind_stacktrace *trace);
 
+static __always_inline void unwind_exit_to_user_mode(void)
+{
+	current->unwind_info.cache.nr_entries = 0;
+}
+
 #else /* !CONFIG_UNWIND_USER */
 
 static inline void unwind_task_init(struct task_struct *task) {}
@@ -19,6 +24,8 @@ static inline void unwind_task_free(struct task_struct *task) {}
 
 static inline int unwind_deferred_trace(struct unwind_stacktrace *trace) { return -ENOSYS; }
 
+static inline void unwind_exit_to_user_mode(void) {}
+
 #endif /* !CONFIG_UNWIND_USER */
 
 #endif /* _LINUX_UNWIND_USER_DEFERRED_H */
diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
index aa32db574e43..b3b7389ee6eb 100644
--- a/include/linux/unwind_deferred_types.h
+++ b/include/linux/unwind_deferred_types.h
@@ -2,8 +2,13 @@
 #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
 #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
 
-struct unwind_task_info {
+struct unwind_cache {
 	unsigned long		*entries;
+	unsigned int		nr_entries;
+};
+
+struct unwind_task_info {
+	struct unwind_cache	cache;
 };
 
 #endif /* _LINUX_UNWIND_USER_DEFERRED_TYPES_H */
diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index 5a3789e38c00..89ed04b1c527 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -12,6 +12,7 @@
 int unwind_deferred_trace(struct unwind_stacktrace *trace)
 {
 	struct unwind_task_info *info = &current->unwind_info;
+	struct unwind_cache *cache = &info->cache;
 
 	/* Should always be called from faultable context */
 	might_fault();
@@ -19,17 +20,29 @@ int unwind_deferred_trace(struct unwind_stacktrace *trace)
 	if (current->flags & PF_EXITING)
 		return -EINVAL;
 
-       if (!info->entries) {
-               info->entries = kmalloc_array(UNWIND_MAX_ENTRIES, sizeof(long),
-                                             GFP_KERNEL);
-               if (!info->entries)
-		       return -ENOMEM;
+	if (!cache->entries) {
+		cache->entries = kmalloc_array(UNWIND_MAX_ENTRIES, sizeof(long),
+					       GFP_KERNEL);
+		if (!cache->entries)
+			return -ENOMEM;
+        }
+
+	trace->entries = cache->entries;
+
+	if (cache->nr_entries) {
+               /*
+                * The user stack has already been previously unwound in this
+                * entry context.  Skip the unwind and use the cache.
+                */
+               trace->nr = cache->nr_entries;
+               return 0;
        }
 
 	trace->nr = 0;
-	trace->entries = info->entries;
 	unwind_user(trace, UNWIND_MAX_ENTRIES);
 
+	cache->nr_entries = trace->nr;
+
 	return 0;
 }
 
@@ -44,5 +57,5 @@ void unwind_task_free(struct task_struct *task)
 {
 	struct unwind_task_info *info = &task->unwind_info;
 
-	kfree(info->entries);
+	kfree(info->cache.entries);
 }
-- 
2.47.2