[PATCH v2 3/3] list_debug: Introduce CONFIG_DEBUG_LIST_MINIMAL

[Marco Elver]

Numerous production kernel configs (see [1, 2]) are choosing to enable
CONFIG_DEBUG_LIST, which is also being recommended by KSPP for hardened
configs [3]. The feature has never been designed with performance in
mind, yet common list manipulation is happening across hot paths all
over the kernel.

Introduce CONFIG_DEBUG_LIST_MINIMAL, which performs list pointer
checking inline, and only upon list corruption delegates to the
reporting slow path.

To generate optimal machine code with CONFIG_DEBUG_LIST_MINIMAL:

  1. Elide checking for pointer values which upon dereference would
     result in an immediate access fault -- therefore "minimal" checks.
     The trade-off is lower-quality error reports.

  2. Use the newly introduced __preserve_most function attribute
     (available with Clang, but not yet with GCC) to minimize the code
     footprint for calling the reporting slow path. As a result,
     function size of callers is reduced by avoiding saving registers
     before calling the rarely called reporting slow path.

     Note that all TUs in lib/Makefile already disable function tracing,
     including list_debug.c, and __preserve_most's implied notrace has
     no effect in this case.

  3. Because the inline checks are a subset of the full set of checks in
     ___list_*_valid(), always return false if the inline checks failed.
     This avoids redundant compare and conditional branch right after
     return from the slow path.

As a side-effect of the checks being inline, if the compiler can prove
some condition to always be true, it can completely elide some checks.

Running netperf with CONFIG_DEBUG_LIST_MINIMAL (using a Clang compiler
with "preserve_most") shows throughput improvements, in my case of ~7%
on average (up to 20-30% on some test cases).

Link: https://r.android.com/1266735 [1]
Link: https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/blob/main/config [2]
Link: https://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project/Recommended_Settings [3]
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
v2:
* Note that lib/Makefile disables function tracing for everything and
  __preserve_most's implied notrace is a noop here.
---
 arch/arm64/kvm/hyp/nvhe/list_debug.c |  2 +
 include/linux/list.h                 | 56 +++++++++++++++++++++++++---
 lib/Kconfig.debug                    | 15 ++++++++
 lib/list_debug.c                     |  2 +
 4 files changed, 69 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/list_debug.c b/arch/arm64/kvm/hyp/nvhe/list_debug.c
index 589284496ac5..df718e29f6d4 100644
--- a/arch/arm64/kvm/hyp/nvhe/list_debug.c
+++ b/arch/arm64/kvm/hyp/nvhe/list_debug.c
@@ -26,6 +26,7 @@ static inline __must_check bool nvhe_check_data_corruption(bool v)
 
 /* The predicates checked here are taken from lib/list_debug.c. */
 
+__list_valid_slowpath
 bool ___list_add_valid(struct list_head *new, struct list_head *prev,
 		       struct list_head *next)
 {
@@ -37,6 +38,7 @@ bool ___list_add_valid(struct list_head *new, struct list_head *prev,
 	return true;
 }
 
+__list_valid_slowpath
 bool ___list_del_entry_valid(struct list_head *entry)
 {
 	struct list_head *prev, *next;
diff --git a/include/linux/list.h b/include/linux/list.h
index e0b2cf904409..a28a215a3eb1 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -39,20 +39,64 @@ static inline void INIT_LIST_HEAD(struct list_head *list)
 }
 
 #ifdef CONFIG_DEBUG_LIST
-extern bool ___list_add_valid(struct list_head *new,
-			      struct list_head *prev,
-			      struct list_head *next);
+
+#ifdef CONFIG_DEBUG_LIST_MINIMAL
+# define __list_valid_slowpath __cold __preserve_most
+#else
+# define __list_valid_slowpath
+#endif
+
+extern bool __list_valid_slowpath ___list_add_valid(struct list_head *new,
+						    struct list_head *prev,
+						    struct list_head *next);
 static __always_inline bool __list_add_valid(struct list_head *new,
 					     struct list_head *prev,
 					     struct list_head *next)
 {
-	return ___list_add_valid(new, prev, next);
+	bool ret = true;
+
+	if (IS_ENABLED(CONFIG_DEBUG_LIST_MINIMAL)) {
+		/*
+		 * In the minimal config, elide checking if next and prev are
+		 * NULL, since the immediate dereference of them below would
+		 * result in a fault if NULL.
+		 *
+		 * With the minimal config we can afford to inline the checks,
+		 * which also gives the compiler a chance to elide some of them
+		 * completely if they can be proven at compile-time. If one of
+		 * the pre-conditions does not hold, the slow-path will show a
+		 * report which pre-condition failed.
+		 */
+		if (likely(next->prev == prev && prev->next == next && new != prev && new != next))
+			return true;
+		ret = false;
+	}
+
+	ret &= ___list_add_valid(new, prev, next);
+	return ret;
 }
 
-extern bool ___list_del_entry_valid(struct list_head *entry);
+extern bool __list_valid_slowpath ___list_del_entry_valid(struct list_head *entry);
 static __always_inline bool __list_del_entry_valid(struct list_head *entry)
 {
-	return ___list_del_entry_valid(entry);
+	bool ret = true;
+
+	if (IS_ENABLED(CONFIG_DEBUG_LIST_MINIMAL)) {
+		struct list_head *prev = entry->prev;
+		struct list_head *next = entry->next;
+
+		/*
+		 * In the minimal config, elide checking if next and prev are
+		 * NULL, LIST_POISON1 or LIST_POISON2, since the immediate
+		 * dereference of them below would result in a fault.
+		 */
+		if (likely(prev->next == entry && next->prev == entry))
+			return true;
+		ret = false;
+	}
+
+	ret &= ___list_del_entry_valid(entry);
+	return ret;
 }
 #else
 static inline bool __list_add_valid(struct list_head *new,
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index fbc89baf7de6..e72cf08af0fa 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1680,6 +1680,21 @@ config DEBUG_LIST
 
 	  If unsure, say N.
 
+config DEBUG_LIST_MINIMAL
+	bool "Minimal linked list debug checks"
+	default !DEBUG_KERNEL
+	depends on DEBUG_LIST
+	help
+	  Only perform the minimal set of checks in the linked-list walking
+	  routines to catch corruptions that are not guaranteed to result in an
+	  immediate access fault.
+
+	  This trades lower quality error reports for improved performance: the
+	  generated code should be more optimal and provide trade-offs that may
+	  better serve safety- and performance- critical environments.
+
+	  If unsure, say Y.
+
 config DEBUG_PLIST
 	bool "Debug priority linked list manipulation"
 	depends on DEBUG_KERNEL
diff --git a/lib/list_debug.c b/lib/list_debug.c
index fd69009cc696..daad32855f0d 100644
--- a/lib/list_debug.c
+++ b/lib/list_debug.c
@@ -17,6 +17,7 @@
  * attempt).
  */
 
+__list_valid_slowpath
 bool ___list_add_valid(struct list_head *new, struct list_head *prev,
 		       struct list_head *next)
 {
@@ -39,6 +40,7 @@ bool ___list_add_valid(struct list_head *new, struct list_head *prev,
 }
 EXPORT_SYMBOL(___list_add_valid);
 
+__list_valid_slowpath
 bool ___list_del_entry_valid(struct list_head *entry)
 {
 	struct list_head *prev, *next;
-- 
2.41.0.640.ga95def55d0-goog