[PATCH 3/3] list_debug: Introduce CONFIG_DEBUG_LIST_MINIMAL
Marco Elver
elver at google.com
Wed Aug 2 08:06:39 PDT 2023
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.
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 at google.com>
---
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;
--
2.41.0.585.gd2178a4bd4-goog
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