[PATCH] ubsan: introducing CONFIG_UBSAN_LOCAL_BOUNDS for Clang

[George-Aurelian Popescu <georgepope@google.com>]

From: George Popescu <georgepope@android.com>

When the kernel is compiled with Clang, -fsanitize=bounds expands to
-fsanitize=array-bounds and -fsanitize=local-bounds.

Enabling -fsanitize=local-bounds with Clang has the unfortunate
side-effect of inserting traps; this goes back to its original intent,
which was as a hardening and not a debugging feature [1]. The same feature
made its way into -fsanitize=bounds, but the traps remained. For that
reason, -fsanitize=bounds was split into 'array-bounds' and
'local-bounds' [2].

Since 'local-bounds' doesn't behave like a normal sanitizer, enable
it with Clang only if trapping behaviour was requested by
CONFIG_UBSAN_TRAP=y.

Add the UBSAN_LOCAL_BOUNDS config to Kconfig.ubsan to enable the
'local-bounds' option by default when UBSAN_TRAP is enabled.

[1] http://lists.llvm.org/pipermail/llvm-dev/2012-May/049972.html
[2] http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20131021/091536.html

Suggested-by: Marco Elver <elver@google.com>
Reviewed-by: David Brazdil <dbrazdil@google.com>
Signed-off-by: George Popescu <georgepope@android.com>

---
v1: changed the name of the config to UBSAN_LOCAL_BOUNDS in Kconfig
---
---
 lib/Kconfig.ubsan      | 14 ++++++++++++++
 scripts/Makefile.ubsan | 10 +++++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index 774315de555a..58f8d03d037b 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -47,6 +47,20 @@ config UBSAN_BOUNDS
 	  to the {str,mem}*cpy() family of functions (that is addressed
 	  by CONFIG_FORTIFY_SOURCE).
 
+config UBSAN_LOCAL_BOUNDS
+	bool "Perform array local bounds checking"
+	depends on UBSAN_TRAP
+	depends on CC_IS_CLANG
+	depends on !UBSAN_KCOV_BROKEN
+	help
+	  This option enables -fsanitize=local-bounds which traps when an
+	  exception/error is detected. Therefore, it should be enabled only
+	  if trapping is expected.
+	  Enabling this option detects errors due to accesses through a
+	  pointer that is derived from an object of a statically-known size,
+	  where an added offset (which may not be known statically) is
+	  out-of-bounds.
+
 config UBSAN_MISC
 	bool "Enable all other Undefined Behavior sanity checks"
 	default UBSAN
diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
index 27348029b2b8..4e3fff0745e8 100644
--- a/scripts/Makefile.ubsan
+++ b/scripts/Makefile.ubsan
@@ -4,7 +4,15 @@ ifdef CONFIG_UBSAN_ALIGNMENT
 endif
 
 ifdef CONFIG_UBSAN_BOUNDS
-      CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
+      ifdef CONFIG_CC_IS_CLANG
+            CFLAGS_UBSAN += -fsanitize=array-bounds
+      else
+            CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
+      endif
+endif
+
+ifdef CONFIG_UBSAN_LOCAL_BOUNDS
+      CFLAGS_UBSAN += -fsanitize=local-bounds
 endif
 
 ifdef CONFIG_UBSAN_MISC
-- 
2.28.0.681.g6f77f65b4e-goog