[PATCH v5 4/4] initify: Mark functions with the __unverified_nocapture attribute

From: Kees Cook
Date: Tue Jan 31 2017 - 15:32:24 EST

From: Emese Revfy <re.emese@xxxxxxxxx>

The initify plugin attempts to analyze function arguments that have been
marked correctly with the __nocapture attribute. However, due to code
complexity, this is not always possible to verify. As a result, some
__nocapture attributes need to be marked as excluded from the automatic
verification. To do this, the __unverified_nocapture attribute is added.
It is intedned to only be used on function parameters that are difficult
for the plugin to analyze.

Signed-off-by: Emese Revfy <re.emese@xxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
include/linux/compiler-gcc.h | 8 ++++++++
include/linux/compiler.h | 4 ++++
lib/vsprintf.c | 4 ++--
3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index cd4e9ffb00a7..fc0495e849ff 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -204,9 +204,17 @@
* been unmapped from memory. In order to identify functions that are confirmed
* to not capture their arguments, the __nocapture() attribute is used so that
* initify can better identify candidate variables.
+ *
+ * Arguments marked in this way are verified by the plugin, but sometimes
+ * code complexity and other limitiations will cause initify to not be able
+ * to check it correctly. For these cases, the __unverified_nocapture
+ * attribute can be added to disable this checking, overriding the plugin
+ * logic for cases that have been manually verified. This should not need
+ * to be used very often.
#define __nocapture(...) __attribute__((nocapture(__VA_ARGS__)))
+#define __unverified_nocapture(...) __attribute__((unverified_nocapture(__VA_ARGS__)))

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 8b3dcc790bb6..1bde420f07bb 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -437,6 +437,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
# define __nocapture(...)

+#ifndef __unverified_nocapture
+# define __unverified_nocapture(...)
* Tell gcc if a function is cold. The compiler will assume any path
* directly leading to the call is unlikely.
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index a192761d338a..cb964b51f9f8 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -118,7 +118,7 @@ long long simple_strtoll(const char *cp, char **endp, unsigned int base)

-static noinline_for_stack __nocapture(1)
+static noinline_for_stack __nocapture(1) __unverified_nocapture(1)
int skip_atoi(const char **s)
int i = 0;
@@ -1570,7 +1570,7 @@ int kptr_restrict __read_mostly;
* function pointers are really function descriptors, which contain a
* pointer to the real address.
-static noinline_for_stack __nocapture(1)
+static noinline_for_stack __nocapture(1) __unverified_nocapture(1)
char *pointer(const char *fmt, char *buf, char *end, void *ptr,
struct printf_spec spec)