[PATCH v6 1/4] Compiler Attributes: Add __pass_object_size for Clang

[Kees Cook <keescook@chromium.org>]

In order to gain greater visibility to type information when using
__builtin_object_size(), Clang has a function attribute "pass_object_size"
that will make size information available for marked arguments in
a function by way of implicit additional function arguments that are
then wired up the __builtin_object_size().

This is needed to implement FORTIFY_SOURCE in Clang, as a workaround
to Clang's __builtin_object_size() having limited visibility[1] into types
across function calls (even inlines).

Since any usage must also be const, include it in the macro.

This attribute has an additional benefit that it can be used even on
non-inline functions to gain argument size information.

[1] https://github.com/llvm/llvm-project/issues/53516

Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: llvm@lists.linux.dev
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/compiler_attributes.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 37e260020221..4ce370094e3a 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -263,6 +263,20 @@
  */
 #define __packed                        __attribute__((__packed__))
 
+/*
+ * Note: the "type" argument should match any __builtin_object_size(p, type) usage.
+ *
+ * Optional: not supported by gcc.
+ * Optional: not supported by icc.
+ *
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size
+ */
+#if __has_attribute(__pass_object_size__)
+# define __pass_object_size(type)	const __attribute__((__pass_object_size__(type)))
+#else
+# define __pass_object_size(type)
+#endif
+
 /*
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute
  */
-- 
2.30.2