BACKPORT: Compiler Attributes: Add __pass_object_size for Clang

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).

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

[1] llvm/llvm-project#53516

Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: llvm@lists.linux.dev
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20220208225350.1331628-3-keescook@chromium.org
Signed-off-by: UtsavBalar1231 <utsavbalar1231@gmail.com>
Change-Id: I02cd8f92bcd829102498be5d9cc4ff7659879ca3
Signed-off-by: UtsavBalar1231 <utsavbalar1231@gmail.com>
Signed-off-by: Diaz1401 <reagor8161@outlook.com>

include/linux/compiler-clang.h

@@ -64,3 +64,10 @@
 #else
 # define __noscs
 #endif
+
+/*
+ * Note: the "type" argument should match any __builtin_object_size(p, type) usage.
+ *
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size
+ */
+#define __pass_object_size(type)	__attribute__((__pass_object_size__(type)))