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Date:   Tue,  1 Feb 2022 16:30:30 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>, llvm@...ts.linux.dev,
        George Burgess IV <gbiv@...gle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: [PATCH 1/4] Compiler Attributes: Add Clang's __pass_object_size

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 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@...nel.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: llvm@...ts.linux.dev
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 include/linux/compiler_attributes.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 37e260020221..cc751e0770f5 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -263,6 +263,17 @@
  */
 #define __packed                        __attribute__((__packed__))
 
+/*
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size
+ *
+ * The "type" argument should match the __builtin_object_size(p, type) usage.
+ */
+#if __has_attribute(__pass_object_size__)
+# define __pass_object_size(type)	__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

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