linux-hardening - [PATCH v7 2/8] Compiler Attributes: Add __pass_object

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Date:   Tue,  8 Feb 2022 14:53:44 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>, llvm@...ts.linux.dev,
        Miguel Ojeda <ojeda@...nel.org>,
        George Burgess IV <gbiv@...gle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v7 2/8] 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] https://github.com/llvm/llvm-project/issues/53516

Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: llvm@...ts.linux.dev
Reviewed-by: Miguel Ojeda <ojeda@...nel.org>
Signed-off-by: Kees Cook <keescook@...omium.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..d0c503772061 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)	__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