linux-hardening - [PATCH v2] Compiler Attributes: Add __counted

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Date:   Wed, 17 May 2023 12:08:44 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Miguel Ojeda <ojeda@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Bill Wendling <morbo@...gle.com>,
        Qing Zhao <qing.zhao@...cle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v2] Compiler Attributes: Add __counted_by macro

In an effort to annotate all flexible array members with their run-time
size information, the "element_count" attribute is being introduced by
Clang[1] and GCC[2] in future releases. This annotation will provide
the CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE features the ability
to perform run-time bounds checking on otherwise unknown-size flexible
arrays.

Even though the attribute is under development, we can start the
annotation process in the kernel. This requires defining a macro for
it, even if we have to change the name of the actual attribute later.
Since it is likely that this attribute may change its name to "counted_by"
in the future (to better align with a future total bytes "sized_by"
attribute), name the wrapper macro "__counted_by", which also reads more
clearly (and concisely) in structure definitions.

[1] https://reviews.llvm.org/D148381
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896

Cc: Miguel Ojeda <ojeda@...nel.org>
Cc: Bill Wendling <morbo@...gle.com>
Cc: Qing Zhao <qing.zhao@...cle.com>
Cc: Gustavo A. R. Silva <gustavoars@...nel.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: Tom Rix <trix@...hat.com>
Cc: llvm@...ts.linux.dev
Signed-off-by: Kees Cook <keescook@...omium.org>
---
v2: - update "Optional" comments
v1: https://lore.kernel.org/all/20230504181636.never.222-kees@kernel.org/
---
 include/linux/compiler_attributes.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index e659cb6fded3..a92d8887e8f0 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -123,6 +123,19 @@
 # define __designated_init
 #endif
 
+/*
+ * Optional: only supported since gcc >= 14
+ * Optional: only supported since clang >= 17
+ *
+ *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
+ * clang: https://reviews.llvm.org/D148381
+ */
+#if __has_attribute(__element_count__)
+# define __counted_by(member)		__attribute__((__element_count__(member)))
+#else
+# define __counted_by(member)
+#endif
+
 /*
  * Optional: only supported since clang >= 14.0
  *
-- 
2.34.1