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R. Silva" , Nathan Chancellor , Peter Zijlstra , Nick Desaulniers , Marco Elver , Przemek Kitszel , linux-hardening@vger.kernel.org, Randy Dunlap , Linus Torvalds , Matthew Wilcox , Christoph Lameter , Vegard Nossum , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Roman Gushchin , Harry Yoo , Bill Wendling , Justin Stitt , Jann Horn , Greg Kroah-Hartman , Sasha Levin , linux-mm@kvack.org, Nick Desaulniers , Jonathan Corbet , Jakub Kicinski , Yafang Shao , Tony Ambardar , Alexander Lobakin , Jan Hendrik Farr , Alexander Potapenko , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, llvm@lists.linux.dev Subject: [PATCH v5 1/4] compiler_types: Introduce __flex_counter() and family Date: Fri, 21 Nov 2025 17:42:57 -0800 Message-Id: <20251122014304.3417954-1-kees@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20251122014258.do.018-kees@kernel.org> References: <20251122014258.do.018-kees@kernel.org> MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=5209; i=kees@kernel.org; h=from:subject; bh=Bm5UctH3mgw7LUjsKVnWiQeM2ZjrjWLRLpRvc3SOhQI=; b=owGbwMvMwCVmps19z/KJym7G02pJDJmKoso7nm58EGTt81C0PG6jhZD/jO6pLRohBZOjykWLY zhERVk7SlkYxLgYZMUUWYLs3ONcPN62h7vPVYSZw8oEMoSBi1MAJvLPmpHhtnWqcu45r7Nt6RxR piEzpIWdq2qOP7DcfZlTa5bZAbadjAwPN/G4mYuxhbHdYPr2JoUnd9p5t+mHfkaHZvQWbL51NZQ FAA== X-Developer-Key: i=kees@kernel.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: D38222000C X-Stat-Signature: fjdrxtbc1xu5m9t1j1mexjbpzc3qg3o1 X-Rspam-User: X-HE-Tag: 1763775786-869130 X-HE-Meta: U2FsdGVkX1/ObjPFVjcNPa7I45Td6c2WWqTFzjh5OWPEMz8I3fv3nyk9OEueXu7GQsvK5UL0SEDYdHWHKjc5qLeVX23Wi+fGfVdnjI9Z410KJLxwEQcwGn61Q6tNzF/F/Q9/wT1KnItLdHz2QEoPC0XngRAd2NHenuIzJwYK5UWxgbVIhng3fOlKSAeoZX4kdgNN5MihQX+VhzU/7vwjnW6OxGwWA2dp9yYEMel+UP852ripxcxF6MnGbqtZTfXxmANnnU/o/O/uC5NtAhGZL3IPMa809qLJTBHRr4GUAcBx0C/qMP5ICFgPfoM09kmg7nM2+DNxY3iJ3UmoPygCfJyf6fIbhROgPlJaHh//Ve/UJxHlkqpllj3NBobRpheeIEdIch/PgfpC4AqC31uc6boeJYeh9gdvCqrm9uyC0bZ+S5oqnD3ngq/AX9Tnl8KJ6INugWdus/BE1IIN8OBjNwDkm5lpyI9stIJibiVhzmb/1XPqftAFUfyWvHuTLGsl/hje6faunrG9odylpWbKMzGLJwdRLS3KaQimDm6/UFrzGj5XI4ZKpBRn3SPLSzpKqQiX2jshgOvPAi5srzYUpSBf1uDITLBOjS7R1DoI/Q14+FO2LVZWks4Uul3/Zr65AMJ+zWTg36zE/BCjAcQOAv5wqS3Y4FdpyhyiAKiolez3XZOfGoB+kE2DJVGNWqzbkKwYktf9W6rLt8auAXOoirhERuzgMfPqy5DUVTqC9LDEWJk348kY3OB7kukNko3FnuuSgjo7wCa0r99HbhOhz4Rgxo8U2soCA5AQO0vfH7NqGEMEKEmTy3kU8468kW1XJoHwxeu80NJ/DCNGs9mrmfmHRoV0n+5M0hz9hTET27vWLbjPuD7sbaFJlrGwwPi0OQFBIzPhKH5d10zH1Z3dhbsU23B72ujapgtpADifLsVtRU0gHT3tt2EYRXH7arl8Bk5UJEGFO48m3/7phxo 69beRkht XYvRBf7hZUEWOBBAT/9qEGIRDmtZpf8/xPK3vW5ZXFZjfrz9Q45mRp0tta7PjHq3g685XaEhHxm28mPn/RX1YUxtIsQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Introduce __flex_counter() which wraps __builtin_counted_by_ref(), as newly introduced by GCC[1] and Clang[2]. Use of __flex_counter() allows access to the counter member of a struct's flexible array member when it has been annotated with __counted_by(). Introduce typeof_flex_counter(), can_set_flex_counter(), and set_flex_counter() to provide the needed _Generic() wrappers to get sane results out of __flex_counter(). For example, with: struct foo { int counter; short array[] __counted_by(counter); } *p; __flex_counter(p->array) will resolve to: &p->counter typeof_flex_counter(p->array) will resolve to "int". (If p->array was not annotated, it would resolve to "size_t".) can_set_flex_counter(p->array, COUNT) is the same as: COUNT <= type_max(p->counter) && COUNT >= type_min(p->counter) (If p->array was not annotated it would return true since everything fits in size_t.) set_flex_counter(p->array, COUNT) is the same as: p->counter = COUNT; (It is a no-op if p->array is not annotated with __counted_by().) Signed-off-by: Kees Cook --- Cc: Miguel Ojeda Cc: "Gustavo A. R. Silva" Cc: Nathan Chancellor Cc: Peter Zijlstra Cc: Nick Desaulniers Cc: Marco Elver Cc: Przemek Kitszel Cc: linux-hardening@vger.kernel.org --- include/linux/compiler_types.h | 31 ++++++++++++++++++++++++++ include/linux/overflow.h | 40 ++++++++++++++++++++++++++++++++++ 2 files changed, 71 insertions(+) diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index c46855162a8a..a31fe3dbf576 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -507,6 +507,37 @@ struct ftrace_likely_data { #define __annotated(var, attr) __builtin_has_attribute(var, attr) #endif +/* + * Optional: only supported since gcc >= 15, clang >= 19 + * + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fcounted_005fby_005fref + * clang: https://clang.llvm.org/docs/LanguageExtensions.html#builtin-counted-by-ref + */ +#if __has_builtin(__builtin_counted_by_ref) +/** + * __flex_counter() - Get pointer to counter member for the given + * flexible array, if it was annotated with __counted_by() + * @FAM: Pointer to flexible array member of an addressable struct instance + * + * For example, with: + * + * struct foo { + * int counter; + * short array[] __counted_by(counter); + * } *p; + * + * __flex_counter(p->array) will resolve to &p->counter. + * + * Note that Clang may not allow this to be assigned to a separate + * variable; it must be used directly. + * + * If p->array is unannotated, this returns (void *)NULL. + */ +#define __flex_counter(FAM) __builtin_counted_by_ref(FAM) +#else +#define __flex_counter(FAM) ((void *)NULL) +#endif + /* * Some versions of gcc do not mark 'asm goto' volatile: * diff --git a/include/linux/overflow.h b/include/linux/overflow.h index 725f95f7e416..12ca286c0f34 100644 --- a/include/linux/overflow.h +++ b/include/linux/overflow.h @@ -540,4 +540,44 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) (__member_size((name)->array) / sizeof(*(name)->array) + \ __must_be_array((name)->array)) +/** + * typeof_flex_counter() - Return the type of the counter variable of a given + * flexible array member annotated by __counted_by(). + * @FAM: Pointer to the flexible array member within a given struct. + * + * Returns: "size_t" if no annotation exists. + */ +#define typeof_flex_counter(FAM) \ + typeof(_Generic(__flex_counter(FAM), \ + void *: (size_t)0, \ + default: *__flex_counter(FAM))) + +/** + * __can_set_flex_counter() - Check if the counter associated with the given + * flexible array member can represent a value. + * @FAM: Pointer to the flexible array member within a given struct. + * @COUNT: Value to check against the __counted_by annotated @FAM's counter. + * + * Returns: true if @COUNT can be represented in the @FAM counter. When + * @FAM is not annotated with __counted_by(), always returns true. + */ +#define __can_set_flex_counter(FAM, COUNT) \ + (!overflows_type(COUNT, typeof_flex_counter(FAM))) + +/** + * __set_flex_counter() - Set the counter associated with the given flexible + * array member that has been annoated by __counted_by(). + * @FAM: Pointer to the flexible array member within a given struct. + * @COUNT: Value to store to the __counted_by annotated @FAM's counter. + * + * This is a no-op if no annotation exists. Count needs to be checked with + * __can_set_flex_counter(@FAM, @COUNT) before using this function. + */ +#define __set_flex_counter(FAM, COUNT) \ +({ \ + *_Generic(__flex_counter(FAM), \ + void *: &(size_t){ 0 }, \ + default: __flex_counter(FAM)) = (COUNT); \ +}) + #endif /* __LINUX_OVERFLOW_H */ -- 2.34.1