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R. Silva" , Nathan Chancellor , Peter Zijlstra , Nick Desaulniers , Marco Elver , Przemek Kitszel , linux-hardening@vger.kernel.org, Linus Torvalds , Randy Dunlap , Matthew Wilcox , John Hubbard , Joe Perches , 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 v6 3/5] compiler_types: Introduce __flex_counter() and family Date: Wed, 3 Dec 2025 15:30:33 -0800 Message-Id: <20251203233036.3212363-3-kees@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20251203233029.it.641-kees@kernel.org> References: <20251203233029.it.641-kees@kernel.org> MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=5281; i=kees@kernel.org; h=from:subject; bh=MfXbJ6vSs1Dx3b2SfnaSAfowCKIWsMTNjHXn3OinNe8=; b=owGbwMvMwCVmps19z/KJym7G02pJDJkGJ6SyquI9VTrT7K0fRN96M/XBg4q2dqVNr7rUPzbWO P1O3Leto5SFQYyLQVZMkSXIzj3OxeNte7j7XEWYOaxMIEMYuDgFYCLe/Qz/M962TshlVb25WTQ3 YnViltHfjsB5KdXZGiIu2ROm6jhNY2R4Op1R/sq13++vuB0wmvLk3Fpvj8u+a03qGFTK09OCWHb zAgA= X-Developer-Key: i=kees@kernel.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 01616C0010 X-Stat-Signature: ga5yerwzjaarp4erzaayfgjp8zeuceta X-Rspam-User: X-HE-Tag: 1764804638-731977 X-HE-Meta: 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 VzvfvIDj QQZEN1u98ls8hTlpP5bzVoEpOyZ7DwoHWTADFRSSpGThDZvtjwLtckjXAf39NCIGeezQDotnQrryJq2mi5Ujyph5NKg== 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(), overflows_flex_counter_type(), 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".) overflows_flex_counter_type(typeof(*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 | 42 ++++++++++++++++++++++++++++++++++ 2 files changed, 73 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..f362e155a7ec 100644 --- a/include/linux/overflow.h +++ b/include/linux/overflow.h @@ -540,4 +540,46 @@ 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: Instance of 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))) + +/** + * overflows_flex_counter_type() - Check if the counter associated with the + * given flexible array member can represent + * a value. + * @TYPE: Type of the struct that contains the @FAM. + * @FAM: Member name of the FAM within @TYPE. + * @COUNT: Value to check against the __counted_by annotated @FAM's counter. + * + * Returns: true if @COUNT can be represented in the @FAM's counter. When + * @FAM is not annotated with __counted_by(), always returns true. + */ +#define overflows_flex_counter_type(TYPE, FAM, COUNT) \ + (!overflows_type(COUNT, typeof_flex_counter(((TYPE *)NULL)->FAM))) + +/** + * __set_flex_counter() - Set the counter associated with the given flexible + * array member that has been annoated by __counted_by(). + * @FAM: Instance of flexible array member within a given struct. + * @COUNT: Value to store to the __counted_by annotated @FAM_PTR's counter. + * + * This is a no-op if no annotation exists. Count needs to be checked with + * overflows_flex_counter_type() 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