@@ -820,6 +820,25 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
x, y)
/**
+ * const_max_t - return maximum of two compile-time constant expressions
+ * @type: type used for evaluation
+ * @x: first compile-time constant expression
+ * @y: second compile-time constant expression
+ *
+ * This has no multi-evaluation defenses, and must only ever be used with
+ * compile-time constant expressions (for example when calculating a stack
+ * array size).
+ */
+size_t __error_non_const_arg(void) \
+__compiletime_error("const_max_t() used with non-constant expression");
+#define const_max_t(type, x, y) \
+ __builtin_choose_expr(__builtin_constant_p(x) && \
+ __builtin_constant_p(y), \
+ (type)(x) > (type)(y) ? \
+ (type)(x) : (type)(y), \
+ __error_non_const_arg())
+
+/**
* min3 - return minimum of three values
* @x: first value
* @y: second value
In the effort to remove all VLAs from the kernel[1], it is desirable to build with -Wvla. However, this warning is overly pessimistic, in that it is only happy with stack array sizes that are declared as constant expressions, and not constant values. One case of this is the evaluation of the max() macro which, due to its construction, ends up converting constant expression arguments into a constant value result. Attempts to adjust the behavior of max() ran afoul of version-dependent compiler behavior[2]. To work around this and still gain -Wvla coverage, this patch introduces a new macro, const_max_t(), for use in these cases of stack array size declaration, where the constant expressions are retained. Since this means losing the double-evaluation protections of the max() macro, this macro is designed to explicitly fail if used on non-constant arguments. Older compilers will fail with the unhelpful message: error: first argument to ‘__builtin_choose_expr’ not a constant Newer compilers will fail with a hopefully more helpful message: error: call to ‘__error_non_const_arg’ declared with attribute error: const_max_t() used with non-constant expression To gain the ability to compare differing types, the desired type must be explicitly declared, as with the existing max_t() macro. This is needed when comparing different enum types and to allow things like: int foo[const_max_t(size_t, 6, sizeof(something))]; [1] https://lkml.org/lkml/2018/3/7/621 [2] https://lkml.org/lkml/2018/3/10/170 Signed-off-by: Kees Cook <keescook@chromium.org> --- include/linux/kernel.h | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+)