Commit 230f6fa2 authored by Kees Cook's avatar Kees Cook
Browse files

overflow: Provide constant expression struct_size 



There have been cases where struct_size() (or flex_array_size()) needs
to be calculated for an initializer, which requires it be a constant
expression. This is possible when the "count" argument is a constant
expression, so provide this ability for the helpers.

Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
Reviewed-by: default avatarGustavo A. R. Silva <gustavoars@kernel.org>
Tested-by: default avatarGustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/lkml/20220210010407.GA701603@embeddedor
parent e1be43d9

include/linux/overflow.h

+7 −3
Original line number Diff line number Diff line
@@ -4,6 +4,7 @@ 


#include <linux/compiler.h>

#include <linux/limits.h>

#include <linux/const.h>



/*

 * We need to compute the minimum and maximum values representable in a given
@@ -221,8 +222,9 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) 
 * Return: number of bytes needed or SIZE_MAX on overflow.

 */

#define flex_array_size(p, member, count)				\

	size_mul(count,							\

		 sizeof(*(p)->member) + __must_be_array((p)->member))

	__builtin_choose_expr(__is_constexpr(count),			\

		(count) * sizeof(*(p)->member) + __must_be_array((p)->member),	\

		size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))



/**

 * struct_size() - Calculate size of structure with trailing flexible array.
@@ -237,6 +239,8 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) 
 * Return: number of bytes needed or SIZE_MAX on overflow.

 */

#define struct_size(p, member, count)					\

	size_add(sizeof(*(p)), flex_array_size(p, member, count))

	__builtin_choose_expr(__is_constexpr(count),			\

		sizeof(*(p)) + flex_array_size(p, member, count),	\

		size_add(sizeof(*(p)), flex_array_size(p, member, count)))



#endif /* __LINUX_OVERFLOW_H */

lib/test_overflow.c

+17 −9
Original line number Diff line number Diff line
@@ -602,10 +602,18 @@ struct __test_flex_array { 


static int __init test_overflow_size_helpers(void)

{

	/* Make sure struct_size() can be used in a constant expression. */

	u8 ce_array[struct_size((struct __test_flex_array *)0, data, 55)];

	struct __test_flex_array *obj;

	int count = 0;

	int err = 0;

	int var;

	volatile int unconst = 0;



	/* Verify constant expression against runtime version. */

	var = 55;

	OPTIMIZER_HIDE_VAR(var);

	err |= sizeof(ce_array) != struct_size(obj, data, var);



#define check_one_size_helper(expected, func, args...)	({	\

	bool __failure = false;					\
@@ -663,28 +671,28 @@ static int __init test_overflow_size_helpers(void) 
				     flex_array_size, obj, data, var++);

	err |= check_one_size_helper(5 * sizeof(*obj->data),

				     flex_array_size, obj, data, var++);

	err |= check_one_size_helper(0, flex_array_size, obj, data, 0);

	err |= check_one_size_helper(0, flex_array_size, obj, data, 0 + unconst);

	err |= check_one_size_helper(sizeof(*obj->data),

				     flex_array_size, obj, data, 1);

				     flex_array_size, obj, data, 1 + unconst);

	err |= check_one_size_helper(7 * sizeof(*obj->data),

				     flex_array_size, obj, data, 7);

				     flex_array_size, obj, data, 7 + unconst);

	err |= check_one_size_helper(SIZE_MAX,

				     flex_array_size, obj, data, -1);

				     flex_array_size, obj, data, -1 + unconst);

	err |= check_one_size_helper(SIZE_MAX,

				     flex_array_size, obj, data, SIZE_MAX - 4);

				     flex_array_size, obj, data, SIZE_MAX - 4 + unconst);



	var = 4;

	err |= check_one_size_helper(sizeof(*obj) + (4 * sizeof(*obj->data)),

				     struct_size, obj, data, var++);

	err |= check_one_size_helper(sizeof(*obj) + (5 * sizeof(*obj->data)),

				     struct_size, obj, data, var++);

	err |= check_one_size_helper(sizeof(*obj), struct_size, obj, data, 0);

	err |= check_one_size_helper(sizeof(*obj), struct_size, obj, data, 0 + unconst);

	err |= check_one_size_helper(sizeof(*obj) + sizeof(*obj->data),

				     struct_size, obj, data, 1);

				     struct_size, obj, data, 1 + unconst);

	err |= check_one_size_helper(SIZE_MAX,

				     struct_size, obj, data, -3);

				     struct_size, obj, data, -3 + unconst);

	err |= check_one_size_helper(SIZE_MAX,

				     struct_size, obj, data, SIZE_MAX - 3);

				     struct_size, obj, data, SIZE_MAX - 3 + unconst);



	pr_info("%d overflow size helper tests finished\n", count);