[PATCH v3 1/7] overflow: Provide constant expression struct_size

Kees Cook
2022-02-27T18:45:11Z

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: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Tested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/lkml/20220210010407.GA701603@embeddedor
---
v1: https://lore.kernel.org/linux-hardening/20220210004326.776574-1-keescook@chromium.org
v2:
- fix broken self-test due to CE optimization
---
include/linux/overflow.h | 10 +++++++---
lib/test_overflow.c | 26 +++++++++++++++++---------
2 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 59d7228104d0..f1221d11f8e5 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -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 */
diff --git a/lib/test_overflow.c b/lib/test_overflow.c
index 712fb2351c27..f6530fce799d 100644
--- a/lib/test_overflow.c
+++ b/lib/test_overflow.c
@@ -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);

--
2.32.0