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: 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

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 59d7228104d0211d8ffb4cd330b9cf6d01992270..f1221d11f8e57a7dfe9c33dd6363b5875ef6b796 100644 (file)
--- 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 712fb2351c271787d989db33a24d722463358eb2..f6530fce799db7ad6e4f7d6c7c4c28be42a7aaf2 100644 (file)
--- 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);