linux-hardening - [PATCH][next] overflow: Fix direct struct member initialization in _DEFINE

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Message-ID: <aBK2TUEeQfCFop9Y@kspp>
Date: Wed, 30 Apr 2025 17:46:21 -0600
From: "Gustavo A. R. Silva" <gustavoars@...nel.org>
To: Kees Cook <kees@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>
Subject: [PATCH][next] overflow: Fix direct struct member initialization in
 _DEFINE_FLEX()

Currently, to statically initialize the struct members of the `type`
object created by _DEFINE_FLEX(), the internal `obj` member must be
explicitly referenced at the call site. See:

struct flex {
	int a;
	int b;
	struct foo flex_array[];
};

_DEFINE_FLEX(struct flex, instance, flex_array,
	     FIXED_SIZE, = {
		.obj = {
			.a = 0,
			.b = 1,
		},
	});

This leaks _DEFINE_FLEX() internal implementation details and make
the helper harder to use and read.

Fix this and allow for a more natural and intuitive C99 init-style:

_DEFINE_FLEX(struct flex, instance, flex_array,
	     FIXED_SIZE, = {
		.a = 0,
		.b = 1,
	});

Also, update "counter" member initialization in DEFINE_FLEX().

Fixes: 26dd68d293fd ("overflow: add DEFINE_FLEX() for on-stack allocs")
Link: https://lore.kernel.org/linux-hardening/c4828c41-e46c-43c9-a73a-38ce8ab2c1c4@embeddedor.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 include/linux/overflow.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 69533e703be5..170d3cfe7ecc 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -404,7 +404,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
 	union {									\
 		u8 bytes[struct_size_t(type, member, count)];			\
 		type obj;							\
-	} name##_u initializer;							\
+	} name##_u = { .obj initializer };					\
 	type *name = (type *)&name##_u
 
 /**
@@ -444,7 +444,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * elements in array @member.
  */
 #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT)	\
-	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
+	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .COUNTER = COUNT, })
 
 /**
  * STACK_FLEX_ARRAY_SIZE() - helper macro for DEFINE_FLEX() family.
-- 
2.43.0