On 8/18/23 09:14, David Laight wrote:
From: Kees Cook
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Sent: Thursday, August 17, 2023 6:00 PM
On Thu, Aug 17, 2023 at 02:35:23PM +0000, David Laight wrote:
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From: Przemek Kitszel
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Sent: Wednesday, August 16, 2023 3:06 PM
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+#define DEFINE_FLEX(type, name, member, count) \
+ union { \
+ u8 bytes[struct_size_t(type, member, count)]; \
+ type obj; \
+ } name##_u __aligned(_Alignof(type)) = {}; \
You shouldn't need the _Alignof() it is the default.
In the sense that since "type" is in the union, it's okay?
The alignment of the union is the larger of the alignments
of all its members.
Which is what you want.
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I'm not sure you should be forcing the memset() either.
This already got discussed: better to fail safe.
Perhaps call it DEFINE_FLEX_Z() to make this clear and
give the option for a non-zeroing version later.
Not everyone wants the expense of zeroing everything.
per Kees, zeroing should be removed by compiler when not needed:
https://lore.kernel.org/intel-wired-lan/202308101128.C4F0FA235@keescook/ (local)
Thanks for education on alignment!
..
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You might want to add:
Static_assert(is_constexpr(count), "DEFINE_FLEX: non-constant count " #count);
That would be nice, though can Static_assert()s live in the middle of
variable definitions?
I checked and it is fine.
(I double-checked by adding a statement and getting an error.)
Static_assert with nice wording definitively would make it better, thanks again!
David >
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