From: Kees Cook <keescook@chromium•org>
To: Przemek Kitszel <przemyslaw.kitszel@intel•com>
Cc: Jacob Keller <jacob.e.keller@intel•com>,
intel-wired-lan@lists•osuosl.org, netdev@vger•kernel.org,
Alexander Lobakin <aleksander.lobakin@intel•com>
Subject: Re: [RFC net-next 1/2] overflow: add DECLARE_FLEX() for on-stack allocs
Date: Fri, 4 Aug 2023 15:39:53 -0700 [thread overview]
Message-ID: <202308041539.888828AC3@keescook> (raw)
In-Reply-To: <e0cb5bf2-2278-b83f-c45c-0556927787a6@intel.com>
On Fri, Aug 04, 2023 at 03:47:48PM +0200, Przemek Kitszel wrote:
> On 8/2/23 00:31, Kees Cook wrote:
>
> [...]
>
> > Initially I was struggling to make __counted_by work, but it seems we can
> > use an initializer for that member, as long as we don't touch the flexible
> > array member in the initializer. So we just need to add the counted-by
> > member to the macro, and use a union to do the initialization. And if
> > we take the address of the union (and not the struct within it), the
> > compiler will see the correct object size with __builtin_object_size:
> >
> > #define DEFINE_FLEX(type, name, flex, counter, count) \
> > union { \
> > u8 bytes[struct_size_t(type, flex, count)]; \
> > type obj; \
> > } name##_u __aligned(_Alignof(type)) = { .obj.counter = count }; \
> > /* take address of whole union to get the correct __builtin_object_size */ \
> > type *name = (type *)&name##_u
> >
> > i.e. __builtin_object_size(name, 1) (as used by FORTIFY_SOURCE, etc)
> > works correctly here, but breaks (sees a zero-sized flex array member)
> > if this macro ends with:
> >
> > type *name = &name##_u.obj
>
> __builtin_object_size(name, 0) works fine for both versions (with and
> without .obj at the end)
Mode 0 will be unchanged, but mode 1 is what most of FORTIFY uses for
keep the scope narrow.
> however it does not work for builds without -O2 switch, so struct_size_t()
> is rather a way to go :/
FORTIFY depends on -O2, so no worries. :)
--
Kees Cook
next prev parent reply other threads:[~2023-08-04 22:39 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-01 11:19 [RFC net-next 0/2] introduce DECLARE_FLEX() macro Przemek Kitszel
2023-08-01 11:19 ` [RFC net-next 1/2] overflow: add DECLARE_FLEX() for on-stack allocs Przemek Kitszel
2023-08-01 13:54 ` Alexander Lobakin
2023-08-01 14:18 ` Przemek Kitszel
2023-08-01 17:15 ` Alexander Lobakin
2023-08-01 22:31 ` Kees Cook
2023-08-04 10:59 ` Przemek Kitszel
2023-08-04 15:49 ` Alexander Lobakin
2023-08-04 13:47 ` Przemek Kitszel
2023-08-04 15:44 ` Alexander Lobakin
2023-08-07 12:42 ` Przemek Kitszel
2023-08-07 14:47 ` Kees Cook
2023-08-04 22:39 ` Kees Cook [this message]
2023-08-01 11:19 ` [RFC net-next 2/2] ice: make use of DECLARE_FLEX() in ice_switch.c Przemek Kitszel
2023-08-01 13:48 ` Alexander Lobakin
2023-08-01 14:36 ` Przemek Kitszel
2023-08-01 17:22 ` Alexander Lobakin
2023-08-01 22:35 ` Kees Cook
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