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Message-ID: <eb6e6198-dc42-4a63-8d0d-35f3061ff388@intel.com>
Date: Mon, 17 Mar 2025 10:26:30 +0100
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Kees Cook <kees@...nel.org>
CC: Vlastimil Babka <vbabka@...e.cz>, Miguel Ojeda <ojeda@...nel.org>,
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Subject: Re: [PATCH v4 1/2] compiler_types: Introduce __flex_counter() and
family
On 3/15/25 04:15, Kees Cook wrote:
> Introduce __flex_counter() which wraps __builtin_counted_by_ref(),
> as newly introduced by GCC[1] and Clang[2]. Use of __flex_counter()
> allows access to the counter member of a struct's flexible array member
> when it has been annotated with __counted_by().
>
> Introduce typeof_flex_counter(), can_set_flex_counter(), and
> set_flex_counter() to provide the needed _Generic() wrappers to get sane
> results out of __flex_counter().
>
> For example, with:
>
> struct foo {
> int counter;
> short array[] __counted_by(counter);
> } *p;
>
> __flex_counter(p->array) will resolve to: &p->counter
>
> typeof_flex_counter(p->array) will resolve to "int". (If p->array was not
> annotated, it would resolve to "size_t".)
>
> can_set_flex_counter(p->array, COUNT) is the same as:
>
> COUNT <= type_max(p->counter) && COUNT >= type_min(p->counter)
>
> (If p->array was not annotated it would return true since everything
> fits in size_t.)
>
> set_flex_counter(p->array, COUNT) is the same as:
>
> p->counter = COUNT;
>
> (It is a no-op if p->array is not annotated with __counted_by().)
>
> Signed-off-by: Kees Cook <kees@...nel.org>
I agree that there is no suitable fallback handy, but I see counter
as integral part of the struct (in contrast to being merely annotation),
IOW, without set_flex_counter() doing the assignment, someone will
reference it later anyway, without any warning when kzalloc()'d
So, maybe BUILD_BUG() instead of no-op?
> +#define set_flex_counter(FAM, COUNT) \
> +({ \
> + *_Generic(__flex_counter(FAM), \
> + void *: &(size_t){ 0 }, \
> + default: __flex_counter(FAM)) = (COUNT); \
> +})
> +
> #endif /* __LINUX_OVERFLOW_H */
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