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Message-ID: <202308101131.D8DEE055@keescook>
Date: Thu, 10 Aug 2023 11:46:11 -0700
From: Kees Cook <keescook@...omium.org>
To: Przemek Kitszel <przemyslaw.kitszel@...el.com>
Cc: netdev@...r.kernel.org, Jacob Keller <jacob.e.keller@...el.com>,
intel-wired-lan@...ts.osuosl.org,
Alexander Lobakin <aleksander.lobakin@...el.com>,
linux-hardening@...r.kernel.org, Steven Zou <steven.zou@...el.com>
Subject: Re: [PATCH net-next v1 1/7] overflow: add DEFINE_FLEX() for on-stack
allocs
On Thu, Aug 10, 2023 at 06:35:03AM -0400, Przemek Kitszel wrote:
> Add DEFINE_FLEX() macro for on-stack allocations of structs with
> flexible array member.
>
> Add also const_flex_size() macro, that reads size of structs
> allocated by DEFINE_FLEX().
>
> Using underlying array for on-stack storage lets us to declare
> known-at-compile-time structures without kzalloc().
>
> Actual usage for ice driver is in following patches of the series.
>
> Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> ---
> v1: change macro name; add macro for size read;
> accept struct type instead of ptr to it; change alignment;
> ---
> include/linux/overflow.h | 27 +++++++++++++++++++++++++++
> 1 file changed, 27 insertions(+)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index f9b60313eaea..21a4410799eb 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -309,4 +309,31 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
> #define struct_size_t(type, member, count) \
> struct_size((type *)NULL, member, count)
>
> +/**
> + * DEFINE_FLEX() - Define a zeroed, on-stack, instance of @type structure with
> + * a trailing flexible array member.
> + *
> + * @type: structure type name, including "struct" keyword.
> + * @name: Name for a variable to define.
> + * @member: Name of the array member.
> + * @count: Number of elements in the array; must be compile-time const.
> + */
> +#define DEFINE_FLEX(type, name, member, count) \
> + union { \
> + u8 bytes[struct_size_t(type, member, count)]; \
> + type obj; \
> + } name##_u __aligned(_Alignof(type)) = {}; \
> + type *name = (type *)&name##_u
We'll need another macro when __counted_by is needed, but yes, if all of
these structs use non-native endian counters, we can't require it in the
base macro. (i.e. not now -- this is fine as-is.)
> +
> +/**
> + * const_flex_size() - Get size of on-stack instance of structure with
> + * a trailing flexible array member.
> + *
> + * @name: Name of the variable, the one defined by DEFINE_FLEX() macro above.
> + *
> + * Get size of @name, which is equivalent to struct_size(name, array, count),
> + * but does not require (repeating) last two arguments.
> + */
> +#define const_flex_size(name) __builtin_object_size(name, 1)
Naming is hard. ;) I don't like "const" here (it's not a storage
class). But more importantly, this calculation ("how big is this thing
actually?") gets used a lot in the fortify routines, so I'd prefer
exposing those macros (from fortify-string.h):
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index c88488715a39..4b788fa0c576 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -352,6 +352,18 @@ struct ftrace_likely_data {
# define __realloc_size(x, ...)
#endif
+/*
+ * When the size of an allocated object is needed, use the best available
+ * mechanism to find it. (For cases where sizeof() cannot be used.)
+ */
+#if __has_builtin(__builtin_dynamic_object_size)
+#define __struct_size(p) __builtin_dynamic_object_size(p, 0)
+#define __member_size(p) __builtin_dynamic_object_size(p, 1)
+#else
+#define __struct_size(p) __builtin_object_size(p, 0)
+#define __member_size(p) __builtin_object_size(p, 1)
+#endif
+
#ifndef asm_volatile_goto
#define asm_volatile_goto(x...) asm goto(x)
#endif
diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index da51a83b2829..1e7711185ec6 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -93,13 +93,9 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
#if __has_builtin(__builtin_dynamic_object_size)
#define POS __pass_dynamic_object_size(1)
#define POS0 __pass_dynamic_object_size(0)
-#define __struct_size(p) __builtin_dynamic_object_size(p, 0)
-#define __member_size(p) __builtin_dynamic_object_size(p, 1)
#else
#define POS __pass_object_size(1)
#define POS0 __pass_object_size(0)
-#define __struct_size(p) __builtin_object_size(p, 0)
-#define __member_size(p) __builtin_object_size(p, 1)
#endif
#define __compiletime_lessthan(bounds, length) ( \
And the way DEFINE_FLEX is built, __struct_size() and __member_size()
will give the same result (which is what I was concerned about for
FORTIFY_SOURCE's use of __member_size not "seeing" the flexible array
members).
In this case, I think using __struct_size() in place of const_flex_size()
in the patch series is the way to go.
--
Kees Cook
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