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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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