Re: [PATCH v7 1/8] overflow: Move and add few utility macros into overflow

Andrzej Hajda
2022-08-22T14:06:33Z

On 18.08.2022 02:12, Kees Cook wrote:
> On Thu, Aug 18, 2022 at 01:07:29AM +0200, Andi Shyti wrote:
>> Hi Kees,
>>
>> would you mind taking a look at this patch?
>
> Hi! Thanks for the heads-up!
>
>>
>> Thanks,
>> Andi
>>
>> On Tue, Aug 16, 2022 at 06:35:18PM +0900, Gwan-gyeong Mun wrote:
>>> It moves overflows_type utility macro into overflow header from i915_utils
>>> header. The overflows_type can be used to catch the truncation between data
>>> types. And it adds safe_conversion() macro which performs a type conversion
>>> (cast) of an source value into a new variable, checking that the
>>> destination is large enough to hold the source value. And the functionality
>>> of overflows_type has been improved to handle the signbit.
>>> The is_unsigned_type macro has been added to check the sign bit of the
>>> built-in type.
>>>
>>> v3: Add is_type_unsigned() macro (Mauro)
>>> Modify overflows_type() macro to consider signed data types (Mauro)
>>> Fix the problem that safe_conversion() macro always returns true
>>> v4: Fix kernel-doc markups
>>> v6: Move macro addition location so that it can be used by other than drm
>>> subsystem (Jani, Mauro, Andi)
>>> Change is_type_unsigned to is_unsigned_type to have the same name form
>>> as is_signed_type macro
>>>
>>> Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
>>> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>>> Cc: Matthew Auld <matthew.auld@intel.com>
>>> Cc: Nirmoy Das <nirmoy.das@intel.com>
>>> Cc: Jani Nikula <jani.nikula@intel.com>
>>> Cc: Andi Shyti <andi.shyti@linux.intel.com>
>>> Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org> (v5)
>>> ---
>>> drivers/gpu/drm/i915/i915_utils.h | 5 +--
>>> include/linux/overflow.h | 54 +++++++++++++++++++++++++++++++
>>> 2 files changed, 55 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
>>> index c10d68cdc3ca..eb0ded23fa9c 100644
>>> --- a/drivers/gpu/drm/i915/i915_utils.h
>>> +++ b/drivers/gpu/drm/i915/i915_utils.h
>>> @@ -32,6 +32,7 @@
>>> #include <linux/types.h>
>>> #include <linux/workqueue.h>
>>> #include <linux/sched/clock.h>
>>> +#include <linux/overflow.h>
>>>
>>> #ifdef CONFIG_X86
>>> #include <asm/hypervisor.h>
>>> @@ -111,10 +112,6 @@ bool i915_error_injected(void);
>>> #define range_overflows_end_t(type, start, size, max) \
>>> range_overflows_end((type)(start), (type)(size), (type)(max))
>>>
>>> -/* Note we don't consider signbits :| */
>>> -#define overflows_type(x, T) \
>>> - (sizeof(x) > sizeof(T) && (x) >> BITS_PER_TYPE(T))
>>> -
>>> #define ptr_mask_bits(ptr, n) ({ \
>>> unsigned long __v = (unsigned long)(ptr); \
>>> (typeof(ptr))(__v & -BIT(n)); \
>>> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
>>> index f1221d11f8e5..462a03454377 100644
>>> --- a/include/linux/overflow.h
>>> +++ b/include/linux/overflow.h
>>> @@ -35,6 +35,60 @@
>>> #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
>>> #define type_min(T) ((T)((T)-type_max(T)-(T)1))
>>>
>>> +/**
>>> + * is_unsigned_type - helper for checking data type which is an unsigned data
>>> + * type or not
>>> + * @x: The data type to check
>>> + *
>>> + * Returns:
>>> + * True if the data type is an unsigned data type, false otherwise.
>>> + */
>>> +#define is_unsigned_type(x) ((typeof(x))-1 >= (typeof(x))0)
>
> I'd rather not have separate logic for this. Instead, I'd like it to be:
>
> #define is_unsigned_type(x) (!is_signed_type(x))
>
>>> +
>>> +/**
>>> + * overflows_type - helper for checking the truncation between data types
>>> + * @x: Source for overflow type comparison
>>> + * @T: Destination for overflow type comparison
>>> + *
>>> + * It compares the values and size of each data type between the first and
>>> + * second argument to check whether truncation can occur when assigning the
>>> + * first argument to the variable of the second argument.
>>> + * Source and Destination can be used with or without sign bit.
>>> + * Composite data structures such as union and structure are not considered.
>>> + * Enum data types are not considered.
>>> + * Floating point data types are not considered.
>>> + *
>>> + * Returns:
>>> + * True if truncation can occur, false otherwise.
>>> + */
>>> +#define overflows_type(x, T) \
>>> + (is_unsigned_type(x) ? \
>>> + is_unsigned_type(T) ? \
>>> + (sizeof(x) > sizeof(T) && (x) >> BITS_PER_TYPE(T)) ? 1 : 0 \
>>> + : (sizeof(x) >= sizeof(T) && (x) >> (BITS_PER_TYPE(T) - 1)) ? 1 : 0 \
>>> + : is_unsigned_type(T) ? \
>>> + ((x) < 0) ? 1 : (sizeof(x) > sizeof(T) && (x) >> BITS_PER_TYPE(T)) ? 1 : 0 \
>>> + : (sizeof(x) > sizeof(T)) ? \
>>> + ((x) < 0) ? (((x) * -1) >> BITS_PER_TYPE(T)) ? 1 : 0 \
>>> + : ((x) >> BITS_PER_TYPE(T)) ? 1 : 0 \
>>> + : 0)
>
> Like the other, I'd much rather this was rephrased in terms of the
> existing macros (e.g. type_min()/type_max().)

I am not sure how it could be rephrased with type_(min|max), but I guess
the shortest could be sth like:

#define overflows_type(x, T) __builtin_add_overflow_p(x, (typeof(T))0,
(typeof(T))0)

Regards
Andrzej

>
>>> +
>>> +/**
>>> + * safe_conversion - perform a type conversion (cast) of an source value into
>>> + * a new variable, checking that the destination is large enough to hold the
>>> + * source value.
>>> + * @ptr: Destination pointer address
>>> + * @value: Source value
>>> + *
>>> + * Returns:
>>> + * If the value would overflow the destination, it returns false.
>>> + */
>>> +#define safe_conversion(ptr, value) ({ \
>>> + typeof(value) __v = (value); \
>>> + typeof(ptr) __ptr = (ptr); \
>>> + overflows_type(__v, *__ptr) ? 0 : ((*__ptr = (typeof(*__ptr))__v), 1); \
>>> +})
>
> I try to avoid "safe" as an adjective for interface names, since it
> doesn't really answer "safe from what?" This looks more like "assign, but
> zero when out of bounds". And it can be built from existing macros here:
>
> if (check_add_overflow(0, value, ptr))
> *ptr = 0;
>
> I actually want to push back on this a bit, because there can still be
> logic bugs built around this kind of primitive. Shouldn't out-of-bounds
> assignments be seen as a direct failure? I would think this would be
> sufficient:
>
> #define check_assign(value, ptr) check_add_overflow(0, value, ptr)
>
> And callers would do:
>
> if (check_assign(value, &var))
> return -EINVAL;
>
> etc.
>
>