12:12 p.m.
On Thu, Nov 18, 2021 at 06:24:09PM +0100, Laszlo Ersek wrote:
> Your analysis proceeds with all of "a", "b",
and "max" of the same
> unsigned types. I understand why you skipped signed types (the
> analysis is harder); but it it worth supporting cases where "a" and
> "b" are possibly different widths? The gnulib file intprops.h does
> this, but of course, due to the incompatible licensing, me repeating
> how it does it here is not in our best interest (however, I will point
> out that it results in a MUCH larger preprocessor expansion than your
> simpler solution that enforces same size inputs).
I imagine we could take each input (i.e., "a", "b") as uintmax_t,
perform the addition / multiplication in uintmax_t (with the safeguards
I proposed earlier adapted to UINTMAX_MAX), and then explicitly compare
the result against the maximum of the desired result type, such as
UINT64_MAX, SIZE_MAX, and so on.
I didn't think of this because I'm acutely aware of types, promotions
etc when I write C code, so "these two types must be the same" is not
something I'd be bothered by.
It's also a tradeoff on how generic we happen to be, vs. whether all
our callers actually do use the same size. If the code is easier to
maintain by insisting on callers matching certain patterns, that's
okay as long as we can easily detect callers that fail those patterns.
But if the cost of detecting out-of-spec callers is worse than just
biting the bullet and writing the code to support mixed types, then
being fully generic wins out.
>
>> (9) And so the C-language functions are just:
>>
>> bool
>> add_size_t_overflow (size_t a, size_t b, size_t *result)
>> {
>> if (a <= SIZE_MAX - b) {
>> *result = a + b;
>> return false;
>> }
>> return true;
>
> The compiler builtins ALSO assign the low-order bits of the
> infinite-precision operation into *result on overflow, behaving more
> like:
>
> *result = a + b;
> return ! (a <= SIZE_MAX - b);
Ah, interesting. I didn't notice that. I figured "output is
indeterminate on error" is generally acceptable. Is there a particular
use for the low-order bits, on overflow?
I suspect there may be some backwards compatibility at play, and most
hardware is already able to give us that for free. Even though you're
right that most sane users of these macros won't use the result on
overflow, I'm more worried about the subtle bugs that would occur on
less-tested platforms if we don't actually provide a deterministic
value (particularly if most of our testing occurs on platforms where
the compiler builtins do have deterministic behavior).
...
Right, I didn't consider types with lesser integer conversion
rank than
that of "unsigned int" -- I don't think we'd use "short" for
memory
allocation, sizes, indexing, file offsets etc.
While it sounds impractical, I do have to wonder if anyone has ever
tried to detect running out of TCP ports (a "short") by using overflow
checking.
I wouldn't want to be more gnulib than gnulib :) (Not that I looked at
it -- I intentionally didn't.)
And that's a good thing - since I'm tainted by knowing what gnulib has
done, I'm trying to make sure I keep it possible for you to come up
with a clean-room implementation if we want it, even though I
shouldn't write it myself.
...
>> Comments?
>
> As long as we have good unit tests for the particular use cases we
> care about detecting, I can live with either a generic macro or with
> type-specific functional interfaces. I'm a bit worried that using
> functions may fail to catch overflow on the implicit type conversions
> of arguments passed to the function; maybe it's possible to do a
> hybrid, by writing a function that checks for type equality but then
> calls a function for the actual overflow check, so that we are ensured
> that callers use the correct types, but have the ease of reading the
> overflow check in a function that doesn't require even more macro
> expansions to see what it is doing.
>
Let me try this (later, only a sketch now):
- use macros for checking that the input and output types are (ii)
unsigned (i) integer types, ignore their (iii) particular ranges
- convert the inputs to uintmax_t and perform the operation in uintmax_t
unconditionally, only *remembering* -- using the same in-advance checks
-- if UINTMAX_MAX is overflowed,
Perhaps you can do this with a function signature like:
bool check_op_overflow (uint64_t a, uint64_t b, uint64_t max, uint64_t *r)
- deduce the actual result limit from ((typeof (*r))-1),
- assign the low-order bits in any case -- if either the uintmax_t
operation overflows or the actual result limit is exceeded, it all
happens in unsigned, so it's well-defined (modular reduction). Return
the overflow status correctly.
and then utilize it something like:
#define CHECK_OP_OVERFLOW(a, b, r) \
({ \
bool overflow; uint64_t tmp; \
CHECK_UNSIGNED_INT (a); CHECK_UNSIGNED_INT (b); \
overflow = check_op_overflow (a, b, (typeof (*(r)))-1, &tmp); \
*(r) = tmp; \
overflow; \
})
and maybe even have CHECK_UNSIGNED_INT() permit signed values >= 0,
rather than insisting that the underlying type be unsigned.
At any rate, the approach sounds good to me.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org