I guess that what Kees wanted, ""fill the array without knowing the actual
final size" code pattern”, as following:
>> struct foo *f;
>> char *p;
>> int i;
>>
>> f = alloc(maximum_possible);
>> f->count = 0;
>> p = f->buf;
>>
>> for (i; data_is_available() && i < maximum_possible; i++) {
>> f->count ++;
>> p[i] = next_data_item();
>> }
actually is a dynamic array, or more accurately, Bounded-size dynamic array: (
but not a dynamic allocated array as we discussed so far)
https://en.wikipedia.org/wiki/Dynamic_array
This dynamic array, also is called growable array, or resizable array, whose
size can
be changed during the lifetime.
For VLA or FAM, I believe that they are both dynamic allocated array, i.e, even
though the size is not know at the compilation time, but the size
will be fixed after the array is allocated.
I am not sure whether C has support to such Dynamic array? Or whether it’s easy
to provide dynamic array support in C?
Qing
> On Oct 26, 2023, at 12:45 PM, Martin Uecker <uec...@tugraz.at> wrote:
>
> Am Donnerstag, dem 26.10.2023 um 09:13 -0700 schrieb Kees Cook:
>> On Thu, Oct 26, 2023 at 10:15:10AM +0200, Martin Uecker wrote:
>>> but not this:
>>>
>
> x->count = 11;
>>> char *p = &x->buf;
>>> x->count = 1;
>>> p[10] = 1; // !
>>
>> This seems fine to me -- it's how I'd expect it to work: "10" is beyond
>> "1".
>
> Note that the store would be allowed.
>
>>
>>> (because the pointer is passed around the
>>> store to the counter)
>>>
>>> and also here the second store is then irrelevant
>>> for the access:
>>>
>>> x->count = 10;
>>> char* p = &x->buf;
>>> ...
>>> x->count = 1; // somewhere else
>>> ----
>>> p[9] = 1; // ok, because count matter when buf was accesssed.
>>
>> This is less great, but I can understand why it happens. "p" loses the
>> association with "x". It'd be nice if "p" had to way to retain that it
>> was just an alias for x->buf, so future p access would check count.
>
> The problem is not to discover that p is an alias to x->buf,
> but that it seems difficult to make sure that stores to
> x->count are not reordered relative to the final access to
> p[i] you want to check, so that you then get the right value.
>
>>
>> But this appears to be an existing limitation in other areas where an
>> assignment will cause the loss of object association. (I've run into
>> this before.) It's just more surprising in the above example because in
>> the past the loss of association would cause __bdos() to revert back to
>> "SIZE_MAX" results ("I don't know the size") rather than an "outdated"
>> size, which may get us into unexpected places...
>>
>>> IMHO this makes sense also from the user side and
>>> are the desirable semantics we discussed before.
>>>
>>> But can you take a look at this?
>>>
>>>
>>> This should simulate it fairly well:
>>> https://godbolt.org/z/xq89aM7Gr
>>>
>>> (the call to the noinline function would go away,
>>> but not necessarily its impact on optimization)
>>
>> Yeah, this example should be a very rare situation: a leaf function is
>> changing the characteristics of the struct but returning a buffer within
>> it to the caller. The more likely glitch would be from:
>>
>> int main()
>> {
>> struct foo *f = foo_alloc(7);
>> char *p = FAM_ACCESS(f, size, buf);
>>
>> printf("%ld\n", __builtin_dynamic_object_size(p, 0));
>> test1(f); // or just "f->count = 10;" no function call needed
>> printf("%ld\n", __builtin_dynamic_object_size(p, 0));
>>
>> return 0;
>> }
>>
>> which reports:
>> 7
>> 7
>>
>> instead of:
>> 7
>> 10
>>
>> This kind of "get an alias" situation is pretty common in the kernel
>> as a way to have a convenient "handle" to the array. In the case of a
>> "fill the array without knowing the actual final size" code pattern,
>> things would immediately break:
>>
>> struct foo *f;
>> char *p;
>> int i;
>>
>> f = alloc(maximum_possible);
>> f->count = 0;
>> p = f->buf;
>>
>> for (i; data_is_available() && i < maximum_possible; i++) {
>> f->count ++;
>> p[i] = next_data_item();
>> }
>>
>> Now perhaps the problem here is that "count" cannot be used for a count
>> of "logically valid members in the array" but must always be a count of
>> "allocated member space in the array", which I guess is tolerable, but
>> isn't ideal -- I'd like to catch logic bugs in addition to allocation
>> bugs, but the latter is certainly much more important to catch.
>
> Maybe we could have a warning when f->buf is not directly
> accessed.
>
> Martin
>
>>
>
