> On Nov 2, 2023, at 3:57 AM, Richard Biener <richard.guent...@gmail.com> wrote:
>
> On Wed, Nov 1, 2023 at 3:47 PM Qing Zhao <qing.z...@oracle.com> wrote:
>>
>>
>>
>>> On Oct 31, 2023, at 6:14 PM, Joseph Myers <jos...@codesourcery.com> wrote:
>>>
>>> On Tue, 31 Oct 2023, Qing Zhao wrote:
>>>
>>>> 2.3 A new semantic requirement in the user documentation of "counted_by"
>>>>
>>>> For the following structure including a FAM with a counted_by attribute:
>>>>
>>>> struct A
>>>> {
>>>> size_t size;
>>>> char buf[] __attribute__((counted_by(size)));
>>>> };
>>>>
>>>> for any object with such type:
>>>>
>>>> struct A *obj = __builtin_malloc (sizeof(struct A) + sz * sizeof(char));
>>>>
>>>> The setting to the size field should be done before the first reference
>>>> to the FAM field.
>>>>
>>>> Such requirement to the user will guarantee that the first reference to
>>>> the FAM knows the size of the FAM.
>>>>
>>>> We need to add this additional requirement to the user document.
>>>
>>> Make sure the manual is very specific about exactly when size is
>>> considered to be an accurate representation of the space available for buf
>>> (given that, after malloc or realloc, it's going to be temporarily
>>> inaccurate). If the intent is that inaccurate size at such a time means
>>> undefined behavior, say so explicitly.
>>
>> Yes, good point. We need to define this clearly in the beginning.
>> We need to explicit say that
>>
>> the size of the FAM is defined by the latest “counted_by” value. And it’s an
>> undefined behavior when the size field is not defined when the FAM is
>> referenced.
>>
>> Is the above good enough?
>>
>>
>>>
>>>> 2.4 Replace FAM field accesses with the new function ACCESS_WITH_SIZE
>>>>
>>>> In C FE:
>>>>
>>>> for every reference to a FAM, for example, "obj->buf" in the small example,
>>>> check whether the corresponding FIELD_DECL has a "counted_by" attribute?
>>>> if YES, replace the reference to "obj->buf" with a call to
>>>> .ACCESS_WITH_SIZE (obj->buf, obj->size, -1);
>>>
>>> This seems plausible - but you should also consider the case of static
>>> initializers - remember the GNU extension for statically allocated objects
>>> with flexible array members (unless you're not allowing it with
>>> counted_by).
>>>
>>> static struct A x = { sizeof "hello", "hello" };
>>> static char *y = &x.buf;
>>>
>>> I'd expect that to be valid - and unless you say such a usage is invalid,
>>
>> At this moment, I think that this should be valid.
>>
>> I,e, the following:
>>
>> struct A
>> {
>> size_t size;
>> char buf[] __attribute__((counted_by(size)));
>> };
>>
>> static struct A x = {sizeof "hello", "hello”};
>>
>> Should be valid, and x.size represents the number of elements of x.buf.
>> Both x.size and x.buf are initialized statically.
>>
>>> you should avoid the replacement in such a static initializer context when
>>> the FAM reference is to an object with a constant address (if
>>> .ACCESS_WITH_SIZE would not act as an lvalue whose address is a constant
>>> expression; if it works fine as a constant-address lvalue, then the
>>> replacement would be OK).
>>
>> Then if such usage for the “counted_by” is valid, we need to replace the FAM
>> reference by a call to .ACCESS_WITH_SIZE as well.
>> Otherwise the “counted_by” relationship will be lost to the Middle end.
>>
>> With the current definition of .ACCESS_WITH_SIZE
>>
>> PTR = .ACCESS_WITH_SIZE (PTR, SIZE, ACCESS_MODE)
>>
>> Isn’t the PTR (return value of the call) a LVALUE?
>
> You probably want to specify that when a pointer to the array is taken the
> pointer has to be to the first array element (or do we want to mangle the
> 'size' accordingly for the instrumentation?).
Yes. Will add this into the user documentation.
> You also want to specify that
> the 'size' associated with such pointer is assumed to be unchanging and
> after changing the size such pointer has to be re-obtained.
What do you mean by “re-obtained”?
> Plus that
> changes to the allocated object/size have to be performed through an
> lvalue where the containing type and thus the 'counted_by' attribute is
> visible.
Through an lvalue with the containing type?
Yes, will add this too.
> That is,
>
> size_t *s = &a.size;
> *s = 1;
>
> is invoking undefined behavior,
right.
> likewise modifying 'buf' (makes it a bit
> awkward since for example that wouldn't support using posix_memalign
> for allocation, though aligned_alloc would be fine).
Is there a small example for the undefined behavior for this?
Qing
>
> Richard.
>
>> Qing
>>>
>>> --
>>> Joseph S. Myers
>>> jos...@codesourcery.com
>>
