Am Donnerstag, dem 02.11.2023 um 13:50 +0000 schrieb Qing Zhao:
>
> > 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.
This shouldn't be necessary. The object-size pass
can track pointer arithmeti if it comes after
inserting the .ACCESS_WITH_SIZE.
https://godbolt.org/z/fvc3aoPfd
>
> > 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.
I do not understand this. It shouldn't matter how
it is updated as long as taking the reference to buf
happens through an lvalue that has the attribute.
>
>
> > That is,
> >
> > size_t *s = &a.size;
> > *s = 1;
> >
> > is invoking undefined behavior,
>
> right.
size_t *s = &a.size;
*s = 1;
char *buf = a.buf;
Should work just fine as long as the reference to buf
comes after the store to the size (which can be indirect).
Martin
>
> > 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?
>
