Re: HELP: Will the reordering happen? Re: [V3][PATCH 0/3] New attribute "counted_by" to annotate bounds for C99 FAM(PR108896)

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Oct 25, 2023 at 8:16 PM Martin Uecker <uecker@tugraz.at> wrote:
>
> Am Mittwoch, dem 25.10.2023 um 13:13 +0200 schrieb Richard Biener:
> >
> > > Am 25.10.2023 um 12:47 schrieb Martin Uecker <uecker@tugraz.at>:
> > >
> > > Am Mittwoch, dem 25.10.2023 um 06:25 -0400 schrieb Siddhesh Poyarekar:
> > > > > On 2023-10-25 04:16, Martin Uecker wrote:
> > > > > Am Mittwoch, dem 25.10.2023 um 08:43 +0200 schrieb Richard Biener:
> > > > > >
> > > > > > > Am 24.10.2023 um 22:38 schrieb Martin Uecker <uecker@tugraz.at>:
> > > > > > >
> > > > > > > Am Dienstag, dem 24.10.2023 um 20:30 +0000 schrieb Qing Zhao:
> > > > > > > > Hi, Sid,
> > > > > > > >
> > > > > > > > Really appreciate for your example and detailed explanation. Very helpful.
> > > > > > > > I think that this example is an excellent example to show (almost) all the issues we need to consider.
> > > > > > > >
> > > > > > > > I slightly modified this example to make it to be compilable and run-able, as following:
> > > > > > > > (but I still cannot make the incorrect reordering or DSE happening, anyway, the potential reordering possibility is there…)
> > > > > > > >
> > > > > > > >  1 #include <malloc.h>
> > > > > > > >  2 struct A
> > > > > > > >  3 {
> > > > > > > >  4  size_t size;
> > > > > > > >  5  char buf[] __attribute__((counted_by(size)));
> > > > > > > >  6 };
> > > > > > > >  7
> > > > > > > >  8 static size_t
> > > > > > > >  9 get_size_from (void *ptr)
> > > > > > > > 10 {
> > > > > > > > 11  return __builtin_dynamic_object_size (ptr, 1);
> > > > > > > > 12 }
> > > > > > > > 13
> > > > > > > > 14 void
> > > > > > > > 15 foo (size_t sz)
> > > > > > > > 16 {
> > > > > > > > 17  struct A *obj = __builtin_malloc (sizeof(struct A) + sz * sizeof(char));
> > > > > > > > 18  obj->size = sz;
> > > > > > > > 19  obj->buf[0] = 2;
> > > > > > > > 20  __builtin_printf (“%d\n", get_size_from (obj->buf));
> > > > > > > > 21  return;
> > > > > > > > 22 }
> > > > > > > > 23
> > > > > > > > 24 int main ()
> > > > > > > > 25 {
> > > > > > > > 26  foo (20);
> > > > > > > > 27  return 0;
> > > > > > > > 28 }
> > > > > > > >
> > > >
> > > > <snip>
> > > >
> > > > > > When it’s set I suppose.  Turn
> > > > > >
> > > > > > X.l = n;
> > > > > >
> > > > > > Into
> > > > > >
> > > > > > X.l = __builtin_with_size (x.buf, n);
> > > > >
> > > > > It would turn
> > > > >
> > > > > some_variable = (&) x.buf
> > > > >
> > > > > into
> > > > >
> > > > > some_variable = __builtin_with_size ( (&) x.buf. x.len)
> > > > >
> > > > >
> > > > > So the later access to x.buf and not the initialization
> > > > > of a member of the struct (which is too early).
> > > > >
> > > >
> > > > Hmm, so with Qing's example above, are you suggesting the transformation
> > > > be to foo like so:
> > > >
> > > > 14 void
> > > > 15 foo (size_t sz)
> > > > 16 {
> > > > 16.5  void * _1;
> > > > 17  struct A *obj = __builtin_malloc (sizeof(struct A) + sz * sizeof(char));
> > > > 18  obj->size = sz;
> > > > 19  obj->buf[0] = 2;
> > > > 19.5  _1 = __builtin_with_size (obj->buf, obj->size);
> > > > 20  __builtin_printf (“%d\n", get_size_from (_1));
> > > > 21  return;
> > > > 22 }
> > > >
> > > > If yes then this could indeed work.  I think I got thrown off by the
> > > > reference to __bdos.
> > >
> > > Yes. I think it is important not to evaluate the size at the
> > > access to buf and not the allocation, because the point is to
> > > recover it from the size member even when the compiler can't
> > > see the original allocation.
> >
> > But if the access is through a pointer without the attribute visible
> > even the Frontend cannot recover?
>
> Yes, if the access is using a struct-with-FAM without the attribute
> the FE would not be insert the builtin.  BDOS could potentially
> still see the original allocation but if it doesn't, then there is
> no information.
>
> > We’d need to force type correctness and give up on indirecting
> > through an int * when it can refer to two diffenent container types.
> > The best we can do I think is mark allocation sites and hope for
> > some basic code hygiene (not clobbering size or array pointer
> > through pointers without the appropriately attributed type)
>
> I am do not fully understand what you are referring to.

struct A { int n; int data[n]; };
struct B { long n; int data[n]; };

int *p = flag ? a->data : b->data;

access *p;

Since we need to allow interoperability of pointers (a->data is
convertible to a non-fat pointer of type int *) this leaves us with
ambiguity we need to conservatively handle to avoid false positives.

We _might_ want to diagnose decay of a->data to int *, but IIRC
there's no way (or proposal) to allow declaring a corresponding
fat pointer, so it's not a good designed feature.

Having __builtin_with_size at allocation would possibly make
the BOS use-def walk discover both objects.  I think you can't
insert __builtin_with_size at the access to *p, but in practice
that would be very much needed.

Richard.

> But yes,
> for full bounds safety we would need the language feature.
> In C people should start to variably-modified types
> more.  I think we can build perfect bounds safety on top of
> them in a very good way with only FE changes.
>
> All these attributes are just a best effort.  But for a while,
> this will be necessary.
>
> Martin
>
> >
> > > Evaluating at this point requires that the size is correctly set
> > > before the access to the FAM and the user has to make sure
> > > this is the case. But to me this requirement would make sense.
> > >
> > > Semantically, it could aöso make sense to evaluate the size at a
> > > later time.  But then the reordering becomes problematic again.
> > >
> > > Also I think this would make this feature generally more useful.
> > > For example, it could work also for others pointers in the struct
> > > and not just for FAMs.  In this case, the struct may already be
> > > freed when  BDOS is called, so it might also not possible to
> > > access the size member at a later time.
> > >
> > > Martin
> > >
> > >
> > > >
> > >
>