[cfe-dev] Addition of function parameter attribute to help __builtin_object_size

[George Burgess IV via cfe-dev]

It's more or less meant to be implemented by passing extra arguments, which
changes how the function is called. This is also meant to work across TUs.
e.g. in

// foo.h
int doIt(void *const p __attribute__((carry_size(0))));

// foo.c
int doIt(void *const p __attribute__((carry_size(0)))) { return
__builtin_object_size(p, 0); }

// main.c
#include "foo.h"
int main() {
  char c[10];
  return doIt(c);
}

Also, the common case for how this is foreseen to be used is "call function
A with the object size if __builtin_object_size succeeds, and function B if
__builtin_object_size fails," so being able to overload on the attribute
may be mildly convenient. :)

On Mon, Sep 14, 2015 at 11:19 AM, PaX Team <pageexec at gmail.com> wrote:

> On 14 Sep 2015 at 10:54, George Burgess IV via cfe-dev wrote:
>
> > Hello friends,
> >
> > I plan to add a function parameter attribute called carry_size (name
> > suggestions welcome). The goal of this attribute is to help Clang/LLVM
> > answer `__builtin_object_size(Ptr, Type)` calls when the object pointed
> to
> > by Ptr isn't visible in the caller of __builtin_object_size.
> >
> > An example of its use:
> > #define CS(N) __attribute__((carry_size(N)))
> > void __attribute__((noinline)) Func(void *const p CS(0)) {
> >   printf("%d\n", __builtin_object_size(p, 0));
> > }
>
> i must be missing something here but why do you need a manually applied
> attr at all when the compiler can just figure it out itself whenever it
> sees a call to __builtin_object_size?
>
>
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