On Mon, Sep 19, 2022 at 02:36:41PM +0200, Florian Weimer wrote:
> * Szabolcs Nagy:
>
> > unlike musl those implementations don't return exact size nor have the
> > same security and memory fragmentation guarantees, so bad comparision.
> >
> > tcmalloc:
> >   // Returns the actual number N of bytes reserved by tcmalloc for the pointer
> >   // p.  This number may be equal to or greater than the number of bytes
> >   // requested when p was allocated.
> >   //
> >   // This function is just useful for statistics collection.  The client must
> >   // *not* read or write from the extra bytes that are indicated by this call.
> >
> > jemalloc:
> >       <para>The <function>malloc_usable_size()</function> function
> >       returns the usable size of the allocation pointed to by
> >       <parameter>ptr</parameter>.  The return value may be larger than the size
> >       that was requested during allocation.  The
> >       <function>malloc_usable_size()</function> function is not a
> >       mechanism for in-place <function>realloc()</function>; rather
> >       it is provided solely as a tool for introspection purposes.  Any
> >       discrepancy between the requested allocation size and the size reported
> >       by <function>malloc_usable_size()</function> should not be
> >       depended on, since such behavior is entirely implementation-dependent.
>
> These implementations are buggy or at least mis-documented.  The
> interface contract is clearly that for that particular object, the extra
> bytes in the allocation are available for reading and writing.  It is
> not guaranteed that the allocator will always provide the same number of
> extra bytes for the same requested size, but they must be there for the
> allocation being examined.  It's even in the name of the function!

I'm not sure I understand what you're saying, but the core problem
that really can't be solved is potential discrepancy between the
malloc implementation's idea of usable and the compiler's. For
example:

        char *p = malloc(1);
        if (malloc_usable_size(p)>1) p[1] = 42;

will cause a compiler that's actively detecting UB to abort the
program when malloc_usable_size returns a value larger than 1.

Rich