Re: [Qemu-devel] [PATCH] Permit zero-sized qemu_malloc() & friends

[Markus Armbruster]

Glauber Costa <address@hidden> writes:

> On Tue, Dec 01, 2009 at 12:57:27PM +0000, Paul Brook wrote:
>> > You might want to have a 'static uint8_t zero_length_malloc[0]' and
>> > return that instead of the magic cookie '1'.  Makes the code more
>> > readable IMHO and you'll also have symbol in gdb when debugging qemu.
>> 
>> Having multiple malloc return the same pointer sounds like a really bad idea.
> And why's that?
>
> Keep in mind that *any* dereference over that address is a bug.
>
> Actually, I very much like Gerd's idea to unmap that address, so the bug
> won't hide from us in any circumnstances.

For what it's worth, it violates the spec for malloc().  For zero-sized
allocations, we may either return a null pointer (but we already decided
we don't want to), or an object different from any other object alive.
Thus, we can't return the same non-null pointer for all zero-sized
allocations.

Chapter and verse: ISO/IEC 9899:1999 7.20.3 Memory management functions

    The order and contiguity of storage allocated by successive calls to
    the calloc, malloc, and realloc functions is unspecified.  The
    pointer returned if the allocation succeeds is suitably aligned so
    that it may be assigned to a pointer to any type of object and then
    used to access such an object or an array of such objects in the
    space allocated (until the space is explicitly deallocated).  The
    lifetime of an allocated object extends from the allocation until
    the deallocation.  Each such allocation shall yield a pointer to an
    object disjoint from any other object.  The pointer returned points
    to the start (lowest byte address) of the allocated space.  If the
    space cannot be allocated, a null pointer is returned.  If the size
    of the space requested is zero, the behavior is implementation-
    defined: either a null pointer is returned, or the behavior is as if
    the size were some nonzero value, except that the returned pointer
    shall not be used to access an object.