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    SubjectRe: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777 warning


    On Fri, 1 Jun 2007, Christoph Lameter wrote:
    >
    > Right it could catch a lot of other bugs as well.

    I'd actually prefer "malloc(0)" to _not_ return NULL, but some known
    (non-NULL) bogus pointer.

    Why?

    Because it's quite sane to have simple logic like

    ptr = malloc(size);
    if (!ptr)
    return -ENOMEM;

    and writing it as

    if (size && !ptr)
    return -ENOMEM;

    is just annoying.

    Also, NULL is _special_. There are absolutely tons of code in the kernel
    (and elsewhere) that just does something *different* from NULL pointers,
    and that totally breaks the whole notion of "you can allocate a zero-sized
    allocation, you just must not dereference it". If people special-case
    NULL as something else, they won't even go through the normal code-path.

    So for *both* of the above reasons, it's actually stupid to return NULL
    for a zero-sized allocation. It would be much better to return another
    pointer that will trap on access. A good candidate might be to return

    #define BADPTR ((void *)16)

    which is a portable-enough (where "portable-enough" is "against strict
    ANSI C rules, but works in practice on all architectures") way to return
    something that will cause the same page fault behaviour as NULL, but will
    _not_ trigger the "NULL is special" code.

    (Of course, you then need to teach "kfree()" to accept it as another
    pointer to be ignored, that's fine).

    I bet you'd find *more* problems that way than by returning NULL, and
    you'd also avoid the whole problem with "if (!ptr) return -ENOMEM".

    Linus
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