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    SubjectRe: [PATCH 2/2] mm: Add kvmalloc_ab_c and kvzalloc_struct
    On Mon, Apr 30, 2018 at 2:29 PM, Rasmus Villemoes
    <linux@rasmusvillemoes.dk> wrote:
    > On 2018-04-30 22:16, Matthew Wilcox wrote:
    >> On Mon, Apr 30, 2018 at 12:02:14PM -0700, Kees Cook wrote:
    >>>
    >>> Getting the constant ordering right could be part of the macro
    >>> definition, maybe? i.e.:
    >>>
    >>> static inline void *kmalloc_ab(size_t a, size_t b, gfp_t flags)
    >>> {
    >>> if (__builtin_constant_p(a) && a != 0 && \
    >>> b > SIZE_MAX / a)
    >>> return NULL;
    >>> else if (__builtin_constant_p(b) && b != 0 && \
    >>> a > SIZE_MAX / b)
    >>> return NULL;
    >>>
    >>> return kmalloc(a * b, flags);
    >>> }
    >>
    >> Ooh, if neither a nor b is constant, it just didn't do a check ;-( This
    >> stuff is hard.
    >>
    >>> (I just wish C had a sensible way to catch overflow...)
    >>
    >> Every CPU I ever worked with had an "overflow" bit ... do we have a
    >> friend on the C standards ctte who might figure out a way to let us
    >> write code that checks it?
    >
    > gcc 5.1+ (I think) have the __builtin_OP_overflow checks that should
    > generate reasonable code. Too bad there's no completely generic
    > check_all_ops_in_this_expression(a+b*c+d/e, or_jump_here). Though it's
    > hard to define what they should be checked against - probably would
    > require all subexpressions (including the variables themselves) to have
    > the same type.
    >
    > plug: https://lkml.org/lkml/2015/7/19/358

    That's a very nice series. Why did it never get taken? It seems to do
    the right things quite correctly.

    Daniel, while this isn't a perfect solution, is this something you'd
    use in graphics-land?

    -Kees

    --
    Kees Cook
    Pixel Security

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