On Wed, 14 Feb 2018, Kees Cook wrote:

> On Wed, Feb 14, 2018 at 10:26 AM, Matthew Wilcox <wi...@infradead.org> wrote:
> > From: Matthew Wilcox <mawil...@microsoft.com>
> >
> > We have kvmalloc_array in order to safely allocate an array with a
> > number of elements specified by userspace (avoiding arithmetic overflow
> > leading to a buffer overrun).  But it's fairly common to have a header
> > in front of that array (eg specifying the length of the array), so we
> > need a helper function for that situation.
> >
> > kvmalloc_ab_c() is the workhorse that does the calculation, but in spite
> > of our best efforts to name the arguments, it's really hard to remember
> > which order to put the arguments in.  kvzalloc_struct() eliminates that
> > effort; you tell it about the struct you're allocating, and it puts the
> > arguments in the right order for you (and checks that the arguments
> > you've given are at least plausible).
> >
> > For comparison between the three schemes:
> >
> >         sev = kvzalloc(sizeof(*sev) + sizeof(struct v4l2_kevent) * elems,
> >                         GFP_KERNEL);
> >         sev = kvzalloc_ab_c(elems, sizeof(struct v4l2_kevent), sizeof(*sev),
> >                         GFP_KERNEL);
> >         sev = kvzalloc_struct(sev, events, elems, GFP_KERNEL);
> >
> > Signed-off-by: Matthew Wilcox <mawil...@microsoft.com>
> > ---
> >  include/linux/mm.h | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 51 insertions(+)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 81bd7f0be286..ddf929c5aaee 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -557,6 +557,57 @@ static inline void *kvmalloc_array(size_t n, size_t 
> > size, gfp_t flags)
> >         return kvmalloc(n * size, flags);
> >  }
> >
> > +/**
> > + * kvmalloc_ab_c() - Allocate memory.
>
> Longer description, maybe? "Allocate a *b + c bytes of memory"?
>
> > + * @n: Number of elements.
> > + * @size: Size of each element (should be constant).
> > + * @c: Size of header (should be constant).
>
> If these should be constant, should we mark them as "const"? Or WARN
> if __builtin_constant_p() isn't true?
>
> > + * @gfp: Memory allocation flags.
> > + *
> > + * Use this function to allocate @n * @size + @c bytes of memory.  This
> > + * function is safe to use when @n is controlled from userspace; it will
> > + * return %NULL if the required amount of memory cannot be allocated.
> > + * Use kvfree() to free the allocated memory.
> > + *
> > + * The kvzalloc_hdr_arr() function is easier to use as it has typechecking
>
> renaming typo? Should this be "kvzalloc_struct()"?
>
> > + * and you do not need to remember which of the arguments should be 
> > constants.
> > + *
> > + * Context: Process context.  May sleep; the @gfp flags should be based on
> > + *         %GFP_KERNEL.
> > + * Return: A pointer to the allocated memory or %NULL.
> > + */
> > +static inline __must_check
> > +void *kvmalloc_ab_c(size_t n, size_t size, size_t c, gfp_t gfp)
> > +{
> > +       if (size != 0 && n > (SIZE_MAX - c) / size)
> > +               return NULL;
> > +
> > +       return kvmalloc(n * size + c, gfp);
> > +}
> > +#define kvzalloc_ab_c(a, b, c, gfp)    kvmalloc_ab_c(a, b, c, gfp | 
> > __GFP_ZERO)
>
> Nit: "(gfp) | __GFP_ZERO" just in case of insane usage.
>
> > +
> > +/**
> > + * kvzalloc_struct() - Allocate and zero-fill a structure containing a
> > + *                    variable length array.
> > + * @p: Pointer to the structure.
> > + * @member: Name of the array member.
> > + * @n: Number of elements in the array.
> > + * @gfp: Memory allocation flags.
> > + *
> > + * Allocate (and zero-fill) enough memory for a structure with an array
> > + * of @n elements.  This function is safe to use when @n is specified by
> > + * userspace as the arithmetic will not overflow.
> > + * Use kvfree() to free the allocated memory.
> > + *
> > + * Context: Process context.  May sleep; the @gfp flags should be based on
> > + *         %GFP_KERNEL.
> > + * Return: Zero-filled memory or a NULL pointer.
> > + */
> > +#define kvzalloc_struct(p, member, n, gfp)                             \
> > +       (typeof(p))kvzalloc_ab_c(n,                                     \
> > +               sizeof(*(p)->member) + __must_be_array((p)->member),    \
> > +               offsetof(typeof(*(p)), member), gfp)
> > +
> >  extern void kvfree(const void *addr);
> >
> >  static inline atomic_t *compound_mapcount_ptr(struct page *page)
>
> It might be nice to include another patch that replaces some of the
> existing/common uses of a*b+c with the new function...
>
> Otherwise, yes, please. We could build a coccinelle rule for
> additional replacements...

Thanks for the suggestion.  I will look into it.

julia
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