linux-kernel - Re: [PATCH 2/2] mm: Add kvmalloc_ab_c and kvzalloc

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Date:   Wed, 14 Feb 2018 11:35:17 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Julia Lawall <julia.lawall@...6.fr>, cocci@...teme.lip6.fr
Subject: Re: [PATCH 2/2] mm: Add kvmalloc_ab_c and kvzalloc_struct

On Wed, Feb 14, 2018 at 11:22:38AM -0800, Kees Cook wrote:
> > +/**
> > + * kvmalloc_ab_c() - Allocate memory.
> 
> Longer description, maybe? "Allocate a *b + c bytes of memory"?

Done!

> > + * @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?

It's only less efficient if they're not const.  Theoretically they could be
variable ... and I've been bitten by __builtin_constant_p() recently
(gcc bug 83653 which I still don't really understand).

> > + * @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()"?

Urgh, yes.  I swear I searched for it ... must've typoed my search string.
Anyway, fixed, because kvzalloc_hdr_arr() wasn't a good name.

> > +#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.

Fixed!

> It might be nice to include another patch that replaces some of the
> existing/common uses of a*b+c with the new function...

Sure!  I have a few examples in my tree, I just didn't want to complicate
things by sending a patch that crossed dozens of maintainer trees.