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Date: Mon, 30 Apr 2018 13:16:07 -0700
From: Matthew Wilcox <willy@...radead.org>
To: Kees Cook <keescook@...omium.org>
Cc: Julia Lawall <julia.lawall@...6.fr>,
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>,
cocci@...teme.lip6.fr, Himanshu Jha <himanshujha199640@...il.com>
Subject: Re: [PATCH 2/2] mm: Add kvmalloc_ab_c and kvzalloc_struct
On Mon, Apr 30, 2018 at 12:02:14PM -0700, Kees Cook wrote:
> On Sun, Apr 29, 2018 at 1:30 PM, Matthew Wilcox <willy@...radead.org> wrote:
> > On Sun, Apr 29, 2018 at 09:59:27AM -0700, Kees Cook wrote:
> >> Did this ever happen?
> >
> > Not yet. I brought it up at LSFMM, and I'll repost the patches soon.
> >
> >> I'd also like to see kmalloc_array_3d() or
> >> something that takes three size arguments. We have a lot of this
> >> pattern too:
> >>
> >> kmalloc(sizeof(foo) * A * B, gfp...)
> >>
> >> And we could turn that into:
> >>
> >> kmalloc_array_3d(sizeof(foo), A, B, gfp...)
> >
> > Are either of A or B constant? Because if so, we could just use
> > kmalloc_array. If not, then kmalloc_array_3d becomes a little more
> > expensive than kmalloc_array because we have to do a divide at runtime
> > instead of compile-time. that's still better than allocating too few
> > bytes, of course.
>
> Yeah, getting the order of the division is nice. Some thoughts below...
>
> >
> > I'm wondering how far down the abc + ab + ac + bc + d rabbit-hole we're
> > going to end up going. As far as we have to, I guess.
>
> Well, the common patterns I've seen so far are:
>
> a
> ab
> abc
> a + bc
> ab + cd
>
> For any longer multiplications, I've only found[1]:
>
> drivers/staging/rtl8188eu/os_dep/osdep_service.c: void **a =
> kzalloc(h * sizeof(void *) + h * w * size, GFP_KERNEL);
That's pretty good, although it's just an atrocious vendor driver and
it turns out all of those things are constants, and it'd be far better
off with just declaring an array. I bet they used to declare one on
the stack ...
> At the end of the day, though, I don't really like having all these
> different names...
>
> kmalloc(), kmalloc_array(), kmalloc_ab_c(), kmalloc_array_3d()
>
> with their "matching" zeroing function:
>
> kzalloc(), kcalloc(), kzalloc_ab_c(), kmalloc_array_3d(..., gfp | __GFP_ZERO)
Yes, it's not very regular.
> For the multiplication cases, I wonder if we could just have:
>
> kmalloc_multN(gfp, a, b, c, ...)
> kzalloc_multN(gfp, a, b, c, ...)
>
> and we can replace all kcalloc() users with kzalloc_mult2(), all
> kmalloc_array() users with kmalloc_mult2(), the abc uses with
> kmalloc_mult3().
I'm reluctant to do away with kcalloc() as it has the obvious heritage
from user-space calloc() with the addition of GFP flags.
> That said, I *do* like kmalloc_struct() as it's a very common pattern...
Thanks! And way harder to misuse than kmalloc_ab_c().
> Or maybe, just leave the pattern in the name? kmalloc_ab(),
> kmalloc_abc(), kmalloc_ab_c(), kmalloc_ab_cd() ?
>
> 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?
> -Kees
>
> [1] git grep -E 'alloc\([^,]+[^(]\*[^)][^,]+[^(]\*[^)][^,]+[^(]\*[^)][^,]+,'
I'm impressed, but it's not going to catch
veryLongPointerNameThatsMeaningfulToMe = kmalloc(initialSize +
numberOfEntries * entrySize + someOtherThing * yourMum,
GFP_KERNEL);
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