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

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Message-ID: <4ad99a55-9c93-5ea1-5954-3cb6e5ba7df9@rasmusvillemoes.dk>
Date:   Mon, 30 Apr 2018 23:29:04 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Matthew Wilcox <willy@...radead.org>,
        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 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

Rasmus