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

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Message-ID: <20180308230512.GD29073@bombadil.infradead.org>
Date:   Thu, 8 Mar 2018 15:05:12 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Kees Cook <keescook@...omium.org>,
        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 Thu, Mar 08, 2018 at 07:24:47AM +0100, Julia Lawall wrote:
> On Wed, 7 Mar 2018, Matthew Wilcox wrote:
> > On Wed, Mar 07, 2018 at 10:18:21PM +0100, Julia Lawall wrote:
> > > > Otherwise, yes, please. We could build a coccinelle rule for
> > > > additional replacements...
> > >
> > > A potential semantic patch and the changes it generates are attached
> > > below.  Himanshu Jha helped with its development.  Working on this
> > > uncovered one bug, where the allocated array is too large, because the
> > > size provided for it was a structure size, but actually only pointers to
> > > that structure were to be stored in it.
> >
> > This is cool!  Thanks for doing the coccinelle patch!  Diffstat:
> >
> >  50 files changed, 81 insertions(+), 124 deletions(-)
> >
> > I find that pretty compelling.  I'll repost the kvmalloc_struct patch
> > imminently.
> 
> Thanks.  So it's OK to replace kmalloc and kzalloc, even though they
> didn't previously consider vmalloc and even though kmalloc doesn't zero?

We'll also need to replace the corresponding places where those structs
are freed with kvfree().  Can coccinelle handle that too?

> There are a few other cases that use GFP_NOFS and GFP_NOWAIT, but I didn't
> transform those because the comment says that the flags should be
> GFP_KERNEL based.  Should those be transformed too?

The problem with non-GFP_KERNEL allocations is that vmalloc may have to
allocate page tables, which is always done with an implicit GFP_KERNEL
allocation.  There's an intent to get rid of GFP_NOFS, but that's not
been realised yet (and I'm not sure of our strategy to eliminate it ...
I'll send a separate email about that).  I'm not sure why anything's
trying to allocate with GFP_NOWAIT; can you send a list of those places?