Subject: Re: [PATCH] bpf: fix a false positive kmemcheck warning

From: Hannes Frederic Sowa <[email-masked]>
Date: Fri, 05 Sep 2014 19:00:57 +0200

On Fr, 2014-09-05 at 18:20 +0200, Daniel Borkmann wrote:
> Hi Mikulas,
> 
> On 09/05/2014 06:01 PM, Mikulas Patocka wrote:
> > This patch fixes false positive kmemcheck warning in bpf.
> >
> > When we try to write the variable len, the compiler generates a code that
> > reads the 32-bit word, modifies the bits belonging to "len" and writes the
> > 32-bit word back. The reading of the word results in kmemcheck warning due
> > to reading uninitialized memory. This patch fixes it by avoiding using bit
> > fields when kmemcheck is enabled.
> >
> > Signed-off-by: Mikulas Patocka <[email-masked]>
> 
> You need to submit this patch to netdev (Cc'ed).
> 
> > ---
> >   include/linux/filter.h |    5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > Index: linux-2.6/include/linux/filter.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/filter.h	2014-09-04 23:04:26.000000000 +0200
> > +++ linux-2.6/include/linux/filter.h	2014-09-04 23:43:05.000000000 +0200
> > @@ -325,8 +325,13 @@ struct sock;
> >   struct seccomp_data;
> >
> >   struct bpf_prog {
> > +#ifdef CONFIG_KMEMCHECK
> > +	bool			jited;
> > +	u32			len;
> > +#else
> >   	u32			jited:1,	/* Is our filter JIT'ed? */
> >   				len:31;		/* Number of filter blocks */
> > +#endif
> >   	struct sock_fprog_kern	*orig_prog;	/* Original BPF program */
> >   	unsigned int		(*bpf_func)(const struct sk_buff *skb,
> >   					    const struct bpf_insn *filter);
> 
> I don't really like this if-def. If you really want to fix it, can't
> you just use :
> 
>    kmemcheck_bitfield_begin(bpf_anc_data)
>    ...
>    kmemcheck_bitfield_end(bpf_anc_data)

you also need to annotate the bitfield after allocation:
struct bpf_prog *prog = kalloc(...);
kmemcheck_annotate_bitfield(prog, bpf_anc_data);

Bye,
Hannes