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

[Daniel Borkmann]

On 09/05/2014 07:00 PM, Hannes Frederic Sowa wrote:
> 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 <mpatocka@xxxxxxxxxx>
> >
> > 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);

Yes, sure, sorry if that was not clear from my side, that was what I
intended to say with kmemcheck /infrastructure/. :)

> Bye,
> Hannes
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