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

[Mikulas Patocka]

On Fri, 5 Sep 2014, Daniel Borkmann wrote:

> 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/. :)

So, change it to use these markings. I'm not an expert in this area, so I
don't know all the places where this structure could be allocated. If you
know them all, mark it in this way.

Mikulas

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