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

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Date:	Fri, 05 Sep 2014 19:10:08 +0200
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Hannes Frederic Sowa <hannes@...hat.com>
CC:	Mikulas Patocka <mpatocka@...hat.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] bpf: fix a false positive kmemcheck warning

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@...hat.com>
>>
>> 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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