From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Borkmann Subject: Re: [PATCH] kernel: bpf: remove dead code Date: Mon, 22 May 2017 16:52:24 +0200 Message-ID: <5922FB28.5070303@iogearbox.net> References: <20170522140746.GA10113@embeddedgus> <20170522.103800.1354089494827582585.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: ast@kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: David Miller , garsilva@embeddedor.com Return-path: In-Reply-To: <20170522.103800.1354089494827582585.davem@davemloft.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 05/22/2017 04:38 PM, David Miller wrote: > From: "Gustavo A. R. Silva" > Date: Mon, 22 May 2017 09:07:46 -0500 > >> Execution cannot reach NET_IP_ALIGN inside the following statement: >> ip_align = strict ? 2 : NET_IP_ALIGN >> >> Addresses-Coverity-ID: 1409762 >> Signed-off-by: Gustavo A. R. Silva >> --- >> NOTE: variable ip_align could also be removed and use value 2 directly. > > Incorrect. > > Some platforms define NET_IP_ALIGN to zero, so the code must remain > as is. In the check_pkt_ptr_alignment(), when !strict you would already return earlier from that function. So, above test in ip_align will always give 2, meaning technically the patch is correct, although hard-coded value less clean. Perhaps something like the below to keep intentions more clear (and it will get resolved during compile time anyway ...): diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index a098d95..3cf1d60 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -2297,8 +2297,10 @@ static inline int pskb_network_may_pull(struct sk_buff *skb, unsigned int len) * Since this trade off varies between architectures, we allow NET_IP_ALIGN * to be overridden. */ +#define NET_IP_ALIGN_DEFAULT 2 + #ifndef NET_IP_ALIGN -#define NET_IP_ALIGN 2 +#define NET_IP_ALIGN NET_IP_ALIGN_DEFAULT #endif /* diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 1eddb71..61f6aaa 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -812,7 +812,7 @@ static int check_pkt_ptr_alignment(const struct bpf_reg_state *reg, * we force this to 2 which is universally what architectures use * when they don't set CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS. */ - ip_align = strict ? 2 : NET_IP_ALIGN; + ip_align = NET_IP_ALIGN ? : NET_IP_ALIGN_DEFAULT; if ((ip_align + reg_off + off) % size != 0) { verbose("misaligned packet access off %d+%d+%d size %d\n", ip_align, reg_off, off, size);