Re: [dpdk-dev] [PATCH v2] app/testpmd: fix IP checksum calculation

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 6 Jan 2021 23:39:39 -0600
George Prekas <prekageo@amazon.com> wrote:

> On 1/6/2021 12:02 PM, Ferruh Yigit wrote:
> > On 12/5/2020 5:42 AM, George Prekas wrote:  
> >> Strict-aliasing rules are violated by cast to uint16_t* in flowgen.c
> >> and the calculated IP checksum is wrong on GCC 9 and GCC 10.
> >>
> >> Signed-off-by: George Prekas <prekageo@amazon.com>
> >> ---
> >> v2:
> >> * Instead of a compiler barrier, use a compiler flag.
> >> ---
> >>   app/test-pmd/meson.build | 1 +
> >>   1 file changed, 1 insertion(+)
> >>
> >> diff --git a/app/test-pmd/meson.build b/app/test-pmd/meson.build
> >> index 7e9c7bdd6..5d24e807f 100644
> >> --- a/app/test-pmd/meson.build
> >> +++ b/app/test-pmd/meson.build
> >> @@ -4,6 +4,7 @@
> >>   # override default name to drop the hyphen
> >>   name = 'testpmd'
> >>   cflags += '-Wno-deprecated-declarations'
> >> +cflags += '-fno-strict-aliasing'
> >>   sources = files('5tswap.c',
> >>       'cmdline.c',
> >>       'cmdline_flow.c',
> >>  
> > 
> > Hi George,
> > 
> > I am trying to understand this, the relevant code is as below:
> > ip_hdr->hdr_checksum = ip_sum((unaligned_uint16_t *)ip_hdr, sizeof(*ip_hdr));
> > 
> > You are suspicious of strict aliasing rule violation, with more details:
> > The concern is the "struct rte_ipv4_hdr *ip_hdr;" aliased to "const
> > unaligned_uint16_t *hdr", and compiler can optimize out the calculations using
> > data pointed by 'hdr' pointer, since the 'hdr' pointer is not used to alter the
> > data and compiler may think data is not changed at all.
> > 
> > 1) But the pointer "hdr" is assigned in the loop, from another pointer whose
> > content is changing, why this is not helping to figure out that the data 'hdr'
> > pointing is changed.
> > 
> > 2) I tried to debug this, but I am not able to reproduce the issue, 'ip_sum()'
> > called each time and checksum calculated correctly. Using gcc 10.2.1-9. Can you
> > able to confirm the case with debug, or from the assembly/object file?
> > 
> > 
> > And if the issue is strict aliasing rule violation as you said, compiler flag is
> > an option but not sure how much it reduces the compiler optimization benefit, I
> > guess other options also not so good, memcpy brings too much work on runtime and
> > union requires bigger change and makes code complex.
> > I wonder if making 'ip_sum()' a non inline function can help, can you please
> > give a try since you can reproduce it?  
> 
> Hi Ferruh,
> 
> Thanks for looking into it.
> 
> I am copy-pasting at the end of this email a minimal reproduction. It calculates a checksum and prints it. The correct value is f8d9. If you compile it with -O0 or -O3 -fno-strict-aliasing, you will get the correct value. If you compile it with gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0 and -O3, you will get f8e8. You can also try it on https://godbolt.org/ and see how different versions behave.
> 
> My understanding is that the code violates the C standard (https://stackoverflow.com/a/99010).
> 
> --- cut here --- 
> 
> #include <stdint.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> 
> struct rte_ipv4_hdr {
> 	uint8_t  version_ihl;
> 	uint8_t  type_of_service;
> 	uint16_t total_length;
> 	uint16_t packet_id;
> 	uint16_t fragment_offset;
> 	uint8_t  time_to_live;
> 	uint8_t  next_proto_id;
> 	uint16_t hdr_checksum;
> 	uint32_t src_addr;
> 	uint32_t dst_addr;
> };
> 
> static inline uint16_t ip_sum(const uint16_t *hdr, int hdr_len)
> {
> 	uint32_t sum = 0;
> 
> 	while (hdr_len > 1)
> 	{
> 		sum += *hdr++;
> 		if (sum & 0x80000000)
> 			sum = (sum & 0xFFFF) + (sum >> 16);
> 		hdr_len -= 2;
> 	}
> 
> 	while (sum >> 16)
> 		sum = (sum & 0xFFFF) + (sum >> 16);
> 
> 	return ~sum;
> }
> 
> static void pkt_burst_flow_gen(void)
> {
> 	struct rte_ipv4_hdr *ip_hdr = (struct rte_ipv4_hdr *) malloc(4096);
> 	memset(ip_hdr, 0, sizeof(*ip_hdr));
> 	ip_hdr->version_ihl	= 1;
> 	ip_hdr->type_of_service	= 2;
> 	ip_hdr->fragment_offset	= 3;
> 	ip_hdr->time_to_live	= 4;
> 	ip_hdr->next_proto_id	= 5;
> 	ip_hdr->packet_id	= 6;
> 	ip_hdr->src_addr	= 7;
> 	ip_hdr->dst_addr	= 8;
> 	ip_hdr->total_length	= 9;
> 	ip_hdr->hdr_checksum	= ip_sum((uint16_t *)ip_hdr, sizeof(*ip_hdr));
> 	printf("%x\n", ip_hdr->hdr_checksum);
> }
> 
> int main(void)
> {
> 	pkt_burst_flow_gen();
> 	return 0;
> }

If I change your code like this to use union, Gcc 10 is still broken.
It is a compiler bug.  It maybe because optimizer is not smart enough
to know that memset has cleared the header.


#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct rte_ipv4_hdr {
	uint8_t  version_ihl;
	uint8_t  type_of_service;
	uint16_t total_length;
	uint16_t packet_id;
	uint16_t fragment_offset;
	uint8_t  time_to_live;
	uint8_t  next_proto_id;
	uint16_t hdr_checksum;
	uint32_t src_addr;
	uint32_t dst_addr;
};

static inline uint16_t ip_sum(const uint16_t *hdr, int hdr_len)
{
	uint32_t sum = 0;

	while (hdr_len > 1)
	{
		sum += *hdr++;
		if (sum & 0x80000000)
			sum = (sum & 0xFFFF) + (sum >> 16);
		hdr_len -= 2;
	}

	while (sum >> 16)
		sum = (sum & 0xFFFF) + (sum >> 16);

	return ~sum;
}

static void pkt_burst_flow_gen(void)
{
	union {
		struct rte_ipv4_hdr ip;
		uint16_t data[10];
	} *hdr;

	hdr = malloc(sizeof(*hdr));

	memset(hdr, 0, sizeof(*hdr));
	hdr->ip.version_ihl	= 1;
	hdr->ip.type_of_service	= 2;
	hdr->ip.fragment_offset	= 3;
	hdr->ip.time_to_live	= 4;
	hdr->ip.next_proto_id	= 5;
	hdr->ip.packet_id	= 6;
	hdr->ip.src_addr	= 7;
	hdr->ip.dst_addr	= 8;
	hdr->ip.total_length	= 9;
	hdr->ip.hdr_checksum	= ip_sum(hdr->data, sizeof(*hdr));
	printf("%x\n", hdr->ip.hdr_checksum);
}

int main(void)
{
	pkt_burst_flow_gen();
	return 0;
}