Re: [PATCH v3 6/9] checksum: use csum_ip4_header() in udp.c and tcp.c

On Fri, 1 Mar 2024 10:10:52 +1100
David Gibson <david(a)gibson.dropbear.id.au> wrote:

...
  On Thu, Feb 29, 2024 at 05:24:06PM +0100, Stefano
Brivio wrote:
  On Sat, 17 Feb 2024 16:07:22 +0100
 Laurent Vivier <lvivier(a)redhat.com> wrote:
   
  We can find the same function to compute the IPv4
header
 checksum in tcp.c, udp.c and tap.c
 
 Use the function defined for tap.c, csum_ip4_header(), but
 with the code used in tcp.c and udp.c as it doesn't need a fully
 initialiazed IPv4 header, only protocol, tot_len, saddr and daddr.
 
 Signed-off-by: Laurent Vivier <lvivier(a)redhat.com>
 ---
 
 Notes:
     v3:
       - function parameters provide tot_len, saddr, daddr and protocol
        rather than an iphdr
     
     v2:
       - use csum_ip4_header() from checksum.c
       - use code from tcp.c and udp.c in csum_ip4_header()
       - use "const struct iphfr *", check is not updated by the
         function but by the caller.
 
  checksum.c | 17 +++++++++++++----
  checksum.h |  3 ++-
  tap.c      |  3 ++-
  tcp.c      | 24 +++---------------------
  udp.c      | 20 ++------------------
  5 files changed, 22 insertions(+), 45 deletions(-)
 
 diff --git a/checksum.c b/checksum.c
 index 74e3742bc6f6..511b296a9a80 100644
 --- a/checksum.c
 +++ b/checksum.c
 @@ -57,6 +57,7 @@
  #include <linux/icmpv6.h>
  
  #include "util.h"
 +#include "ip.h"
  #include "checksum.h"
  
  /* Checksums are optional for UDP over IPv4, so we usually just set
 @@ -116,13 +117,21 @@ uint16_t csum_fold(uint32_t sum)
  uint16_t csum(const void *buf, size_t len, uint32_t init);
  
  /**
 - * csum_ip4_header() - Calculate and set IPv4 header checksum
 + * csum_ip4_header() - Calculate IPv4 header checksum
   * @ip4h:	IPv4 header
   */
 -void csum_ip4_header(struct iphdr *ip4h)
 +uint16_t csum_ip4_header(uint16_t tot_len, uint8_t protocol,
 +			 uint32_t saddr, uint32_t daddr)
  {
 -	ip4h->check = 0;
 -	ip4h->check = csum(ip4h, (size_t)ip4h->ihl * 4, 0);
 +	uint32_t sum = L2_BUF_IP4_PSUM(protocol);   
 
 Now that we use this macro, Coverity Scan realises that it's broken:
 
 #define L2_BUF_IP4_PSUM(proto) ((uint32_t)htons_constant(0x4500) +     \
 				(uint32_t)htons_constant(0xff00 | (proto)))
 
 ...but proto is eight (lower) bits, so this actually ignores 'proto'.   

 Uh... how so? 
Oops, sorry, it's not broken, and this is a false positive due to the
fact that __bswap_constant_16() (which htons_constant() resolves to, on
little-endian) is defined, for example in glibc, as:

#define __bswap_constant_16(x) \
     ((((x) >> 8) & 0xff) | (((x) & 0xff) << 8))

and in this case the first term of the | resolves to a constant value,
0xff, because 0xffxx >> 8 is 0xff for any value of xx.

I couldn't think of a "solution", yet.

-- 
Stefano