Re: [Qemu-devel] [PATCH 3/5] slirp: Add dns6 resolution

[Thomas Huth <thuth@redhat.com>]

On 29.03.2016 01:44, Samuel Thibault wrote:
> This makes get_dns_addr address family-agnostic, thus allowing to add the
> IPv6 case.
> 
> Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
> ---
>  slirp/ip6.h      |  9 +++++++
>  slirp/libslirp.h |  1 +
>  slirp/slirp.c    | 72 ++++++++++++++++++++++++++++++++++++++++----------------
>  slirp/socket.c   |  4 ++--
>  4 files changed, 64 insertions(+), 22 deletions(-)
[...]
> diff --git a/slirp/slirp.c b/slirp/slirp.c
> index 2f0d3a3..3558b47 100644
> --- a/slirp/slirp.c
> +++ b/slirp/slirp.c
> @@ -45,7 +45,9 @@ static QTAILQ_HEAD(slirp_instances, Slirp) slirp_instances =
>      QTAILQ_HEAD_INITIALIZER(slirp_instances);
>  
>  static struct in_addr dns_addr;
> +static struct in6_addr dns6_addr;
>  static u_int dns_addr_time;
> +static u_int dns6_addr_time;
>  
>  #define TIMEOUT_FAST 2  /* milliseconds */
>  #define TIMEOUT_SLOW 499  /* milliseconds */
> @@ -99,6 +101,11 @@ int get_dns_addr(struct in_addr *pdns_addr)
>      return 0;
>  }
>  
> +int get_dns6_addr(struct in6_addr *pdns_addr6)
> +{
> +    return -1;
> +}
> +
>  static void winsock_cleanup(void)
>  {
>      WSACleanup();
> @@ -106,36 +113,40 @@ static void winsock_cleanup(void)
>  
>  #else
>  
> -static struct stat dns_addr_stat;
> -
> -static int get_dns_addr_cached(struct in_addr *pdns_addr)
> +static int get_dns_addr_cached(void *pdns_addr, void *cached_addr,
> +                               socklen_t addrlen,
> +                               struct stat *cached_stat, u_int *cached_time)
>  {
>      struct stat old_stat;
>      if ((curtime - dns_addr_time) < TIMEOUT_DEFAULT) {

Shouldn't that be "*cached_time" now instead of "dns_addr_time"? Or what
is this cached_time parameter now good for?

> -        *pdns_addr = dns_addr;
> +        memcpy(pdns_addr, cached_addr, addrlen);
>          return 0;
>      }
> -    old_stat = dns_addr_stat;
> -    if (stat("/etc/resolv.conf", &dns_addr_stat) != 0) {
> +    old_stat = *cached_stat;
> +    if (stat("/etc/resolv.conf", cached_stat) != 0) {
>          return -1;
>      }
> -    if ((dns_addr_stat.st_dev == old_stat.st_dev)
> -        && (dns_addr_stat.st_ino == old_stat.st_ino)
> -        && (dns_addr_stat.st_size == old_stat.st_size)
> -        && (dns_addr_stat.st_mtime == old_stat.st_mtime)) {
> -        *pdns_addr = dns_addr;
> +    if ((cached_stat->st_dev == old_stat.st_dev)
> +        && (cached_stat->st_ino == old_stat.st_ino)
> +        && (cached_stat->st_size == old_stat.st_size)
> +        && (cached_stat->st_mtime == old_stat.st_mtime)) {
> +        memcpy(pdns_addr, cached_addr, addrlen);
>          return 0;
>      }
>      return 1;
>  }
>  
> -static int get_dns_addr_resolv_conf(struct in_addr *pdns_addr)
> +static int get_dns_addr_resolv_conf(int af, void *pdns_addr, void *cached_addr,
> +                                    socklen_t addrlen, u_int *cached_time)
>  {
>      char buff[512];
>      char buff2[257];
>      FILE *f;
>      int found = 0;
> -    struct in_addr tmp_addr;
> +    void *tmp_addr = alloca(addrlen);
> +#ifdef DEBUG
> +    char s[INET6_ADDRSTRLEN];
> +#endif
>  
>      f = fopen("/etc/resolv.conf", "r");
>      if (!f)
> @@ -146,13 +157,14 @@ static int get_dns_addr_resolv_conf(struct in_addr *pdns_addr)
>  #endif
>      while (fgets(buff, 512, f) != NULL) {
>          if (sscanf(buff, "nameserver%*[ \t]%256s", buff2) == 1) {
> -            if (!inet_aton(buff2, &tmp_addr))
> +            if (!inet_pton(af, buff2, tmp_addr)) {
>                  continue;
> +            }
>              /* If it's the first one, set it to dns_addr */
>              if (!found) {
> -                *pdns_addr = tmp_addr;
> -                dns_addr = tmp_addr;
> -                dns_addr_time = curtime;
> +                memcpy(pdns_addr, tmp_addr, addrlen);
> +                memcpy(cached_addr, tmp_addr, addrlen);
> +                *cached_time = curtime;
>              }
>  #ifdef DEBUG
>              else
> @@ -166,7 +178,7 @@ static int get_dns_addr_resolv_conf(struct in_addr *pdns_addr)
>              }
>  #ifdef DEBUG
>              else

Maybe you could also add proper curly braces for this else-path now, and
then add the "char s[INET6_ADDRSTRLEN]" definition here instead ... that
way you don't need the separate "#ifdef DEBUG" at the beginning of this
function.

> -                fprintf(stderr, "%s", inet_ntoa(tmp_addr));
> +                fprintf(stderr, "%s", inet_ntop(af, tmp_addr, s, sizeof(s)));

While inet_ntoa never returns NULL, inet_ntop might return NULL on
errors. I think glibc fprintf will likely survive such a NULL pointer,
but it might still be better to check the return value and print
something else if the return value was NULL ?

>  #endif
>          }
>      }
> @@ -176,16 +188,36 @@ static int get_dns_addr_resolv_conf(struct in_addr *pdns_addr)
>      return 0;
>  }
>  
> +static struct stat dns_addr_stat;

You could also move this definition into the get_dns_addr() function.

>  int get_dns_addr(struct in_addr *pdns_addr)
>  {
>      if (dns_addr.s_addr != 0) {
>          int ret;
> -        ret = get_dns_addr_cached(pdns_addr);
> +        ret = get_dns_addr_cached(pdns_addr, &dns_addr, sizeof(dns_addr),
> +                                  &dns_addr_stat, &dns_addr_time);
> +        if (ret <= 0) {
> +            return ret;
> +        }
> +    }
> +    return get_dns_addr_resolv_conf(AF_INET, pdns_addr, &dns_addr,
> +                                    sizeof(dns_addr), &dns_addr_time);
> +}
> +
> +static struct stat dns6_addr_stat;

You could also move this definition into the get_dns6_addr() function.

> +int get_dns6_addr(struct in6_addr *pdns6_addr)
> +{
> +    if (!in6_zero(&dns6_addr)) {
> +        int ret;
> +        ret = get_dns_addr_cached(pdns6_addr, &dns6_addr, sizeof(dns6_addr),
> +                                  &dns6_addr_stat, &dns6_addr_time);
>          if (ret <= 0) {
>              return ret;
>          }
>      }
> -    return get_dns_addr_resolv_conf(pdns_addr);
> +    return get_dns_addr_resolv_conf(AF_INET6, pdns6_addr, &dns6_addr,
> +                                    sizeof(dns6_addr), &dns6_addr_time);
>  }

 Thomas