Re: [PATCH v3 03/10] net: dhcp: Allow overloading legacy bootfile and name field

[Daniel Kiper]

On Thu, Mar 07, 2019 at 03:14:09PM +0000, Andre Przywara wrote:
> From: Andrei Borzenkov <address@hidden>
>
> DHCP specifies a special dummy option OVERLOAD, to allow DHCP options to
> spill over into the (legacy) BOOTFILE and SNAME fields.
>
> Parse and handle this option properly.
>
> Signed-off-by: Andre Przywara <address@hidden>

In general Reviewed-by: Daniel Kiper <address@hidden> but one
nitpick below...

> ---
>  grub-core/net/bootp.c | 59 ++++++++++++++++++++++++++++++++++++++++++-
>  include/grub/net.h    |  1 +
>  2 files changed, 59 insertions(+), 1 deletion(-)
>
> diff --git a/grub-core/net/bootp.c b/grub-core/net/bootp.c
> index 8d6763689..860d9c80d 100644
> --- a/grub-core/net/bootp.c
> +++ b/grub-core/net/bootp.c
> @@ -25,11 +25,19 @@
>  #include <grub/net/udp.h>
>  #include <grub/datetime.h>
>
> +enum
> +{
> +  GRUB_DHCP_OPT_OVERLOAD_FILE = 1,
> +  GRUB_DHCP_OPT_OVERLOAD_SNAME = 2,
> +};
> +
>  static const void *
>  find_dhcp_option (const struct grub_net_bootp_packet *bp, grub_size_t size,
>                 grub_uint8_t opt_code, grub_uint8_t *opt_len)
>  {
>    const grub_uint8_t *ptr;
> +  grub_uint8_t overload = 0;
> +  int end = 0;
>    grub_size_t i;
>
>    if (opt_len)
> @@ -54,6 +62,7 @@ find_dhcp_option (const struct grub_net_bootp_packet *bp, 
> grub_size_t size,
>    size -= sizeof (*bp);
>    i = sizeof (grub_uint32_t);
>
> +again:
>    while (i < size)
>      {
>        grub_uint8_t tagtype;
> @@ -67,7 +76,10 @@ find_dhcp_option (const struct grub_net_bootp_packet *bp, 
> grub_size_t size,
>
>        /* End tag.  */
>        if (tagtype == GRUB_NET_BOOTP_END)
> -     break;
> +     {
> +       end = 1;
> +       break;
> +     }
>
>        if (i >= size)
>       return NULL;
> @@ -84,9 +96,54 @@ find_dhcp_option (const struct grub_net_bootp_packet *bp, 
> grub_size_t size,
>         return &ptr[i];
>       }
>
> +      if (tagtype == GRUB_NET_DHCP_OVERLOAD && taglength == 1)
> +     overload = ptr[i];
> +
>        i += taglength;
>      }
>
> +  if (!end)
> +    return NULL;
> +
> +  /* RFC2131, 4.1, 23ff:
> +   * If the options in a DHCP message extend into the 'sname' and 'file'
> +   * fields, the 'option overload' option MUST appear in the 'options'
> +   * field, with value 1, 2 or 3, as specified in RFC 1533.  If the
> +   * 'option overload' option is present in the 'options' field, the
> +   * options in the 'options' field MUST be terminated by an 'end' option,
> +   * and MAY contain one or more 'pad' options to fill the options field.
> +   * The options in the 'sname' and 'file' fields (if in use as indicated
> +   * by the 'options overload' option) MUST begin with the first octet of
> +   * the field, MUST be terminated by an 'end' option, and MUST be
> +   * followed by 'pad' options to fill the remainder of the field.  Any
> +   * individual option in the 'options', 'sname' and 'file' fields MUST be
> +   * entirely contained in that field.  The options in the 'options' field
> +   * MUST be interpreted first, so that any 'option overload' options may
> +   * be interpreted.  The 'file' field MUST be interpreted next (if the
> +   * 'option overload' option indicates that the 'file' field contains
> +   * DHCP options), followed by the 'sname' field.
> +   *
> +   * FIXME: We do not explicitly check for trailing 'pad' options here.
> +   */
> +  end = 0;

It seems to me that this line is not needed... I can drop it before
committing the patch if you are OK with it.

> +  if (overload & GRUB_DHCP_OPT_OVERLOAD_FILE)
> +  {
> +    overload &= ~GRUB_DHCP_OPT_OVERLOAD_FILE;
> +    ptr = (grub_uint8_t *) &bp->boot_file[0];
> +    size = sizeof (bp->boot_file);
> +    i = 0;
> +    goto again;
> +  }
> +
> +  if (overload & GRUB_DHCP_OPT_OVERLOAD_SNAME)
> +  {
> +    overload &= ~GRUB_DHCP_OPT_OVERLOAD_SNAME;
> +    ptr = (grub_uint8_t *) &bp->server_name[0];
> +    size = sizeof (bp->server_name);
> +    i = 0;
> +    goto again;
> +  }
> +
>    return NULL;
>  }

Daniel