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

[Daniel Kiper]

On Fri, Mar 08, 2019 at 11:51:27AM +0000, Andre Przywara wrote:
> On Fri, 8 Mar 2019 12:34:18 +0100
> Daniel Kiper <address@hidden> wrote:
>
> > 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.
>
> I was eyeballing this line as well, but I think it *is* needed:
> It resets end to 0, before possibly jumping back to the again label, inside 
> the if conditions below.
> Does that make sense?

Ahhh... Right, I have missed that. So,
  Reviewed-by: Daniel Kiper <address@hidden>

Daniel

> Cheers,
> Andre.
>
> >
> > > +  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