dhcpopts.c

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/*
 * Copyright (C) 2008 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
FILE_SECBOOT ( PERMITTED );

#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <ipxe/dhcp.h>
#include <ipxe/dhcpopts.h>

/** @file
 *
 * DHCP options
 *
 */

/**
 * Obtain printable version of a DHCP option tag
 *
 * @v tag               DHCP option tag
 * @ret name            String representation of the tag
 *
 */
static inline char * dhcp_tag_name ( unsigned int tag ) {
        static char name[8];

        if ( DHCP_IS_ENCAP_OPT ( tag ) ) {
                snprintf ( name, sizeof ( name ), "%d.%d",
                           DHCP_ENCAPSULATOR ( tag ),
                           DHCP_ENCAPSULATED ( tag ) );
        } else {
                snprintf ( name, sizeof ( name ), "%d", tag );
        }
        return name;
}

/**
 * Get pointer to DHCP option
 *
 * @v options           DHCP options block
 * @v offset            Offset within options block
 * @ret option          DHCP option
 */
static inline __attribute__ (( always_inline )) struct dhcp_option *
dhcp_option ( struct dhcp_options *options, unsigned int offset ) {
        return ( ( struct dhcp_option * ) ( options->data + offset ) );
}

/**
 * Get offset of a DHCP option
 *
 * @v options           DHCP options block
 * @v option            DHCP option
 * @ret offset          Offset within options block
 */
static inline __attribute__ (( always_inline )) int
dhcp_option_offset ( struct dhcp_options *options,
                     struct dhcp_option *option ) {
        return ( ( ( void * ) option ) - options->data );
}

/**
 * Calculate length of any DHCP option
 *
 * @v option            DHCP option
 * @ret len             Length (including tag and length field)
 */
static unsigned int dhcp_option_len ( struct dhcp_option *option ) {
        if ( ( option->tag == DHCP_END ) || ( option->tag == DHCP_PAD ) ) {
                return 1;
        } else {
                return ( option->len + DHCP_OPTION_HEADER_LEN );
        }
}

/**
 * Find DHCP option within DHCP options block, and its encapsulator (if any)
 *
 * @v options           DHCP options block
 * @v tag               DHCP option tag to search for
 * @ret encap_offset    Offset of encapsulating DHCP option
 * @ret offset          Offset of DHCP option, or negative error
 *
 * Searches for the DHCP option matching the specified tag within the
 * DHCP option block.  Encapsulated options may be searched for by
 * using DHCP_ENCAP_OPT() to construct the tag value.
 *
 * If the option is encapsulated, and @c encap_offset is non-NULL, it
 * will be filled in with the offset of the encapsulating option.
 *
 * This routine is designed to be paranoid.  It does not assume that
 * the option data is well-formatted, and so must guard against flaws
 * such as options missing a @c DHCP_END terminator, or options whose
 * length would take them beyond the end of the data block.
 */
static int find_dhcp_option_with_encap ( struct dhcp_options *options,
                                         unsigned int tag,
                                         int *encap_offset ) {
        unsigned int original_tag __attribute__ (( unused )) = tag;
        struct dhcp_option *option;
        int offset = 0;
        ssize_t remaining = options->used_len;
        unsigned int option_len;

        /* Sanity check */
        if ( tag == DHCP_PAD )
                return -ENOENT;

        /* Search for option */
        while ( remaining ) {
                /* Calculate length of this option.  Abort processing
                 * if the length is malformed (i.e. takes us beyond
                 * the end of the data block).
                 */
                option = dhcp_option ( options, offset );
                option_len = dhcp_option_len ( option );
                remaining -= option_len;
                if ( remaining < 0 )
                        break;
                /* Check for explicit end marker */
                if ( option->tag == DHCP_END ) {
                        if ( tag == DHCP_END )
                                /* Special case where the caller is interested
                                 * in whether we have this marker or not.
                                 */
                                return offset;
                        else
                                break;
                }
                /* Check for matching tag */
                if ( option->tag == tag ) {
                        DBGC ( options, "DHCPOPT %p found %s (length %d)\n",
                               options, dhcp_tag_name ( original_tag ),
                               option_len );
                        return offset;
                }
                /* Check for start of matching encapsulation block */
                if ( DHCP_IS_ENCAP_OPT ( tag ) &&
                     ( option->tag == DHCP_ENCAPSULATOR ( tag ) ) ) {
                        if ( encap_offset )
                                *encap_offset = offset;
                        /* Continue search within encapsulated option block */
                        tag = DHCP_ENCAPSULATED ( tag );
                        remaining = option_len;
                        offset += DHCP_OPTION_HEADER_LEN;
                        continue;
                }
                offset += option_len;
        }

        return -ENOENT;
}

/**
 * Refuse to reallocate DHCP option block
 *
 * @v options           DHCP option block
 * @v len               New length
 * @ret rc              Return status code
 */
int dhcpopt_no_realloc ( struct dhcp_options *options, size_t len ) {
        return ( ( len <= options->alloc_len ) ? 0 : -ENOSPC );
}

/**
 * Resize a DHCP option
 *
 * @v options           DHCP option block
 * @v offset            Offset of option to resize
 * @v encap_offset      Offset of encapsulating offset (or -ve for none)
 * @v old_len           Old length (including header)
 * @v new_len           New length (including header)
 * @ret rc              Return status code
 */
static int resize_dhcp_option ( struct dhcp_options *options,
                                int offset, int encap_offset,
                                size_t old_len, size_t new_len ) {
        struct dhcp_option *encapsulator;
        struct dhcp_option *option;
        ssize_t delta = ( new_len - old_len );
        size_t old_alloc_len;
        size_t new_used_len;
        size_t new_encapsulator_len;
        void *source;
        void *dest;
        int rc;

        /* Check for sufficient space */
        if ( new_len > DHCP_MAX_LEN ) {
                DBGC ( options, "DHCPOPT %p overlength option\n", options );
                return -ENOSPC;
        }
        new_used_len = ( options->used_len + delta );

        /* Expand options block, if necessary */
        if ( new_used_len > options->alloc_len ) {
                /* Reallocate options block */
                old_alloc_len = options->alloc_len;
                if ( ( rc = options->realloc ( options, new_used_len ) ) != 0 ){
                        DBGC ( options, "DHCPOPT %p could not reallocate to "
                               "%zd bytes\n", options, new_used_len );
                        return rc;
                }
                /* Clear newly allocated space */
                memset ( ( options->data + old_alloc_len ), 0,
                         ( options->alloc_len - old_alloc_len ) );
        }

        /* Update encapsulator, if applicable */
        if ( encap_offset >= 0 ) {
                encapsulator = dhcp_option ( options, encap_offset );
                new_encapsulator_len = ( encapsulator->len + delta );
                if ( new_encapsulator_len > DHCP_MAX_LEN ) {
                        DBGC ( options, "DHCPOPT %p overlength encapsulator\n",
                               options );
                        return -ENOSPC;
                }
                encapsulator->len = new_encapsulator_len;
        }

        /* Update used length */
        options->used_len = new_used_len;

        /* Move remainder of option data */
        option = dhcp_option ( options, offset );
        source = ( ( ( void * ) option ) + old_len );
        dest = ( ( ( void * ) option ) + new_len );
        memmove ( dest, source, ( new_used_len - offset - new_len ) );

        /* Shrink options block, if applicable */
        if ( new_used_len < options->alloc_len ) {
                if ( ( rc = options->realloc ( options, new_used_len ) ) != 0 ){
                        DBGC ( options, "DHCPOPT %p could not reallocate to "
                               "%zd bytes\n", options, new_used_len );
                        return rc;
                }
        }

        return 0;
}

/**
 * Set value of DHCP option
 *
 * @v options           DHCP option block
 * @v tag               DHCP option tag
 * @v data              New value for DHCP option
 * @v len               Length of value, in bytes
 * @ret offset          Offset of DHCP option, or negative error
 *
 * Sets the value of a DHCP option within the options block.  The
 * option may or may not already exist.  Encapsulators will be created
 * (and deleted) as necessary.
 *
 * This call may fail due to insufficient space in the options block.
 * If it does fail, and the option existed previously, the option will
 * be left with its original value.
 */
static int set_dhcp_option ( struct dhcp_options *options, unsigned int tag,
                             const void *data, size_t len ) {
        static const uint8_t empty_encap[] = { DHCP_END };
        int offset;
        int encap_offset = -1;
        int creation_offset;
        struct dhcp_option *option;
        unsigned int encap_tag = DHCP_ENCAPSULATOR ( tag );
        size_t old_len = 0;
        size_t new_len = ( len ? ( len + DHCP_OPTION_HEADER_LEN ) : 0 );
        int rc;

        /* Sanity check */
        if ( tag == DHCP_PAD )
                return -ENOTTY;

        creation_offset = find_dhcp_option_with_encap ( options, DHCP_END,
                                                        NULL );
        if ( creation_offset < 0 )
                creation_offset = options->used_len;
        /* Find old instance of this option, if any */
        offset = find_dhcp_option_with_encap ( options, tag, &encap_offset );
        if ( offset >= 0 ) {
                old_len = dhcp_option_len ( dhcp_option ( options, offset ) );
                DBGC ( options, "DHCPOPT %p resizing %s from %zd to %zd\n",
                       options, dhcp_tag_name ( tag ), old_len, new_len );
        } else {
                DBGC ( options, "DHCPOPT %p creating %s (length %zd)\n",
                       options, dhcp_tag_name ( tag ), new_len );
        }

        /* Ensure that encapsulator exists, if required */
        if ( encap_tag ) {
                if ( encap_offset < 0 ) {
                        encap_offset =
                                set_dhcp_option ( options, encap_tag,
                                                  empty_encap,
                                                  sizeof ( empty_encap ) );
                }
                if ( encap_offset < 0 )
                        return encap_offset;
                creation_offset = ( encap_offset + DHCP_OPTION_HEADER_LEN );
        }

        /* Create new option if necessary */
        if ( offset < 0 )
                offset = creation_offset;

        /* Resize option to fit new data */
        if ( ( rc = resize_dhcp_option ( options, offset, encap_offset,
                                         old_len, new_len ) ) != 0 )
                return rc;

        /* Copy new data into option, if applicable */
        if ( len ) {
                option = dhcp_option ( options, offset );
                option->tag = tag;
                option->len = len;
                memcpy ( &option->data, data, len );
        }

        /* Delete encapsulator if there's nothing else left in it */
        if ( encap_offset >= 0 ) {
                option = dhcp_option ( options, encap_offset );
                if ( option->len <= 1 )
                        set_dhcp_option ( options, encap_tag, NULL, 0 );
        }

        return offset;
}

/**
 * Check applicability of DHCP option setting
 *
 * @v tag               Setting tag number
 * @ret applies         Setting applies to this option block
 */
int dhcpopt_applies ( unsigned int tag ) {

        return ( tag && ( tag <= DHCP_ENCAP_OPT ( DHCP_MAX_OPTION,
                                                  DHCP_MAX_OPTION ) ) );
}

/**
 * Store value of DHCP option setting
 *
 * @v options           DHCP option block
 * @v tag               Setting tag number
 * @v data              Setting data, or NULL to clear setting
 * @v len               Length of setting data
 * @ret rc              Return status code
 */
int dhcpopt_store ( struct dhcp_options *options, unsigned int tag,
                    const void *data, size_t len ) {
        int offset;

        offset = set_dhcp_option ( options, tag, data, len );
        if ( offset < 0 )
                return offset;
        return 0;
}

/**
 * Fetch value of DHCP option setting
 *
 * @v options           DHCP option block
 * @v tag               Setting tag number
 * @v data              Buffer to fill with setting data
 * @v len               Length of buffer
 * @ret len             Length of setting data, or negative error
 */
int dhcpopt_fetch ( struct dhcp_options *options, unsigned int tag,
                    void *data, size_t len ) {
        int offset;
        struct dhcp_option *option;
        size_t option_len;

        offset = find_dhcp_option_with_encap ( options, tag, NULL );
        if ( offset < 0 )
                return offset;

        option = dhcp_option ( options, offset );
        option_len = option->len;
        if ( len > option_len )
                len = option_len;
        memcpy ( data, option->data, len );

        return option_len;
}

/**
 * Recalculate length of DHCP options block
 *
 * @v options           Uninitialised DHCP option block
 *
 * The "used length" field will be updated based on scanning through
 * the block to find the end of the options.
 */
void dhcpopt_update_used_len ( struct dhcp_options *options ) {
        struct dhcp_option *option;
        int offset = 0;
        ssize_t remaining = options->alloc_len;
        unsigned int option_len;

        /* Find last non-pad option */
        options->used_len = 0;
        while ( remaining ) {
                option = dhcp_option ( options, offset );
                option_len = dhcp_option_len ( option );
                remaining -= option_len;
                if ( remaining < 0 )
                        break;
                offset += option_len;
                if ( option->tag != DHCP_PAD )
                        options->used_len = offset;
        }
}

/**
 * Initialise prepopulated block of DHCP options
 *
 * @v options           Uninitialised DHCP option block
 * @v data              Memory for DHCP option data
 * @v alloc_len         Length of memory for DHCP option data
 * @v realloc           DHCP option block reallocator
 *
 * The memory content must already be filled with valid DHCP options.
 * A zeroed block counts as a block of valid DHCP options.
 */
void dhcpopt_init ( struct dhcp_options *options, void *data, size_t alloc_len,
                    int ( * realloc ) ( struct dhcp_options *options,
                                        size_t len ) ) {

        /* Fill in fields */
        options->data = data;
        options->alloc_len = alloc_len;
        options->realloc = realloc;

        /* Update length */
        dhcpopt_update_used_len ( options );

        DBGC ( options, "DHCPOPT %p created (data %p lengths %#zx,%#zx)\n",
               options, options->data, options->used_len, options->alloc_len );
}