path: root/EmbeddedPkg/Library/FdtLib/fdt_overlay.c

/*
 * libfdt - Flat Device Tree manipulation
 * Copyright (C) 2016 Free Electrons
 * Copyright (C) 2016 NextThing Co.
 *
 * libfdt is dual licensed: you can use it either under the terms of
 * the GPL, or the BSD license, at your option.
 *
 *  a) This library 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 (at your option) any later version.
 *
 *     This library 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 library; if not, write to the Free
 *     Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
 *     MA 02110-1301 USA
 *
 * Alternatively,
 *
 *  b) Redistribution and use in source and binary forms, with or
 *     without modification, are permitted provided that the following
 *     conditions are met:
 *
 *     1. Redistributions of source code must retain the above
 *        copyright notice, this list of conditions and the following
 *        disclaimer.
 *     2. Redistributions in binary form must reproduce the above
 *        copyright notice, this list of conditions and the following
 *        disclaimer in the documentation and/or other materials
 *        provided with the distribution.
 *
 *     THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
 *     CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
 *     INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 *     MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 *     DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 *     CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 *     SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 *     NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 *     LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 *     HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 *     CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
 *     OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
 *     EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
#include "libfdt_env.h"

#include <fdt.h>
#include <libfdt.h>

#include "libfdt_internal.h"

/**
 * overlay_get_target_phandle - retrieves the target phandle of a fragment
 * @fdto: pointer to the device tree overlay blob
 * @fragment: node offset of the fragment in the overlay
 *
 * overlay_get_target_phandle() retrieves the target phandle of an
 * overlay fragment when that fragment uses a phandle (target
 * property) instead of a path (target-path property).
 *
 * returns:
 *      the phandle pointed by the target property
 *      0, if the phandle was not found
 *      -1, if the phandle was malformed
 */
static uint32_t
overlay_get_target_phandle (
  const void  *fdto,
  int         fragment
  )
{
  const fdt32_t  *val;
  int            len;

  val = fdt_getprop (fdto, fragment, "target", &len);
  if (!val) {
    return 0;
  }

  if ((len != sizeof (*val)) || (fdt32_to_cpu (*val) == (uint32_t)-1)) {
    return (uint32_t)-1;
  }

  return fdt32_to_cpu (*val);
}

/**
 * overlay_get_target - retrieves the offset of a fragment's target
 * @fdt: Base device tree blob
 * @fdto: Device tree overlay blob
 * @fragment: node offset of the fragment in the overlay
 * @pathp: pointer which receives the path of the target (or NULL)
 *
 * overlay_get_target() retrieves the target offset in the base
 * device tree of a fragment, no matter how the actual targetting is
 * done (through a phandle or a path)
 *
 * returns:
 *      the targetted node offset in the base device tree
 *      Negative error code on error
 */
static int
overlay_get_target (
  const void  *fdt,
  const void  *fdto,
  int         fragment,
  char const  **pathp
  )
{
  uint32_t    phandle;
  const char  *path = NULL;
  int         path_len = 0, ret;

  /* Try first to do a phandle based lookup */
  phandle = overlay_get_target_phandle (fdto, fragment);
  if (phandle == (uint32_t)-1) {
    return -FDT_ERR_BADPHANDLE;
  }

  /* no phandle, try path */
  if (!phandle) {
    /* And then a path based lookup */
    path = fdt_getprop (fdto, fragment, "target-path", &path_len);
    if (path) {
      ret = fdt_path_offset (fdt, path);
    } else {
      ret = path_len;
    }
  } else {
    ret = fdt_node_offset_by_phandle (fdt, phandle);
  }

  /*
  * If we haven't found either a target or a
  * target-path property in a node that contains a
  * __overlay__ subnode (we wouldn't be called
  * otherwise), consider it a improperly written
  * overlay
  */
  if ((ret < 0) && (path_len == -FDT_ERR_NOTFOUND)) {
    ret = -FDT_ERR_BADOVERLAY;
  }

  /* return on error */
  if (ret < 0) {
    return ret;
  }

  /* return pointer to path (if available) */
  if (pathp) {
    *pathp = path ? path : NULL;
  }

  return ret;
}

/**
 * overlay_phandle_add_offset - Increases a phandle by an offset
 * @fdt: Base device tree blob
 * @node: Device tree overlay blob
 * @name: Name of the property to modify (phandle or linux,phandle)
 * @delta: offset to apply
 *
 * overlay_phandle_add_offset() increments a node phandle by a given
 * offset.
 *
 * returns:
 *      0 on success.
 *      Negative error code on error
 */
static int
overlay_phandle_add_offset (
  void        *fdt,
  int         node,
  const char  *name,
  uint32_t    delta
  )
{
  const fdt32_t  *val;
  uint32_t       adj_val;
  int            len;

  val = fdt_getprop (fdt, node, name, &len);
  if (!val) {
    return len;
  }

  if (len != sizeof (*val)) {
    return -FDT_ERR_BADPHANDLE;
  }

  adj_val = fdt32_to_cpu (*val);
  if ((adj_val + delta) < adj_val) {
    return -FDT_ERR_NOPHANDLES;
  }

  adj_val += delta;
  if (adj_val == (uint32_t)-1) {
    return -FDT_ERR_NOPHANDLES;
  }

  return fdt_setprop_inplace_u32 (fdt, node, name, adj_val);
}

/**
 * overlay_adjust_node_phandles - Offsets the phandles of a node
 * @fdto: Device tree overlay blob
 * @node: Offset of the node we want to adjust
 * @delta: Offset to shift the phandles of
 *
 * overlay_adjust_node_phandles() adds a constant to all the phandles
 * of a given node. This is mainly use as part of the overlay
 * application process, when we want to update all the overlay
 * phandles to not conflict with the overlays of the base device tree.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_adjust_node_phandles (
  void      *fdto,
  int       node,
  uint32_t  delta
  )
{
  int  child;
  int  ret;

  ret = overlay_phandle_add_offset (fdto, node, "phandle", delta);
  if (ret && (ret != -FDT_ERR_NOTFOUND)) {
    return ret;
  }

  ret = overlay_phandle_add_offset (fdto, node, "linux,phandle", delta);
  if (ret && (ret != -FDT_ERR_NOTFOUND)) {
    return ret;
  }

  fdt_for_each_subnode (child, fdto, node) {
    ret = overlay_adjust_node_phandles (fdto, child, delta);
    if (ret) {
      return ret;
    }
  }

  return 0;
}

/**
 * overlay_adjust_local_phandles - Adjust the phandles of a whole overlay
 * @fdto: Device tree overlay blob
 * @delta: Offset to shift the phandles of
 *
 * overlay_adjust_local_phandles() adds a constant to all the
 * phandles of an overlay. This is mainly use as part of the overlay
 * application process, when we want to update all the overlay
 * phandles to not conflict with the overlays of the base device tree.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_adjust_local_phandles (
  void      *fdto,
  uint32_t  delta
  )
{
  /*
   * Start adjusting the phandles from the overlay root
   */
  return overlay_adjust_node_phandles (fdto, 0, delta);
}

/**
 * overlay_update_local_node_references - Adjust the overlay references
 * @fdto: Device tree overlay blob
 * @tree_node: Node offset of the node to operate on
 * @fixup_node: Node offset of the matching local fixups node
 * @delta: Offset to shift the phandles of
 *
 * overlay_update_local_nodes_references() update the phandles
 * pointing to a node within the device tree overlay by adding a
 * constant delta.
 *
 * This is mainly used as part of a device tree application process,
 * where you want the device tree overlays phandles to not conflict
 * with the ones from the base device tree before merging them.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_update_local_node_references (
  void      *fdto,
  int       tree_node,
  int       fixup_node,
  uint32_t  delta
  )
{
  int  fixup_prop;
  int  fixup_child;
  int  ret;

  fdt_for_each_property_offset (fixup_prop, fdto, fixup_node) {
    const fdt32_t  *fixup_val;
    const char     *tree_val;
    const char     *name;
    int            fixup_len;
    int            tree_len;
    int            i;

    fixup_val = fdt_getprop_by_offset (
                  fdto,
                  fixup_prop,
                  &name,
                  &fixup_len
                  );
    if (!fixup_val) {
      return fixup_len;
    }

    if (fixup_len % sizeof (uint32_t)) {
      return -FDT_ERR_BADOVERLAY;
    }

    tree_val = fdt_getprop (fdto, tree_node, name, &tree_len);
    if (!tree_val) {
      if (tree_len == -FDT_ERR_NOTFOUND) {
        return -FDT_ERR_BADOVERLAY;
      }

      return tree_len;
    }

    for (i = 0; i < (fixup_len / sizeof (uint32_t)); i++) {
      fdt32_t   adj_val;
      uint32_t  poffset;

      poffset = fdt32_to_cpu (fixup_val[i]);

      /*
       * phandles to fixup can be unaligned.
       *
       * Use a memcpy for the architectures that do
       * not support unaligned accesses.
       */
      memcpy (&adj_val, tree_val + poffset, sizeof (adj_val));

      adj_val = cpu_to_fdt32 (fdt32_to_cpu (adj_val) + delta);

      ret = fdt_setprop_inplace_namelen_partial (
              fdto,
              tree_node,
              name,
              strlen (name),
              poffset,
              &adj_val,
              sizeof (adj_val)
              );
      if (ret == -FDT_ERR_NOSPACE) {
        return -FDT_ERR_BADOVERLAY;
      }

      if (ret) {
        return ret;
      }
    }
  }

  fdt_for_each_subnode (fixup_child, fdto, fixup_node) {
    const char  *fixup_child_name = fdt_get_name (
                                      fdto,
                                      fixup_child,
                                      NULL
                                      );
    int  tree_child;

    tree_child = fdt_subnode_offset (
                   fdto,
                   tree_node,
                   fixup_child_name
                   );
    if (tree_child == -FDT_ERR_NOTFOUND) {
      return -FDT_ERR_BADOVERLAY;
    }

    if (tree_child < 0) {
      return tree_child;
    }

    ret = overlay_update_local_node_references (
            fdto,
            tree_child,
            fixup_child,
            delta
            );
    if (ret) {
      return ret;
    }
  }

  return 0;
}

/**
 * overlay_update_local_references - Adjust the overlay references
 * @fdto: Device tree overlay blob
 * @delta: Offset to shift the phandles of
 *
 * overlay_update_local_references() update all the phandles pointing
 * to a node within the device tree overlay by adding a constant
 * delta to not conflict with the base overlay.
 *
 * This is mainly used as part of a device tree application process,
 * where you want the device tree overlays phandles to not conflict
 * with the ones from the base device tree before merging them.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_update_local_references (
  void      *fdto,
  uint32_t  delta
  )
{
  int  fixups;

  fixups = fdt_path_offset (fdto, "/__local_fixups__");
  if (fixups < 0) {
    /* There's no local phandles to adjust, bail out */
    if (fixups == -FDT_ERR_NOTFOUND) {
      return 0;
    }

    return fixups;
  }

  /*
   * Update our local references from the root of the tree
   */
  return overlay_update_local_node_references (
           fdto,
           0,
           fixups,
           delta
           );
}

/**
 * overlay_fixup_one_phandle - Set an overlay phandle to the base one
 * @fdt: Base Device Tree blob
 * @fdto: Device tree overlay blob
 * @symbols_off: Node offset of the symbols node in the base device tree
 * @path: Path to a node holding a phandle in the overlay
 * @path_len: number of path characters to consider
 * @name: Name of the property holding the phandle reference in the overlay
 * @name_len: number of name characters to consider
 * @poffset: Offset within the overlay property where the phandle is stored
 * @label: Label of the node referenced by the phandle
 *
 * overlay_fixup_one_phandle() resolves an overlay phandle pointing to
 * a node in the base device tree.
 *
 * This is part of the device tree overlay application process, when
 * you want all the phandles in the overlay to point to the actual
 * base dt nodes.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_fixup_one_phandle (
  void        *fdt,
  void        *fdto,
  int         symbols_off,
  const char  *path,
  uint32_t    path_len,
  const char  *name,
  uint32_t    name_len,
  int         poffset,
  const char  *label
  )
{
  const char  *symbol_path;
  uint32_t    phandle;
  fdt32_t     phandle_prop;
  int         symbol_off, fixup_off;
  int         prop_len;

  if (symbols_off < 0) {
    return symbols_off;
  }

  symbol_path = fdt_getprop (
                  fdt,
                  symbols_off,
                  label,
                  &prop_len
                  );
  if (!symbol_path) {
    return prop_len;
  }

  symbol_off = fdt_path_offset (fdt, symbol_path);
  if (symbol_off < 0) {
    return symbol_off;
  }

  phandle = fdt_get_phandle (fdt, symbol_off);
  if (!phandle) {
    return -FDT_ERR_NOTFOUND;
  }

  fixup_off = fdt_path_offset_namelen (fdto, path, path_len);
  if (fixup_off == -FDT_ERR_NOTFOUND) {
    return -FDT_ERR_BADOVERLAY;
  }

  if (fixup_off < 0) {
    return fixup_off;
  }

  phandle_prop = cpu_to_fdt32 (phandle);
  return fdt_setprop_inplace_namelen_partial (
           fdto,
           fixup_off,
           name,
           name_len,
           poffset,
           &phandle_prop,
           sizeof (phandle_prop)
           );
}

unsigned long
strtoul (
  const char  *nptr,
  char        **endptr,
  int         base
  );

/**
 * overlay_fixup_phandle - Set an overlay phandle to the base one
 * @fdt: Base Device Tree blob
 * @fdto: Device tree overlay blob
 * @symbols_off: Node offset of the symbols node in the base device tree
 * @property: Property offset in the overlay holding the list of fixups
 *
 * overlay_fixup_phandle() resolves all the overlay phandles pointed
 * to in a __fixups__ property, and updates them to match the phandles
 * in use in the base device tree.
 *
 * This is part of the device tree overlay application process, when
 * you want all the phandles in the overlay to point to the actual
 * base dt nodes.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_fixup_phandle (
  void  *fdt,
  void  *fdto,
  int   symbols_off,
  int   property
  )
{
  const char  *value;
  const char  *label;
  int         len;

  value = fdt_getprop_by_offset (
            fdto,
            property,
            &label,
            &len
            );
  if (!value) {
    if (len == -FDT_ERR_NOTFOUND) {
      return -FDT_ERR_INTERNAL;
    }

    return len;
  }

  do {
    const char  *path, *name, *fixup_end;
    const char  *fixup_str = value;
    uint32_t    path_len, name_len;
    uint32_t    fixup_len;
    char        *sep, *endptr;
    int         poffset, ret;

    fixup_end = memchr (value, '\0', len);
    if (!fixup_end) {
      return -FDT_ERR_BADOVERLAY;
    }

    fixup_len = fixup_end - fixup_str;

    len   -= fixup_len + 1;
    value += fixup_len + 1;

    path = fixup_str;
    sep  = memchr (fixup_str, ':', fixup_len);
    if (!sep || (*sep != ':')) {
      return -FDT_ERR_BADOVERLAY;
    }

    path_len = sep - path;
    if (path_len == (fixup_len - 1)) {
      return -FDT_ERR_BADOVERLAY;
    }

    fixup_len -= path_len + 1;
    name       = sep + 1;
    sep        = memchr (name, ':', fixup_len);
    if (!sep || (*sep != ':')) {
      return -FDT_ERR_BADOVERLAY;
    }

    name_len = sep - name;
    if (!name_len) {
      return -FDT_ERR_BADOVERLAY;
    }

    poffset = strtoul (sep + 1, &endptr, 10);
    if ((*endptr != '\0') || (endptr <= (sep + 1))) {
      return -FDT_ERR_BADOVERLAY;
    }

    ret = overlay_fixup_one_phandle (
            fdt,
            fdto,
            symbols_off,
            path,
            path_len,
            name,
            name_len,
            poffset,
            label
            );
    if (ret) {
      return ret;
    }
  } while (len > 0);

  return 0;
}

/**
 * overlay_fixup_phandles - Resolve the overlay phandles to the base
 *                          device tree
 * @fdt: Base Device Tree blob
 * @fdto: Device tree overlay blob
 *
 * overlay_fixup_phandles() resolves all the overlay phandles pointing
 * to nodes in the base device tree.
 *
 * This is one of the steps of the device tree overlay application
 * process, when you want all the phandles in the overlay to point to
 * the actual base dt nodes.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_fixup_phandles (
  void  *fdt,
  void  *fdto
  )
{
  int  fixups_off, symbols_off;
  int  property;

  /* We can have overlays without any fixups */
  fixups_off = fdt_path_offset (fdto, "/__fixups__");
  if (fixups_off == -FDT_ERR_NOTFOUND) {
    return 0;             /* nothing to do */
  }

  if (fixups_off < 0) {
    return fixups_off;
  }

  /* And base DTs without symbols */
  symbols_off = fdt_path_offset (fdt, "/__symbols__");
  if (((symbols_off < 0) && (symbols_off != -FDT_ERR_NOTFOUND))) {
    return symbols_off;
  }

  fdt_for_each_property_offset (property, fdto, fixups_off) {
    int  ret;

    ret = overlay_fixup_phandle (fdt, fdto, symbols_off, property);
    if (ret) {
      return ret;
    }
  }

  return 0;
}

/**
 * overlay_apply_node - Merges a node into the base device tree
 * @fdt: Base Device Tree blob
 * @target: Node offset in the base device tree to apply the fragment to
 * @fdto: Device tree overlay blob
 * @node: Node offset in the overlay holding the changes to merge
 *
 * overlay_apply_node() merges a node into a target base device tree
 * node pointed.
 *
 * This is part of the final step in the device tree overlay
 * application process, when all the phandles have been adjusted and
 * resolved and you just have to merge overlay into the base device
 * tree.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_apply_node (
  void  *fdt,
  int   target,
  void  *fdto,
  int   node
  )
{
  int  property;
  int  subnode;

  fdt_for_each_property_offset (property, fdto, node) {
    const char  *name;
    const void  *prop;
    int         prop_len;
    int         ret;

    prop = fdt_getprop_by_offset (
             fdto,
             property,
             &name,
             &prop_len
             );
    if (prop_len == -FDT_ERR_NOTFOUND) {
      return -FDT_ERR_INTERNAL;
    }

    if (prop_len < 0) {
      return prop_len;
    }

    ret = fdt_setprop (fdt, target, name, prop, prop_len);
    if (ret) {
      return ret;
    }
  }

  fdt_for_each_subnode (subnode, fdto, node) {
    const char  *name = fdt_get_name (fdto, subnode, NULL);
    int         nnode;
    int         ret;

    nnode = fdt_add_subnode (fdt, target, name);
    if (nnode == -FDT_ERR_EXISTS) {
      nnode = fdt_subnode_offset (fdt, target, name);
      if (nnode == -FDT_ERR_NOTFOUND) {
        return -FDT_ERR_INTERNAL;
      }
    }

    if (nnode < 0) {
      return nnode;
    }

    ret = overlay_apply_node (fdt, nnode, fdto, subnode);
    if (ret) {
      return ret;
    }
  }

  return 0;
}

/**
 * overlay_merge - Merge an overlay into its base device tree
 * @fdt: Base Device Tree blob
 * @fdto: Device tree overlay blob
 *
 * overlay_merge() merges an overlay into its base device tree.
 *
 * This is the next to last step in the device tree overlay application
 * process, when all the phandles have been adjusted and resolved and
 * you just have to merge overlay into the base device tree.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_merge (
  void  *fdt,
  void  *fdto
  )
{
  int  fragment;

  fdt_for_each_subnode (fragment, fdto, 0) {
    int  overlay;
    int  target;
    int  ret;

    /*
     * Each fragments will have an __overlay__ node. If
     * they don't, it's not supposed to be merged
     */
    overlay = fdt_subnode_offset (fdto, fragment, "__overlay__");
    if (overlay == -FDT_ERR_NOTFOUND) {
      continue;
    }

    if (overlay < 0) {
      return overlay;
    }

    target = overlay_get_target (fdt, fdto, fragment, NULL);
    if (target < 0) {
      return target;
    }

    ret = overlay_apply_node (fdt, target, fdto, overlay);
    if (ret) {
      return ret;
    }
  }

  return 0;
}

static int
get_path_len (
  const void  *fdt,
  int         nodeoffset
  )
{
  int         len = 0, namelen;
  const char  *name;

  FDT_CHECK_HEADER (fdt);

  for ( ; ;) {
    name = fdt_get_name (fdt, nodeoffset, &namelen);
    if (!name) {
      return namelen;
    }

    /* root? we're done */
    if (namelen == 0) {
      break;
    }

    nodeoffset = fdt_parent_offset (fdt, nodeoffset);
    if (nodeoffset < 0) {
      return nodeoffset;
    }

    len += namelen + 1;
  }

  /* in case of root pretend it's "/" */
  if (len == 0) {
    len++;
  }

  return len;
}

/**
 * overlay_symbol_update - Update the symbols of base tree after a merge
 * @fdt: Base Device Tree blob
 * @fdto: Device tree overlay blob
 *
 * overlay_symbol_update() updates the symbols of the base tree with the
 * symbols of the applied overlay
 *
 * This is the last step in the device tree overlay application
 * process, allowing the reference of overlay symbols by subsequent
 * overlay operations.
 *
 * returns:
 *      0 on success
 *      Negative error code on failure
 */
static int
overlay_symbol_update (
  void  *fdt,
  void  *fdto
  )
{
  int         root_sym, ov_sym, prop, path_len, fragment, target;
  int         len, frag_name_len, ret, rel_path_len;
  const char  *s, *e;
  const char  *path;
  const char  *name;
  const char  *frag_name;
  const char  *rel_path;
  const char  *target_path;
  char        *buf;
  void        *p;

  ov_sym = fdt_subnode_offset (fdto, 0, "__symbols__");

  /* if no overlay symbols exist no problem */
  if (ov_sym < 0) {
    return 0;
  }

  root_sym = fdt_subnode_offset (fdt, 0, "__symbols__");

  /* it no root symbols exist we should create them */
  if (root_sym == -FDT_ERR_NOTFOUND) {
    root_sym = fdt_add_subnode (fdt, 0, "__symbols__");
  }

  /* any error is fatal now */
  if (root_sym < 0) {
    return root_sym;
  }

  /* iterate over each overlay symbol */
  fdt_for_each_property_offset (prop, fdto, ov_sym) {
    path = fdt_getprop_by_offset (fdto, prop, &name, &path_len);
    if (!path) {
      return path_len;
    }

    /* verify it's a string property (terminated by a single \0) */
    if ((path_len < 1) || (memchr (path, '\0', path_len) != &path[path_len - 1])) {
      return -FDT_ERR_BADVALUE;
    }

    /* keep end marker to avoid strlen() */
    e = path + path_len;

    /* format: /<fragment-name>/__overlay__/<relative-subnode-path> */

    if (*path != '/') {
      return -FDT_ERR_BADVALUE;
    }

    /* get fragment name first */
    s = strchr (path + 1, '/');
    if (!s) {
      return -FDT_ERR_BADOVERLAY;
    }

    frag_name     = path + 1;
    frag_name_len = s - path - 1;

    /* verify format; safe since "s" lies in \0 terminated prop */
    len = sizeof ("/__overlay__/") - 1;
    if (((e - s) < len) || memcmp (s, "/__overlay__/", len)) {
      return -FDT_ERR_BADOVERLAY;
    }

    rel_path     = s + len;
    rel_path_len = e - rel_path;

    /* find the fragment index in which the symbol lies */
    ret = fdt_subnode_offset_namelen (
            fdto,
            0,
            frag_name,
            frag_name_len
            );
    /* not found? */
    if (ret < 0) {
      return -FDT_ERR_BADOVERLAY;
    }

    fragment = ret;

    /* an __overlay__ subnode must exist */
    ret = fdt_subnode_offset (fdto, fragment, "__overlay__");
    if (ret < 0) {
      return -FDT_ERR_BADOVERLAY;
    }

    /* get the target of the fragment */
    ret = overlay_get_target (fdt, fdto, fragment, &target_path);
    if (ret < 0) {
      return ret;
    }

    target = ret;

    /* if we have a target path use */
    if (!target_path) {
      ret = get_path_len (fdt, target);
      if (ret < 0) {
        return ret;
      }

      len = ret;
    } else {
      len = strlen (target_path);
    }

    ret = fdt_setprop_placeholder (
            fdt,
            root_sym,
            name,
            len + (len > 1) + rel_path_len + 1,
            &p
            );
    if (ret < 0) {
      return ret;
    }

    if (!target_path) {
      /* again in case setprop_placeholder changed it */
      ret = overlay_get_target (fdt, fdto, fragment, &target_path);
      if (ret < 0) {
        return ret;
      }

      target = ret;
    }

    buf = p;
    if (len > 1) {
      /* target is not root */
      if (!target_path) {
        ret = fdt_get_path (fdt, target, buf, len + 1);
        if (ret < 0) {
          return ret;
        }
      } else {
        memcpy (buf, target_path, len + 1);
      }
    } else {
      len--;
    }

    buf[len] = '/';
    memcpy (buf + len + 1, rel_path, rel_path_len);
    buf[len + 1 + rel_path_len] = '\0';
  }

  return 0;
}

int
fdt_overlay_apply (
  void  *fdt,
  void  *fdto
  )
{
  uint32_t  delta = fdt_get_max_phandle (fdt);
  int       ret;

  FDT_CHECK_HEADER (fdt);
  FDT_CHECK_HEADER (fdto);

  ret = overlay_adjust_local_phandles (fdto, delta);
  if (ret) {
    goto err;
  }

  ret = overlay_update_local_references (fdto, delta);
  if (ret) {
    goto err;
  }

  ret = overlay_fixup_phandles (fdt, fdto);
  if (ret) {
    goto err;
  }

  ret = overlay_merge (fdt, fdto);
  if (ret) {
    goto err;
  }

  ret = overlay_symbol_update (fdt, fdto);
  if (ret) {
    goto err;
  }

  /*
   * The overlay has been damaged, erase its magic.
   */
  fdt_set_magic (fdto, ~0);

  return 0;

err:

  /*
   * The overlay might have been damaged, erase its magic.
   */
  fdt_set_magic (fdto, ~0);

  /*
   * The base device tree might have been damaged, erase its
   * magic.
   */
  fdt_set_magic (fdt, ~0);

  return ret;
}