dtoverlay: Add support for string escape sequences

Add the ability to embed arbitrary byte data in strings. The change was
motivated by the desire to allow parameters to supply multiple compatible
strings, but there may be other uses.

The acceptable escape sequences are most of the range supported by C, but
with the hexadecimal ('\x??`) escape limited to precisely two digits.

See: https://forums.raspberrypi.com/viewtopic.php?t=330792

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

helpers/dtoverlay/dtoverlay.c

@@ -1453,7 +1453,7 @@ const char *dtoverlay_find_override(DTBLOB_T *dtb, const char *override_name,
    return data;
 }
 
-int hex_digit(char c)
+static int hex_digit(char c)
 {
    if (c >= '0' && c <= '9')
       return c - '0';
@@ -1465,6 +1465,19 @@ int hex_digit(char c)
       return -1;
 }
 
+static int hex_byte(const char *p)
+{
+   int nib1, nib2;
+
+   nib1 = hex_digit(p[0]);
+   if (nib1 < 0)
+      return -1;
+   nib2 = hex_digit(p[1]);
+   if (nib2 < 0)
+      return -1;
+   return (nib1 << 4) | nib2;
+}
+
 int dtoverlay_override_one_target(int override_type,
                                   const char *override_value,
                                   DTBLOB_T *dtb, int node_off,
@@ -1476,24 +1489,75 @@ int dtoverlay_override_one_target(int override_type,
 
    if (override_type == DTOVERRIDE_STRING)
    {
-      char *prop_val;
+      char unescaped_value[256];
+      char *prop_val, *q;
+      const char *p;
       int prop_len;
 
-      /* Replace the whole property with the string */
+      p = override_value;
+      q = unescaped_value;
+      while (*p)
+      {
+         int c = *(p++);
+         if (c == '\\')
+         {
+            c = *(p++);
+            if (c >= '0' && c <= '7')
+            {
+               c -= '0';
+               if (*p >= '0' && *p <= '7')
+               {
+                  c = (c << 3) + *(p++) - '0';
+                  if (*p >= '0' && *p <= '7')
+                  {
+                     c = (c << 3) + *(p++) - '0';
+                     if (c > 255)
+                        c = -1;
+                  }
+               }
+            }
+            else if (c == 'a')
+               c = '\x07';
+            else if (c == 'b')
+               c = '\b';
+            else if (c == 'f')
+               c = '\f';
+            else if (c == 'n')
+               c = '\n';
+            else if (c == 'r')
+               c = '\r';
+            else if (c == 't')
+               c = '\t';
+            else if (c == 'v')
+               c = '\v';
+            else if (c == 'x')
+               c = hex_byte(p), p += 2;
+            else if (c != '\\' && c != '\'' && c != '"')
+               c = -1;
+            if (c < 0)
+            {
+               dtoverlay_error("invalid escape in '%s'", override_value);
+               return NON_FATAL(FDT_ERR_BADVALUE);
+            }
+         }
+         *(q++) = (char)c;
+      }
+      *(q++) = '\0';
+      /* Append to or replace the property string */
       if ((strcmp(prop_name, "bootargs") == 0) &&
           ((prop_val = fdt_getprop_w(dtb->fdt, node_off, prop_name,
                                      &prop_len)) != NULL) &&
           (prop_len > 0) && prop_val[0])
       {
          prop_val[prop_len - 1] = ' ';
-         err = fdt_appendprop_string(dtb->fdt, node_off, prop_name, override_value);
+         err = fdt_appendprop(dtb->fdt, node_off, prop_name, unescaped_value, q - unescaped_value);
       }
       else if (strcmp(prop_name, "name") == 0) // "name" is a pseudo-property
       {
-         err = dtoverlay_set_node_name(dtb, node_off, override_value);
+         err = dtoverlay_set_node_name(dtb, node_off, unescaped_value);
       }
       else
-         err = fdt_setprop_string(dtb->fdt, node_off, prop_name, override_value);
+         err = fdt_setprop(dtb->fdt, node_off, prop_name, unescaped_value, q - unescaped_value);
    }
    else if (override_type == DTOVERRIDE_BYTE_STRING)
    {
@@ -1504,18 +1568,17 @@ int dtoverlay_override_one_target(int override_type,
 
       while (*p)
       {
-          int nib1, nib2;
+          int byteval;
           // whitespace and colons are legal separators
           if (*p == ':' || *p == ' ' || *p == '\t')
           {
              p++;
              continue;
           }
-          nib1 = hex_digit(*p++);
+          byteval = hex_byte(p);
-          nib2 = hex_digit(*p++);
+          if (byteval < 0)
-          if (nib1 < 0 || nib2 < 0)
           {
-             dtoverlay_error("invalid bytestring '%s'", override_value);
+             dtoverlay_error("invalid bytestring at '%s'", p);
              return NON_FATAL(FDT_ERR_BADVALUE);
           }
           if (byte_count == sizeof(bytes_buf))
@@ -1523,7 +1586,8 @@ int dtoverlay_override_one_target(int override_type,
              dtoverlay_error("bytestring '%s' too long", override_value);
              return NON_FATAL(FDT_ERR_BADVALUE);
           }
-          bytes_buf[byte_count++] = (nib1 << 4) | nib2;
+          bytes_buf[byte_count++] = byteval;
+          p += 2;
       }
 
       err = fdt_setprop(dtb->fdt, node_off, prop_name, bytes_buf, byte_count);