udhcpc: fix 6rd option formatting (was using 4 more bytes than there is)

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 23 insertions, 27 deletions

diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index d1e40ee65..d9269f277 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -161,8 +161,8 @@ static NOINLINE char *xmalloc_optname_optval(uint8_t *option, const struct dhcp_
         int len, type, optlen;
         char *dest, *ret;
 
-        /* option points to OPT_DATA, need to go back and get OPT_LEN */
-        len = option[OPT_LEN - OPT_DATA];
+        /* option points to OPT_DATA, need to go back to get OPT_LEN */
+        len = option[-OPT_DATA + OPT_LEN];
 
         type = optflag->flags & OPTION_TYPE_MASK;
         optlen = dhcp_option_lengths[type];
@@ -252,39 +252,29 @@ static NOINLINE char *xmalloc_optname_optval(uint8_t *option, const struct dhcp_
 
                         return ret;
                 }
-                case OPTION_6RD: {
+                case OPTION_6RD:
                         /* Option binary format (see RFC 5969):
-                         * 0                   1                   2                   3
-                         * 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+                         *  0                   1                   2                   3
+                         *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                          * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          * |  OPTION_6RD   | option-length |  IPv4MaskLen  |  6rdPrefixLen |
                          * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-                         * |                                                               |
                          * |                           6rdPrefix                           |
-                         * |                          (16 octets)                          |
-                         * |                                                               |
-                         * |                                                               |
-                         * |                                                               |
+                         * ...                        (16 octets)                        ...
                          * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-                         * |                     6rdBRIPv4Address(es)                      |
-                         * .                                                               .
-                         * .                                                               .
-                         * .                                                               .
+                         * ...                   6rdBRIPv4Address(es)                    ...
                          * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-                         *
                          * We convert it to a string
                          * "IPv4MaskLen 6rdPrefixLen 6rdPrefix 6rdBRIPv4Address..."
-                         */
-
-                        /* Sanity check: ensure that our length is at least 22 bytes, that
+                         *
+                         * Sanity check: ensure that our length is at least 22 bytes, that
                          * IPv4MaskLen <= 32,
                          * 6rdPrefixLen <= 128,
                          * 6rdPrefixLen + (32 - IPv4MaskLen) <= 128
                          * (2nd condition need no check - it follows from 1st and 3rd).
-                         * If any of these requirements is not fulfilled, return with empty
-                         * value.
+                         * Else, return envvar with empty value ("optname=")
                          */
-                        if (len >= 22
+                        if (len >= (1 + 1 + 16 + 4)
                          && option[0] <= 32
                          && (option[1] + 32 - option[0]) <= 128
                         ) {
@@ -295,19 +285,20 @@ static NOINLINE char *xmalloc_optname_optval(uint8_t *option, const struct dhcp_
                                 /* 6rdPrefix */
                                 dest += sprint_nip6(dest, /* "", */ option);
                                 option += 16;
-                                len -= 1 + 1 + 16;
-                                /* 6rdBRIPv4Address(es) */
+                                len -= 1 + 1 + 16 + 4;
+                                /* "+ 4" above corresponds to the length of IPv4 addr
+                                 * we consume in the loop below */
                                 while (1) {
+                                        /* 6rdBRIPv4Address(es) */
                                         dest += sprint_nip(dest, " ", option);
                                         option += 4;
-                                        len -= 4;
+                                        len -= 4; /* do we have yet another 4+ bytes? */
                                         if (len < 0)
-                                                break;
+                                                break; /* no */
                                 }
                         }
 
                         return ret;
-                }
 #if ENABLE_FEATURE_UDHCP_RFC3397
                 case OPTION_DNS_STRING:
                         /* unpack option into dest; use ret for prefix (i.e., "optname=") */
@@ -347,6 +338,10 @@ static NOINLINE char *xmalloc_optname_optval(uint8_t *option, const struct dhcp_
                         return ret;
 #endif
                 } /* switch */
+
+                /* If we are here, try to format any remaining data
+                 * in the option as another, similarly-formatted option
+                 */
                 option += optlen;
                 len -= optlen;
 // TODO: it can be a list only if (optflag->flags & OPTION_LIST).
@@ -356,7 +351,8 @@ static NOINLINE char *xmalloc_optname_optval(uint8_t *option, const struct dhcp_
                         break;
                 *dest++ = ' ';
                 *dest = '\0';
-        }
+        } /* while */
+
         return ret;
 }