+
+#define EOIE_SIZE (4 + GIT_SHA1_RAWSZ) /* <4-byte offset> + <20-byte hash> */
+#define EOIE_SIZE_WITH_HEADER (4 + 4 + EOIE_SIZE) /* <4-byte signature> + <4-byte length> + EOIE_SIZE */
+
+static size_t read_eoie_extension(const char *mmap, size_t mmap_size)
+{
+ /*
+ * The end of index entries (EOIE) extension is guaranteed to be last
+ * so that it can be found by scanning backwards from the EOF.
+ *
+ * "EOIE"
+ * <4-byte length>
+ * <4-byte offset>
+ * <20-byte hash>
+ */
+ const char *index, *eoie;
+ uint32_t extsize;
+ size_t offset, src_offset;
+ unsigned char hash[GIT_MAX_RAWSZ];
+ git_hash_ctx c;
+
+ /* ensure we have an index big enough to contain an EOIE extension */
+ if (mmap_size < sizeof(struct cache_header) + EOIE_SIZE_WITH_HEADER + the_hash_algo->rawsz)
+ return 0;
+
+ /* validate the extension signature */
+ index = eoie = mmap + mmap_size - EOIE_SIZE_WITH_HEADER - the_hash_algo->rawsz;
+ if (CACHE_EXT(index) != CACHE_EXT_ENDOFINDEXENTRIES)
+ return 0;
+ index += sizeof(uint32_t);
+
+ /* validate the extension size */
+ extsize = get_be32(index);
+ if (extsize != EOIE_SIZE)
+ return 0;
+ index += sizeof(uint32_t);
+
+ /*
+ * Validate the offset we're going to look for the first extension
+ * signature is after the index header and before the eoie extension.
+ */
+ offset = get_be32(index);
+ if (mmap + offset < mmap + sizeof(struct cache_header))
+ return 0;
+ if (mmap + offset >= eoie)
+ return 0;
+ index += sizeof(uint32_t);
+
+ /*
+ * The hash is computed over extension types and their sizes (but not
+ * their contents). E.g. if we have "TREE" extension that is N-bytes
+ * long, "REUC" extension that is M-bytes long, followed by "EOIE",
+ * then the hash would be:
+ *
+ * SHA-1("TREE" + <binary representation of N> +
+ * "REUC" + <binary representation of M>)
+ */
+ src_offset = offset;
+ the_hash_algo->init_fn(&c);
+ while (src_offset < mmap_size - the_hash_algo->rawsz - EOIE_SIZE_WITH_HEADER) {
+ /* After an array of active_nr index entries,
+ * there can be arbitrary number of extended
+ * sections, each of which is prefixed with
+ * extension name (4-byte) and section length
+ * in 4-byte network byte order.
+ */
+ uint32_t extsize;
+ memcpy(&extsize, mmap + src_offset + 4, 4);
+ extsize = ntohl(extsize);
+
+ /* verify the extension size isn't so large it will wrap around */
+ if (src_offset + 8 + extsize < src_offset)
+ return 0;
+
+ the_hash_algo->update_fn(&c, mmap + src_offset, 8);
+
+ src_offset += 8;
+ src_offset += extsize;
+ }
+ the_hash_algo->final_fn(hash, &c);
+ if (!hasheq(hash, (const unsigned char *)index))
+ return 0;
+
+ /* Validate that the extension offsets returned us back to the eoie extension. */
+ if (src_offset != mmap_size - the_hash_algo->rawsz - EOIE_SIZE_WITH_HEADER)
+ return 0;
+
+ return offset;
+}
+
+static void write_eoie_extension(struct strbuf *sb, git_hash_ctx *eoie_context, size_t offset)
+{
+ uint32_t buffer;
+ unsigned char hash[GIT_MAX_RAWSZ];
+
+ /* offset */
+ put_be32(&buffer, offset);
+ strbuf_add(sb, &buffer, sizeof(uint32_t));
+
+ /* hash */
+ the_hash_algo->final_fn(hash, eoie_context);
+ strbuf_add(sb, hash, the_hash_algo->rawsz);
+}