Support Zip archives not starting at zero offset

Zip archives are usually read from the back to the front, making it
possible to append them to other files (e.g. executables) in order to
provide some kind of embedded filesystem.

We can actually extract the start of the archive by looking at the
central directory offset, by comparing the specified and the actual file
offset.

The Zip64 format adds another challenge as the EOCD locator specifies
the offset to the EOCD starting from the beginning of the archive. Such
relative offset cannot be directly used for archives not starting at
position zero, hence the addition of another step that tries to locate
the 64bit EOCD right before the EOCD locator. The added overhead is
pretty small (and could be made smaller by reading both the presumed
EOCD and EOCD locator at once) and works pretty well for 90% of the
archives I've tested it with, if that heuristic fails the old behaviour
is preserved.

miniz_zip.c

@@ -647,10 +647,23 @@ static int mz_stat64(const char *path, struct __stat64 *buffer)
         return MZ_TRUE;
     }
 
+    static mz_bool mz_zip_reader_eocd64_valid(mz_zip_archive *pZip, uint64_t offset, uint8_t *buf)
+    {
+        if (pZip->m_pRead(pZip->m_pIO_opaque, offset, buf, MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE) == MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE)
+        {
+            if (MZ_READ_LE32(buf + MZ_ZIP64_ECDH_SIG_OFS) == MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIG)
+            {
+                return MZ_TRUE;
+            }
+        }
+
+        return MZ_FALSE;
+    }
+
     static mz_bool mz_zip_reader_read_central_dir(mz_zip_archive *pZip, mz_uint flags)
     {
         mz_uint cdir_size = 0, cdir_entries_on_this_disk = 0, num_this_disk = 0, cdir_disk_index = 0;
-        mz_uint64 cdir_ofs = 0;
+        mz_uint64 cdir_ofs = 0, eocd_ofs = 0, archive_ofs = 0;
         mz_int64 cur_file_ofs = 0;
         const mz_uint8 *p;
 
@@ -672,6 +685,7 @@ static int mz_stat64(const char *path, struct __stat64 *buffer)
         if (!mz_zip_reader_locate_header_sig(pZip, MZ_ZIP_END_OF_CENTRAL_DIR_HEADER_SIG, MZ_ZIP_END_OF_CENTRAL_DIR_HEADER_SIZE, &cur_file_ofs))
             return mz_zip_set_error(pZip, MZ_ZIP_FAILED_FINDING_CENTRAL_DIR);
 
+        eocd_ofs = cur_file_ofs;
         /* Read and verify the end of central directory record. */
         if (pZip->m_pRead(pZip->m_pIO_opaque, cur_file_ofs, pBuf, MZ_ZIP_END_OF_CENTRAL_DIR_HEADER_SIZE) != MZ_ZIP_END_OF_CENTRAL_DIR_HEADER_SIZE)
             return mz_zip_set_error(pZip, MZ_ZIP_FILE_READ_FAILED);
@@ -685,21 +699,40 @@ static int mz_stat64(const char *path, struct __stat64 *buffer)
             {
                 if (MZ_READ_LE32(pZip64_locator + MZ_ZIP64_ECDL_SIG_OFS) == MZ_ZIP64_END_OF_CENTRAL_DIR_LOCATOR_SIG)
                 {
-                    zip64_end_of_central_dir_ofs = MZ_READ_LE64(pZip64_locator + MZ_ZIP64_ECDL_REL_OFS_TO_ZIP64_ECDR_OFS);
+                    pZip->m_pState->m_zip64 = MZ_TRUE;
-                    if (zip64_end_of_central_dir_ofs > (pZip->m_archive_size - MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE))
-                        return mz_zip_set_error(pZip, MZ_ZIP_NOT_AN_ARCHIVE);
-
-                    if (pZip->m_pRead(pZip->m_pIO_opaque, zip64_end_of_central_dir_ofs, pZip64_end_of_central_dir, MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE) == MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE)
-                    {
-                        if (MZ_READ_LE32(pZip64_end_of_central_dir + MZ_ZIP64_ECDH_SIG_OFS) == MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIG)
-                        {
-                            pZip->m_pState->m_zip64 = MZ_TRUE;
-                        }
-                    }
                 }
             }
         }
 
+        if (pZip->m_pState->m_zip64)
+        {
+            /* Try locating the EOCD64 right before the EOCD64 locator. This works even
+             * when the effective start of the zip header is not yet known. */
+            if (cur_file_ofs < MZ_ZIP64_END_OF_CENTRAL_DIR_LOCATOR_SIZE +
+                                   MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE)
+                return mz_zip_set_error(pZip, MZ_ZIP_NOT_AN_ARCHIVE);
+
+            zip64_end_of_central_dir_ofs = cur_file_ofs -
+                                           MZ_ZIP64_END_OF_CENTRAL_DIR_LOCATOR_SIZE -
+                                           MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE;
+
+            if (!mz_zip_reader_eocd64_valid(pZip, zip64_end_of_central_dir_ofs,
+                                            pZip64_end_of_central_dir))
+            {
+                /* That failed, try reading where the locator tells us to. */
+                zip64_end_of_central_dir_ofs = MZ_READ_LE64(
+                    pZip64_locator + MZ_ZIP64_ECDL_REL_OFS_TO_ZIP64_ECDR_OFS);
+
+                if (zip64_end_of_central_dir_ofs >
+                    (pZip->m_archive_size - MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE))
+                    return mz_zip_set_error(pZip, MZ_ZIP_NOT_AN_ARCHIVE);
+
+                if (!mz_zip_reader_eocd64_valid(pZip, zip64_end_of_central_dir_ofs,
+                                                pZip64_end_of_central_dir))
+                    return mz_zip_set_error(pZip, MZ_ZIP_NOT_AN_ARCHIVE);
+            }
+        }
+
         pZip->m_total_files = MZ_READ_LE16(pBuf + MZ_ZIP_ECDH_CDIR_TOTAL_ENTRIES_OFS);
         cdir_entries_on_this_disk = MZ_READ_LE16(pBuf + MZ_ZIP_ECDH_CDIR_NUM_ENTRIES_ON_DISK_OFS);
         num_this_disk = MZ_READ_LE16(pBuf + MZ_ZIP_ECDH_NUM_THIS_DISK_OFS);
@@ -757,6 +790,26 @@ static int mz_stat64(const char *path, struct __stat64 *buffer)
         if ((cdir_ofs + (mz_uint64)cdir_size) > pZip->m_archive_size)
             return mz_zip_set_error(pZip, MZ_ZIP_INVALID_HEADER_OR_CORRUPTED);
 
+        /* The end of central dir follows the central dir, unless the zip file has
+         * some trailing data (e.g. it is appended to an executable file). */
+        archive_ofs = eocd_ofs - (cdir_ofs + cdir_size);
+        if (pZip->m_pState->m_zip64)
+        {
+            if (archive_ofs < MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE +
+                                  MZ_ZIP64_END_OF_CENTRAL_DIR_LOCATOR_SIZE)
+                return mz_zip_set_error(pZip, MZ_ZIP_INVALID_HEADER_OR_CORRUPTED);
+
+            archive_ofs -= MZ_ZIP64_END_OF_CENTRAL_DIR_HEADER_SIZE +
+                           MZ_ZIP64_END_OF_CENTRAL_DIR_LOCATOR_SIZE;
+        }
+
+        /* Update the archive start position, but only if not specified. */
+        if (pZip->m_pState->m_file_archive_start_ofs == 0)
+        {
+            pZip->m_pState->m_file_archive_start_ofs = archive_ofs;
+            pZip->m_archive_size -= archive_ofs;
+        }
+
         pZip->m_central_directory_file_ofs = cdir_ofs;
 
         if (pZip->m_total_files)