diff --git a/ChangeLog b/ChangeLog
index b54c9dd05a..3871ad1abd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sat Jun  7 12:51:51 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (read_all): truncate the buffer before appending read data,
+	  instead of truncating before reading.
+	  [ruby-core:55951] [Bug #8625]
+
 Sat Jun  7 12:28:53 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/digest/digest.c (rb_digest_instance_equal): no need to call
diff --git a/io.c b/io.c
index 653034dd58..9900118818 100644
--- a/io.c
+++ b/io.c
@@ -2398,23 +2398,27 @@ read_all(rb_io_t *fptr, long siz, VALUE str)
     int cr;
 
     if (NEED_READCONV(fptr)) {
+	int first = !NIL_P(str);
 	SET_BINARY_MODE(fptr);
 	io_setstrbuf(&str,0);
         make_readconv(fptr, 0);
         while (1) {
             VALUE v;
             if (fptr->cbuf.len) {
+		if (first) rb_str_set_len(str, first = 0);
                 io_shift_cbuf(fptr, fptr->cbuf.len, &str);
             }
             v = fill_cbuf(fptr, 0);
             if (v != MORE_CHAR_SUSPENDED && v != MORE_CHAR_FINISHED) {
                 if (fptr->cbuf.len) {
+		    if (first) rb_str_set_len(str, first = 0);
                     io_shift_cbuf(fptr, fptr->cbuf.len, &str);
                 }
                 rb_exc_raise(v);
             }
             if (v == MORE_CHAR_FINISHED) {
                 clear_readconv(fptr);
+		if (first) rb_str_set_len(str, first = 0);
                 return io_enc_str(str, fptr);
             }
         }
diff --git a/test/ruby/test_pipe.rb b/test/ruby/test_pipe.rb
index 34f231ad8c..bcea91bebb 100644
--- a/test/ruby/test_pipe.rb
+++ b/test/ruby/test_pipe.rb
@@ -13,4 +13,17 @@ class TestPipe < Test::Unit::TestCase
       r.close
     end
   end
+  class WithConversion < self
+    def open_file(content)
+      r, w = IO.pipe
+      w << content
+      w.close
+      r.set_encoding("us-ascii:utf-8")
+      begin
+        yield r
+      ensure
+        r.close
+      end
+    end
+  end
 end