From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from neon.ruby-lang.org (neon.ruby-lang.org [221.186.184.75]) by dcvr.yhbt.net (Postfix) with ESMTP id BFE001F5AE for ; Fri, 10 Jul 2020 19:51:40 +0000 (UTC) Received: from neon.ruby-lang.org (localhost [IPv6:::1]) by neon.ruby-lang.org (Postfix) with ESMTP id 4772C120A40; Sat, 11 Jul 2020 04:51:08 +0900 (JST) Received: from xtrwkhkc.outbound-mail.sendgrid.net (xtrwkhkc.outbound-mail.sendgrid.net [167.89.16.28]) by neon.ruby-lang.org (Postfix) with ESMTPS id 01B50120A39 for ; Sat, 11 Jul 2020 04:51:05 +0900 (JST) Received: by filterdrecv-p3iad2-5b55dcd864-5dqzw with SMTP id filterdrecv-p3iad2-5b55dcd864-5dqzw-19-5F08C6C1-4C 2020-07-10 19:51:29.595294313 +0000 UTC m=+1219322.470059144 Received: from herokuapp.com (unknown) by ismtpd0001p1iad1.sendgrid.net (SG) with ESMTP id XvDO0XUqR5W_ZfGG9I8cbA for ; Fri, 10 Jul 2020 19:51:29.445 +0000 (UTC) Date: Fri, 10 Jul 2020 19:51:29 +0000 (UTC) From: chris@chrisseaton.com Message-ID: References: Mime-Version: 1.0 X-Redmine-MailingListIntegration-Message-Ids: 74906 X-Redmine-Project: ruby-master X-Redmine-Issue-Tracker: Bug X-Redmine-Issue-Id: 17023 X-Redmine-Issue-Author: larskanis X-Redmine-Sender: chrisseaton X-Mailer: Redmine X-Redmine-Host: bugs.ruby-lang.org X-Redmine-Site: Ruby Issue Tracking System X-Auto-Response-Suppress: All Auto-Submitted: auto-generated X-SG-EID: =?us-ascii?Q?NC3uwXThXFpCxeI+EU+QkxuM4rZrp+UkiX2tCBF1HHtT4M265Txoyp9a4Emq+z?= =?us-ascii?Q?vKEve4v01EBzZX0aLVqT=2FIZAIZiPz9C4eOv2vuE?= =?us-ascii?Q?n=2F8uwYLCB2N5dohsyVFZpUdYCLOZVnHx20Z+VVq?= =?us-ascii?Q?otf76eGvRANJ6CJNADSFiypBGJh9XMsrtB0ovif?= =?us-ascii?Q?IobK7Ou0DO1DaCeLcDvfuoPW9sMP5V4tO6EW=2FQK?= =?us-ascii?Q?PbSi5xQA=2F57Jc+A74=3D?= To: ruby-core@ruby-lang.org X-ML-Name: ruby-core X-Mail-Count: 99116 Subject: [ruby-core:99116] [Ruby master Bug#17023] How to prevent String memory to be relocated in ruby-ffi X-BeenThere: ruby-core@ruby-lang.org X-Mailman-Version: 2.1.15 Precedence: list Reply-To: Ruby developers List-Id: Ruby developers List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: ruby-core-bounces@ruby-lang.org Sender: "ruby-core" Issue #17023 has been updated by chrisseaton (Chris Seaton). > The user has to ensure on Ruby level that the string isn't GC'ed The contract has always been more than that - the user must also not modify the string, as this may cause the character pointer to be reallocated. ---------------------------------------- Bug #17023: How to prevent String memory to be relocated in ruby-ffi https://bugs.ruby-lang.org/issues/17023#change-86491 * Author: larskanis (Lars Kanis) * Status: Open * Priority: Normal * ruby -v: ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux] * Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN ---------------------------------------- [ruby-ffi](https://github.com/ffi/ffi) allows to pass String objects to C by using the `:string` argument type. This way the string memory returned by `RSTRING_PTR` is passed to the C function. The user has to ensure on Ruby level that the string isn't GC'ed - as long as it is used on C level. That's the contract and this worked with all past ruby versions, but ruby-2.7 introduced `GC.compact`, which can relocate strings to another memory location. This example shows the situation and that the string is relocated although it is still referenced in ruby code: ```ruby File.write "string-relocate.c", <<-EOC static char *g_str; void set(char* str) { g_str = str; } char* get() { return g_str; } EOC system "gcc -shared -fPIC string-relocate.c -o string-relocate.so" require 'ffi' class Foo extend FFI::Library ffi_lib File.expand_path('string-relocate.so') attach_function :set, [:string], :void attach_function :get, [], :string def initialize(count) proc {} # necessary to trigger relocation a = "a" * count set(a) GC.verify_compaction_references(toward: :empty, double_heap: true) puts "get(#{count}): #{get} (should be: #{a})" end end Foo.new(23) Foo.new(24) ``` The output looks like so on ruby-2.7.1: ``` get(23): (should be: aaaaaaaaaaaaaaaaaaaaaaa) get(24): aaaaaaaaaaaaaaaaaaaaaaaa (should be: aaaaaaaaaaaaaaaaaaaaaaaa) ``` So using `GC.compact` while a string parameter is in use, both on Ruby and on C level, can cause invalid memory access. How can this prevented? A C extension is expected to use `rb_gc_mark()` in order to pin the VALUE to a memory location. But I couldn't find a way to pin a `VALUE` at the time the argument is passed to the C function, which is the only point in time ruby-ffi has access to it. ---Files-------------------------------- string-relocate.rb (653 Bytes) -- https://bugs.ruby-lang.org/