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=-4.1 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,MAILING_LIST_MULTI, SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from sourceware.org (server2.sourceware.org [8.43.85.97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by dcvr.yhbt.net (Postfix) with ESMTPS id E935D1F934 for ; Tue, 6 Oct 2020 12:13:09 +0000 (UTC) Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id 144463851C2F; Tue, 6 Oct 2020 12:13:09 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 144463851C2F DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1601986389; bh=NWRcIl4H7NR5wjNYHStWtcM865vMj76wc7Y4EJHzHWk=; h=References:In-Reply-To:Date:Subject:To:List-Id:List-Unsubscribe: List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:Cc: From; b=l7E9V4rR0hjS1L9qHyuHUP5MuWFjZZ3OyrIBTireVxZEP9VgJa1DkO9Nle4PaecYs KNOMJtWvqfHRYGAYniwIuC+38PRK0Sb0SQmWrwQU3Glwuwj+n1dSZGkIUa6phwzfsv NUcI5D7xIJwtH7pTf/iyg8kSQUquJXE4nEW8vJqI= Received: from mail-ot1-x342.google.com (mail-ot1-x342.google.com [IPv6:2607:f8b0:4864:20::342]) by sourceware.org (Postfix) with ESMTPS id B73793857013 for ; Tue, 6 Oct 2020 12:13:05 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org B73793857013 Received: by mail-ot1-x342.google.com with SMTP id s66so12059458otb.2 for ; Tue, 06 Oct 2020 05:13:05 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=NWRcIl4H7NR5wjNYHStWtcM865vMj76wc7Y4EJHzHWk=; b=joQ6nsBvrc7H/8zSgrI1F1kuLIMVTRI0PM2tZHEM6qRC8AUUkAqGFE7GRZbxxpqWmJ HWzuk6cFrhOiiUr/0t+owT/M2BpYLzmRCAac6bMxVhLbappgUhl9GzvUHzeLBCnJKKIz i7R8V43ksjoZ+MzVM71rCzvVMlAbUSAt95yLVJjVSO5lNsIe6tF9i83mqbE5ycgC6osH 5Y662kKLdM+QvpEvlXMA+5JeRKGc9xO/32bRf+y3xk/jCVjIeW9mtV0o+qGQwvP0yN33 ET9zpn2fJP9rq6R1t2Mc3r8uJHvHLuSxPhzUiggCyP6zn3FHdneSx8ALzdAPBZiUSqbH 1k/A== X-Gm-Message-State: AOAM531lEy4rJ1O19Ibb8AnnY0YQEQ6NWPbvYV9ifnPkrwygB8vhM16R hCvnIALmu/a7WIVXl5vxvVWnRdyLABhra1fNoTs= X-Google-Smtp-Source: ABdhPJwjE+8Vth572W+dNh6A8WM0UhDVuIqFRfy1nr1NrlLlFhSLdMEmG9Fhy30SKKlKIoIyIuRGQqzFANhVi6gsMbc= X-Received: by 2002:a9d:6498:: with SMTP id g24mr2742514otl.179.1601986385044; Tue, 06 Oct 2020 05:13:05 -0700 (PDT) MIME-Version: 1.0 References: <20200929205746.6763-1-chang.seok.bae@intel.com> <20201005134534.GT6642@arm.com> <20201006092532.GU6642@arm.com> In-Reply-To: <20201006092532.GU6642@arm.com> Date: Tue, 6 Oct 2020 05:12:29 -0700 Message-ID: Subject: Re: [RFC PATCH 0/4] x86: Improve Minimum Alternate Stack Size To: Dave Martin Content-Type: text/plain; charset="UTF-8" X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: "H.J. Lu via Libc-alpha" Reply-To: "H.J. Lu" Cc: linux-arch , Len Brown , Tony Luck , GNU C Library , "Ravi V. Shankar" , "Chang S. Bae" , the arch/x86 maintainers , LKML , Dave Hansen , Andy Lutomirski , Linux API , Thomas Gleixner , Borislav Petkov , Ingo Molnar Errors-To: libc-alpha-bounces@sourceware.org Sender: "Libc-alpha" On Tue, Oct 6, 2020 at 2:25 AM Dave Martin wrote: > > On Mon, Oct 05, 2020 at 10:17:06PM +0100, H.J. Lu wrote: > > On Mon, Oct 5, 2020 at 6:45 AM Dave Martin wrote: > > > > > > On Tue, Sep 29, 2020 at 01:57:42PM -0700, Chang S. Bae wrote: > > > > During signal entry, the kernel pushes data onto the normal userspace > > > > stack. On x86, the data pushed onto the user stack includes XSAVE state, > > > > which has grown over time as new features and larger registers have been > > > > added to the architecture. > > > > > > > > MINSIGSTKSZ is a constant provided in the kernel signal.h headers and > > > > typically distributed in lib-dev(el) packages, e.g. [1]. Its value is > > > > compiled into programs and is part of the user/kernel ABI. The MINSIGSTKSZ > > > > constant indicates to userspace how much data the kernel expects to push on > > > > the user stack, [2][3]. > > > > > > > > However, this constant is much too small and does not reflect recent > > > > additions to the architecture. For instance, when AVX-512 states are in > > > > use, the signal frame size can be 3.5KB while MINSIGSTKSZ remains 2KB. > > > > > > > > The bug report [4] explains this as an ABI issue. The small MINSIGSTKSZ can > > > > cause user stack overflow when delivering a signal. > > > > > > > > In this series, we suggest a couple of things: > > > > 1. Provide a variable minimum stack size to userspace, as a similar > > > > approach to [5] > > > > 2. Avoid using a too-small alternate stack > > > > > > I can't comment on the x86 specifics, but the approach followed in this > > > series does seem consistent with the way arm64 populates > > > AT_MINSIGSTKSZ. > > > > > > I need to dig up my glibc hacks for providing a sysconf interface to > > > this... > > > > Here is my proposal for glibc: > > > > https://sourceware.org/pipermail/libc-alpha/2020-September/118098.html > > Thanks for the link. > > Are there patches yet? I already had some hacks in the works, but I can > drop them if there's something already out there. I am working on it. > > > 1. Define SIGSTKSZ and MINSIGSTKSZ to 64KB. > > Can we do this? IIUC, this is an ABI break and carries the risk of > buffer overruns. > > The reason for not simply increasing the kernel's MINSIGSTKSZ #define > (apart from the fact that it is rarely used, due to glibc's shadowing > definitions) was that userspace binaries will have baked in the old > value of the constant and may be making assumptions about it. > > For example, the type (char [MINSIGSTKSZ]) changes if this #define > changes. This could be a problem if an newly built library tries to > memcpy() or dump such an object defined by and old binary. > Bounds-checking and the stack sizes passed to things like sigaltstack() > and makecontext() could similarly go wrong. With my original proposal: https://sourceware.org/pipermail/libc-alpha/2020-September/118028.html char [MINSIGSTKSZ] won't compile. The feedback is to increase the constants: https://sourceware.org/pipermail/libc-alpha/2020-September/118092.html > > > 2. Add _SC_RSVD_SIG_STACK_SIZE for signal stack size reserved by the kernel. > > How about "_SC_MINSIGSTKSZ"? This was my initial choice since only the > discovery method is changing. The meaning of the value is exactly the > same as before. > > If we are going to rename it though, it could make sense to go for > something more directly descriptive, say, "_SC_SIGNAL_FRAME_SIZE". > > The trouble with including "STKSZ" is that is sounds like a > recommendation for your stack size. While the signal frame size is > relevant to picking a stack size, it's not the only thing to > consider. The problem is that AT_MINSIGSTKSZ is the signal frame size used by kernel. The minimum stack size for a signal handler is more likely AT_MINSIGSTKSZ + 1.5KB unless AT_MINSIGSTKSZ returns the signal frame size used by kernel + 6KB for user application. > > Also, do we need a _SC_SIGSTKSZ constant, or should the entire concept > of a "recommended stack size" be abandoned? glibc can at least make a > slightly more informed guess about suitable stack sizes than the kernel > (and glibc already has to guess anyway, in order to determine the > default thread stack size). Glibc should try to deduct signal frame size if AT_MINSIGSTKSZ isn't available. > > > 3. Deprecate SIGSTKSZ and MINSIGSTKSZ if _SC_RSVD_SIG_STACK_SIZE > > is in use. > > Great if we can do it. I was concerned that this might be > controversial. > > Would this just be a recommendation, or can we enforce it somehow? It is just an idea. We need to move away from constant SIGSTKSZ and MINSIGSTKSZ. -- H.J.