From owner-freebsd-hackers@freebsd.org Thu Apr 1 04:06:42 2021 Return-Path: Delivered-To: freebsd-hackers@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id D1B685BB2AD for ; Thu, 1 Apr 2021 04:06:42 +0000 (UTC) (envelope-from asomers@gmail.com) Received: from mailman.nyi.freebsd.org (mailman.nyi.freebsd.org [IPv6:2610:1c1:1:606c::50:13]) by mx1.freebsd.org (Postfix) with ESMTP id 4F9qPZ3tMHz3KK7 for ; Thu, 1 Apr 2021 04:06:42 +0000 (UTC) (envelope-from asomers@gmail.com) Received: by mailman.nyi.freebsd.org (Postfix) id 8518F5BB342; Thu, 1 Apr 2021 04:06:42 +0000 (UTC) Delivered-To: hackers@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 84DE95BB515 for ; Thu, 1 Apr 2021 04:06:42 +0000 (UTC) (envelope-from asomers@gmail.com) Received: from mail-oi1-f176.google.com (mail-oi1-f176.google.com [209.85.167.176]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256 client-signature RSA-PSS (2048 bits) client-digest SHA256) (Client CN "smtp.gmail.com", Issuer "GTS CA 1O1" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4F9qPZ3JT6z3KM6 for ; Thu, 1 Apr 2021 04:06:42 +0000 (UTC) (envelope-from asomers@gmail.com) Received: by mail-oi1-f176.google.com with SMTP id n140so499915oig.9 for ; Wed, 31 Mar 2021 21:06:42 -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=gK2xXEgiaD9YN9uGKRXbL5c9u9mid9V0go19xba7Eog=; b=d/OaGm9GI28upiPMadYshB3NXI7zeAIeLhW6QdA3/3nmCGdT4l5/B+T8qLbzbBYBTq mCqgd4A/3PVIhePkCdRcspSmJ7M83gQ2pUGYDlI81F7gvPoMcy1N4ibmVculwXU0mc11 ox5woRZ5SWv4L0MrkuSjAwPR/WswUrZD03uh9boLwjTVb8Kx9OQqanO+HO7IN+m0nAuh Axo42qv0EvLMXXWS7WuNWvOpB7vAXXn4YPIE9lRaUvUq5Bz6trIdFcnjurkHoxwAQzd+ lqfoDs1yfTcbO6kAgDRvUW1HuhI7sXRM6qzy9448Yw7JPdSFfaQENQNRcM0bf7ZXqSt+ B2LA== X-Gm-Message-State: AOAM531f9Vu60tNXEycSkxgPIO+8zX9FglXFJD6uIT3Ebmkx3pSG1ujz aTg/aM8aSgsErcQr1vDFrHXnY1QF6iS1Uxcxbyo= X-Google-Smtp-Source: ABdhPJx8SPkJEiz31i2fBqrJtsDCYT2Mz2kWXpa3KGkOWQUTOsqMSgZMMcntl9kE/dePlTgf2LP2DgGc7hQ+HmEOB34= X-Received: by 2002:aca:b06:: with SMTP id 6mr4564043oil.73.1617250001141; Wed, 31 Mar 2021 21:06:41 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Alan Somers Date: Wed, 31 Mar 2021 22:06:30 -0600 Message-ID: Subject: Re: How does the stack's guard page work on amd64? To: Konstantin Belousov Cc: "freebsd-hackers@freebsd.org" X-Rspamd-Queue-Id: 4F9qPZ3JT6z3KM6 X-Spamd-Bar: ---- Authentication-Results: mx1.freebsd.org; none X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[] Content-Type: text/plain; charset="UTF-8" X-Content-Filtered-By: Mailman/MimeDel 2.1.34 X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Technical discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 01 Apr 2021 04:06:42 -0000 On Wed, Mar 31, 2021 at 5:21 AM Konstantin Belousov wrote: > On Tue, Mar 30, 2021 at 08:28:09PM -0600, Alan Somers wrote: > > On Tue, Mar 30, 2021 at 3:35 AM Konstantin Belousov > > > wrote: > > > > > On Mon, Mar 29, 2021 at 11:06:36PM -0600, Alan Somers wrote: > > > > Rust tries to detect stack overflow and handles it differently than > other > > > > segfaults, but it's currently broken on FreeBSD/amd64. I've got a > patch > > > > that fixes the problem, but I would like someone to confirm my > reasoning. > > > > > > > > It seems like FreeBSD's main thread stacks include a guard page at > the > > > > bottom. However, when Rust tries to create its own guard page (by > > > > re-mmap()ping and mprotect()ing it), it seems like FreeBSD's guard > page > > > > automatically moves up into the un-remapped region. At least, > that's how > > > > it behaves, based on the addresses that segfault. Is that correct? > > > Show the facts. For instance, procstat -v (and a note which > > > mapping was established by runtime for the 'guard') would tell the > whole > > > story. > > > > > > My guess would be that procctl(PROC_STACKGAP_CTL, > &PROC_STACKGAP_DISABLE) > > > would be enough. Cannot tell without specific data. > > > > > > > > > > > For other threads, Rust doesn't try to remap the guard page, it just > > > relies > > > > on the guard page created by libthr in _thr_stack_alloc. > > > > > > > > Finally, what changed in between FreeBSD 10.3 and 11.4? Rust's stack > > > > overflow detection worked in 10.3. > > > > > > > > -Alan > > > > _______________________________________________ > > > > freebsd-hackers@freebsd.org mailing list > > > > https://lists.freebsd.org/mailman/listinfo/freebsd-hackers > > > > To unsubscribe, send any mail to " > > > freebsd-hackers-unsubscribe@freebsd.org" > > > > > > > Here is the relevant portion of procstat -v for a test program built with > > the buggy rustc: > > 651 0x801554000 0x80155d000 rw- 0 17 3 0 ----- > df > > 651 0x801600000 0x801e00000 rw- 30 30 1 0 ----- > df > > 651 0x7fffdfffd000 0x7fffdfffe000 --- 0 0 0 0 ----- > -- > > 651 0x7fffdfffe000 0x7fffdffff000 --- 0 0 0 0 ----- > -- > > <--- What Rustc thinks is the guard page > > 651 0x7fffdffff000 0x7fffe0000000 --- 0 0 0 0 ----- > -- > > <--- Where did this come from? > This is the stack grow area, occupied by 'elastic' guard entry. > It serves two purposes: > 1. it keeps the space, preventing other non-fixed mappings from selecting > the grow area for mapping. > 2. it prevents stack from growing down to the next mapping below it, > preventing issues like StackClash. > > See mmap(2) esp. MAP_STACK part of it. > I saw that. And I even saw where libthr uses MAP_STACK when creating new threads. However, this program is single-threaded. Where does the stack get created for a process's main thread? I couldn't find that. > > > 651 0x7fffe0000000 0x7fffe001e000 rw- 30 30 1 0 ---D- > df > > 651 0x7fffe001e000 0x7fffe003e000 rw- 32 32 1 0 ---D- > df > > > > Rustc tries to create that guard page by finding the base address of the > > stack, reallocating one page, then mprotect()ing it, like this: > > > mmap(0x7fffdfffe000,0x1000,0x3,0x1012,0xffffffff,0) > > mprotect(0x7fffdfffe000,0x1000,0) > > > > If I patch rustc to not attempt to allocate a guard page, then its memory > > map looks like this. Notice that 0x7fffdffff000 is now accessible > It is accessible because stack grown down into this address. > > > 662 0x801531000 0x80155b000 rw- 3 17 3 0 ----- > df > > 662 0x801600000 0x801e00000 rw- 30 30 1 0 ----- > df > > 662 0x7fffdfffd000 0x7fffdfffe000 --- 0 0 0 0 ----- > -- > > 662 0x7fffdfffe000 0x7fffdffff000 --- 0 0 0 0 ----- > -- > > 662 0x7fffdffff000 0x7fffe001e000 rw- 31 31 1 0 ---D- > df > > 662 0x7fffe001e000 0x7fffe003e000 rw- 32 32 1 0 ---D- > df > > > > So the real question is, why does 0x7fffdffff000 become protected when > > rustc protects 0x7fffdfffe000 ? > See above. > > As I said in earlier response, if you want fully shrinkable stack guard, > set procctl(PROC_STACKGAP_CTL, &PROC_STACKGAP_DISABLE) during runtime > initialization. > > Or better, do not create custom guard page at all, relying on system guard. > That's what my patch does. But I've only tested it on amd64, and I don't have access to alternative architectures. Does every architecture create a stack guard this way? -Alan